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CLOSE THIS BOOKAppropriate Building Materials: a Catalogue of Potential Solutions (SKAT, 1988, 430 p.)
Examples of roof materials
VIEW THE DOCUMENTEarth reel roofs
VIEW THE DOCUMENTSoil brick roof
VIEW THE DOCUMENTClay tile roofs
VIEW THE DOCUMENTGypsum-sisal conoid
VIEW THE DOCUMENTPrecast concrete channel roof
VIEW THE DOCUMENTFerrocement roofs
VIEW THE DOCUMENTCorrugated fibre concrete roofing sheets
VIEW THE DOCUMENTFibre and micro concrete tiles
VIEW THE DOCUMENTDurable thatch with stiff-stem grasses
VIEW THE DOCUMENTBamboo roof structure
VIEW THE DOCUMENTPole timber roof structures
VIEW THE DOCUMENTBamboo and wood shingles
VIEW THE DOCUMENTCorrugated metal sheet roofiing

Appropriate Building Materials: a Catalogue of Potential Solutions (SKAT, 1988, 430 p.)

Examples of roof materials

Earth reel roofs

KEYWORDS:

Special properties

Heavy, high thermal capacity roof

Economical aspects

Low cost

Stability

Good

Skills required

Experience in earth construction

Equipment required

Standard construction equipment

Resistance to earthquake

Low

Resistance to hurricane

Good

Resistance to rain

Depends on finishing coat

Resistance to insects

Low

Climatic stability

Hot dry or highland climates

Stage of experience

Traditional

SHORT DESCRIPTION:

· This roof construction system is suitable for sloping and flat roofs.

· Its density and heat retaining capacity make it well suited for hot dry or highland regions, where days are hot and nights are cool.

· The main component is a reel, made by rolling long vegetable fibrous material (generally straw) and a wet clayey soil around a wooden spindle (3 - 5 cm 0, 80 - 100 cm long).

· The reels are laid between the timber purlins when still moist and pressed against each other, the space between them being filled with a fibre-soil mix.

· After drying, the cracks are filled with a mud slurry, on top of which a 2 cm layer of soil, stabilized with finely chopped fibres and lime is applied.

· Finally, the roof is covered with a bitumen roofing felt and a layer of sand or fine gravel.

· On account of the large proportion of vegetable fibres and timber, the risk of termite attack is great.

Further information: Bibl. 02.19, 23.24.


Preparation of Earth Reels and Construction of Roof (Drawings: Vorhauer, Bibl. 23.24)

Soil brick roof

KEYWORDS:

Special properties

Simple self-help prefabrication system

Economical aspects

Low to medium costs

Stability

Good

Skills required

Average construction skills

Equipment required

CINVA-Ram block press, formwork for beams

Resistance to earthquake

Low

Resistance to hurricane

Good

Resistance to rain

Depends on finishing coat

Resistance to insects

Good

Climatic stability

Hot climates, highland climates

Stage of experience

Experimental, numerous houses built in Tunisia

SHORT DESCRIPTION:

· This roof construction method was developed by the Swedish Association for Development of Low-Cost Housing, Lund University, Sweden, for a pilot project in Rohia, Tunisia, based on "organized do-it-yourself building".

· Apart from the self-help aspect, the aim was to design a strong roof (that could be walked on), using local materials other than timber, which is in short supply and expensive.

· The principal material chosen was the local soil, called Torba, a finely grained soil, containing 60 % CaO (lime). This was used to make soil-cement blocks with a CINVA-Ram block press.

· The slightly sloped roofs were constructed with precast concrete beams placed very accurately in parallel, at a distance just sufficient to place two soil-cement blocks such that they lean against each other (for which the blocks were made with one short end slanting). The block pairs were bonded with a lime-cement mortar. The completed roof received a coat of cement slurry and later a roughly 5 cm thick layer of compacted soil-cement, which was finally whitewashed.

Further information: SADEL, Arkitekrur 1, P.O. Box 118, 221 00 Lund, Sweden, Bibl. 00.01.

Clay tile roofs

KEYWORDS:

Special properties

Durable, waterproof cladding for sloped roofs

Economical aspects

Low to medium costs

Stability

Good

Skills required

Skilled labour

Equipment required

Clay tile production unit, roof construction equipment

Resistance to earthquake

Low

Resistance to hurricane

Medium to good

Resistance to rain

Very good

Resistance to insects

Very good

Climatic suitability

All climates, but most common in humid areas

Stage of experience

Traditional

SHORT DESCRIPTION:

· Burnt clay tile roofs are only used for sloping roofs between about 20° and 50° inclination of rafter, and the tile shapes differ for each range of slope. It should be remembered that the rafter pitch is always steeper than the tile pitch (see illustration overleaf).

· Clay tile production is a traditional village craft in many regions, but uniform shapes and qualities are difficult to achieve. Mechanized plants produce good quality tiles, but at higher costs. An appropriate intermediate solution is provided by mobile presses with interchangeable moulds for different tile shapes (see ANNEX: Machines and Equipment).

· Depending on the clay type and production method, a major problem of clay tiles is the immense loss (in India about 35 %) due to cracking and breakage. A good remedy has been found in the use of ammonium chloride as an admixture varying between 0.1 and 1.0 %, depending on the type of soil (Bibl. 00.41).

· Clay tiles are heavy, requiring a strong substructure and closely spaced battens. Therefore, tile designs (eg Mangalore tiles), which require wider spacing of battens, are lighter and more economical. But generally, the weight of the roof and loose connection of tiles, make them susceptible to destruction in earthquakes.

· Good quality tiles with good overlaps are perfectly waterproof. The red colour, however, tends to absorb solar radiation, so theta suspended ceiling may be needed for indoor comfort.


Relation of Rafter Pitch and Tile Pitch; Other Clay Roofing Elements; Some Typical Clay Roofing Tiles and their Minimum Rafter Pitch (reduced by 5°, if the tiles are placed over a waterproof membrane)

Gypsum-sisal conoid

KEYWORDS:

Special properties

Innovative material and design

Economical aspects

Low to medium costs

Stability

Good

Skills required

Special training

Equipment required

Simple wooden framework

Resistance to earthquake

Good

Resistance to hurricane

Good, if protected from rain

Resistance to rain

Low

Resistance to insects

Good

Climatic suitability

Dry climates

Stage of experience

Experimental

SHORT DESCRIPTION:

· This is an experimental unit, developed by Prof. Roberto Mattone and Gloria Pasero at the Turin Polytechnic, Italy.

· The conoid unit has a shape which makes it suitable for use as roofing as well as wall components.

· The aim was to produce a strong, versatile component from gypsum and sisal (which are abundantly available in some regions), using simple formwork and equipment.

· Laboratory tests showed a good strength to weight performance, since the fibres have high tensile strength and bond well with the gypsum. Furthermore, resistance to fire and biological attack is good.

· The main drawback is the solubility of gypsum in water, which calls for a completely waterproof surface protection.

Further information: Prof. Roberto Mattone, Facolta di Architettura, Politecnico di Torino,

Viale Mattioli 39, Torino 10125, Italy; Bibl. 23.15.


Preparing the formwork: the timber frame is filled with broken bricks and stone, first large pieces, then small pieces and finally a fine sand, which is smoothed to the desired shape, and covered with a polythene sheet. On this the gypsum-sisal mortar is spread to form the conoid.


Potential assembly of the modules

Precast concrete channel roof

KEYWORDS:

Special properties

High production rate, minimum formwork and space

Economical aspects

Medium to high costs

Stability

Very good

Skills required

Average construction skills

Equipment required

Special steel moulds

Resistance to earthquake

Good

Resistance to hurricane

Good

Resistance to rain

Good

Resistance to insects

Very good

Climatic suitability

All climates

Stage of experience

Experimental

SHORT DESCRIPTION:

· This roofing system, developed at the National Building Research Institute, Pretoria, South Africa, is based on a precast concrete trough-shaped element, which is cast with great speed and ease, requiring very little working space.

· The cross-sectional dimensions are shown in the diagram overleaf and the length used in the project was 4.27 m, resulting in a total weight of about 107 kg (or 25 kg/m). Seven 4 mm steel bars provide reinforcement along its length, and stirrups of 3.3 mm steel are placed every 30 cm. The elements are self-supporting, and can span 3.50 m with a cantilever on either side of the walls.

· The assembly of the roof is done manually. After placing the boughs side by side, the gaps between them and the top of the walls are closed by inserting precast filler blocks and sealed around the edges. A polythene sheet is laid over the troughs, which are covered with a 20 mm layer of loose gravel, for improved thermal performance and to protect the sheet. The gravel is kept in place by precast, shaped, no-fines concrete blocks placed dry at the ends of the troughs. Rainwater that collects in the troughs percolates through the no-fines concrete and can be collected. Hence, a 5 % slope is suitable.

Further information: Jorge L. Arrigone, Senior Chief Research Officer, National Building Research Institute, P.O. Box 395, Pretoria 0001, South Africa; Bibl. 23.02.

Precasting the Trough-Shaped Units

The steel mould consists of a trough-shaped base with supporting ribs, fixed to the concrete floor, a swell as moveable parts, ie side risers and end closer plates. The inner surface of the mould is covered with a polythene sheet and pushed in place with a steel trough-shaped form. The side and end risers are bolted into position, and a fairly dry mortar mix 1: 3 (cement: coarse sand) poured end distributed evenly, 33 mm thick on the horizontal parts and 22 mm thick on the sloping sides. The reinforcing grid of 4 mm steel bars is placed on the mortar, pushed down, and the surface evened out by tapping the sides of the mould.

About an hour later, a new polythene sheet is placed over the element, pushed in place with the steel form, the side and end risers bolted down and the procedure repeated as before. Up to 10 units are cast one on top of the other, each one taking about 20 minutes to complete. On average, six roofing units are made per mould per 8-hour working day. The units are cured wet for two weeks and dry for another two weeks.


FIGURE

Ferrocement roofs

KEYWORDS:

Special properties

Higher strenght: weight ratio than reinforced concrete

Economical aspects

High costs

Stability

Very good

Skills required

Special training

Equipment required

Formwork, masonry tools

Resistance to earthquake

Very good

Resistance to hurricane

Very good

Resistance to rain

Very good

Resistance to insects

Very good

Climatic suitability

All climates

Stage of experience

Experimental

SHORT DESCRIPTION:

· Ferrocement components are extremely thin (15 to 25 mm), but have a higher percentage of reinforcement than reinforced concrete, thus achieving a higher tensile-strength-to-weight ratio. Further strength and rigidity is achieved by curvature or folds.

· Ferrocement roofs can be made in situ or with precast components, the former being useful for free forms, the latter being appropriate for modular and repetitive constructions.

· Depending on the design, ferrocement roofs can be made to span large areas without supporting structures, thus saving costs and providing unobstructed covered areas. If the ferrocement surface is properly executed (complete cover of wire mesh, dense and smooth finish, cracks sealed) no surface protection is needed, thus saving further costs. However, it is advantageous to apply a reflective coat on the outer surface to reduce solar heat absorption.

Further information: Bibl. 10.02, 10.03, 10.04, 23.01, 23.13, 23.22.

Framed Ferrocement Roof (Bibl. 23.01)


FIGURE

· Once the walls are erected, no reinforced concrete ring beam is required, as the roof is designed to clamp the walls together.

· Around the top, outer edge of the walls, a timber frame (6 x 6 cm) is fixed, as well as two tripod frames above the floor area. The surfaces described by these frames are hyperbolic paraboloids (hypars), which are made up of straight lines. This simplifies the fixing of the wire mesh.

· The mesh (2 or 3 layers) is stretched over the frame and nailed or stapled onto it. The frame is only needed to hold the mesh during construction, as the structure will be self-supporting once plastered.

· Reinforcing bars are fixed around the wall and along the folds of the roof.

· The roof is plastered by a team on top forcing the mortar through the mesh, while another team below recovers the falling mortar to plaster the inside.

· This curved roof system, developed by P. Ambacher, France, permits the wind to blow around smoothly, making it very suitable for hurricane prone areas.

Precast Trough Element (Bibl. 23.22)

· These elements function on the principle that folded plates have much higher strength than plates of the same thickness but without folds.

· The roofing element shown here, developed at the Structural Engineering Research Centre, Roorkee, is made either on a stationary brick-and-concrete mould or on a portable wooden mould, and can be in the form of a trough or inverted.

· A reinforcement cage is prepared on the mould.

· Before placing the mortar, a thin coat of rich cement slurry is applied to the reinforcement cage with a brush. The mortar is then applied and pressed into the reinforcement. This is done in 2 or 3 layers. A specially designed vibrator, operated by two men, compacts the mortar.

· The finished element is moist cured for one week, before it is removed from the mould. The lower side is finished with a coat of cement slurry and cured for at least another week, before handling and installation.


FIGURE

Precast Segmental Element (Bibl. 23.13)

· The alternative to trough elements, shown on the previous page, is a segmental element, made principally in the same way.

· The segmental element shown here was developed at the Regional Research Laboratory, Jorhat, India.

· The element is 60 cm wide, 250 cm long and 2 cm thick. The reinforcement in each element consists of 5 bars of 6 mm 0 in the longitudinal direction and 10 bars of the same diameter in the transverse direction, with two layers of hexagonal chicken wire mesh. The mortar comprised 1 part cement: 2 parts sand by weight.

· Long-term performance tests have shown very satisfactory results.

Corrugated fibre concrete roofing sheets

KEYWORDS:

Special properties

Local, low-cost method

Economical aspects

Inexpensive durable roofing material

Stability

Good, if properly manufactured and installed

Skills required

Thorough training and constant quality control

Equipment required

Simple, locally made, transportable moulds

Resistance to earthquake

Uncertain

Resistance to hurricane

Good, if well installed and secured

Resistance to rain

Good

Resistance to insects

Good

Climatic suitability

All climates

Stage of experience

Fairly mature technology

SHORT DESCRIPTION:

Corrugated FC sheets

· were the first FC roofing elements to be developed, as the aim was to substitute gci and ac sheets;

· require fairly simple, locally made equipment and a very well coordinated working team of at least two workers;

· consume about the same amount of cement es asbestos cements sheets (15 kg per m2), on account of their greater thickness and production method by manual tamping, but require no electricity;

· are difficult to handle when fresh and to cure in water tanks, because of their large size;

· are difficult to transport and install without breakage, and do not tolerate inaccurately constructed and flexing supporting structures;

· withstand strong wind forces because they are heavy and have few overlaps.

In most cases FC/MC tiles are easier to produce and install than FC sheets and therefore represent the more appropriate solution.

Further information: RAS c/o SKAT, Vadianstrasse 42, CH-9000 St. Gall, Switzerland;
Bibl. 11.03,11.05,11.07,11.08,11.12, 11.15.

Production of Corrugated FC Sheets


FIGURE

Materials and equipment

· Cement: ordinary portland cement (Skgper 10 mm thick corrugated sheet of 60 x 60 cm) corresponding to cement: sand ratio of 1: 1; a pozzolana (eg rice husk ash) can be added to improve fibre durability and reduce cement content, but causes slow setting, which necessitates a larger number of moulds and larger workspace.

· Sand: (5 kg per sheet) preferably with angular particles and good grain size distribution between 0.06 and 2 mm, free from silt and clay.

· Fibre: (0.1 kg per sheet) mainly natural, such as sisal, jute, coir, or banana fibre, but also synthetic fibres, eg polypropelene or glass fibre, can be used. Long fibres can be used, but require a different (more difficult) manufacturing process and result in weaker products. Short fibres, chopped to lengths of 12 to 25 mm, are easy to process, provide cohesiveness to the wet mortar, permitting reshaping without cracking, and also help to prevent cracking due to drying shrinkage.

· Water: preferably drinkable water, just enough to make the mortar mix workable (water: cement ratio 0.5-0.65 by weight).

· Admixtures: such as waterproofers may be used, if the sand is not well graded, and colorants, if the grey cement colour is not desired.

· Screeding board: a flat horizontal board with outer frame, to define the FC sheet size and clamp down the polythene interface sheet.

· Corrugated setting moulds: gci or ac sheets, enough for two days production. All sheets should be obtained from a single batch made from a single master mould, as sheets from different batches or different producers are likely to have dissimilar corrugations. Accuracy in the corrugations is vital for proper installation and trouble-free performance.

· Other equipment: standard workshop tools.

Moulding and curing

· The correctly proportioned and well-mixed mortar is trowelled evenly onto the polythene sheet, which is fixed on the screeding board; the mortar is tamped, levelled to a uniform thickness of 10 mm and smoothed off with the trowel.

· The frame is removed, the edges of the mortar layer trimmed and the screeding board tilted, such that the polythene sheet with the wet fibre concrete is allowed to gradually slide onto the corrugated mould held below.

· The fresh FC sheet and mould is placed on a stack for primary curing for 24 hours, after which they are hard enough to be demoulded and placed upright for further curing (by regular watering), or completely immersed in water tanks for about 2 weeks.

· Demoulding should not be done later than 48 hours after moulding, as the sheets tend to shrink on drying, and will crack if resisted by the setting mould.

Production of FC Ridge Tiles

· Materials and equipment: same as for sheets, but different shape of frame, and screeding board made with hinges, so that it can be bent and used as the setting mould, held in a template.

· Moulding and curing: same as for sheets.


FIGURE

Installation of FC Roofing with Corrugated Sheets

The corrugated FC sheets are laid on timber roof structures in much the same way as gci and ac sheets. However, FC sheets are less flexible and can be damaged if the loads are not evenly distributed. Therefore, care must be taken in constructing the substructure, to ensure that the top edges of all members are properly aligned. If nails or bolts are used, holes (of slightly larger diameter) should be drilled beforehand. Alternatively, nibs with wire loops can be cast-in during moulding, avoiding the need for drilling. Mitred corners are essential for a weathertight fit.


FIGURE

Fibre and micro concrete tiles

KEYWORDS:

Special properties

Promising, local, low-cost method

Economical aspects

Inexpensive locally produced durable roofing material

Stability

Good, if properly manufactured and installed

Skills required

Thorough training and constant quality control

Equipment required

Imported, transportable production kit

Resistance to earthquake

Good

Resistance to hurricane

Satisfactory, if well installed and secured

Resistance to rain

Good

Resistance to insects

Good

Climatic suitability

All climates

Stage of experience

Mature technology

SHORT DESCRIPTION:

FC/MC tiles

· were developed to overcome most of the problems encountered in producing and installing corrugated FC sheets (previous example);

· are made most efficiently on a small vibrating table (hand powered or run by electricity, ea. a car battery), which can be operated by a single trained worker;

· can be made thinner (6 mm) than FC sheets (10 mm), and their cement: sand ratio (between 1: 2 and 1: 3) is less than for FC sheets (1: 1), so that the cement used for making tiles is only between 5 and 7 kg per m2 of roofing;

· are easy to handle when fresh and to cure in water tanks; do not tend to break as easily as sheets during transport and installation, and minor inaccuracies in the supporting structure have no negative effects; are easily torn off by strong wind forces, if they are not well fixed to the substructure.

Further information: RAS c/o SKAT, Vadianstrasse 42, CH-9000 St. Gall, Switzerland; Bibl. 11.03,11.05,11.07,11.08, 11.15.

Production of FC/MC tiles

Materials and equipment

· Cement: same as for FC sheets, but about 0.4 kg per 6 mm thick pantile of 50 x 25 cm, corresponding to cement: sand ratio of 1: 3.

· Sand: same as for sheets, but 1.2 kg per tile.

· Fibre: same as for sheets, but 0.02 kg per tile, used in FC tiles only.

· Aggregate: for MC tiles aggregate is used instead of fibre. The ratio sand to aggregate is between 2:1 and 1:1.

· Water and admixtures: same as for sheets.

· Screeding machine: comprising a vibrating screeding surface and interchangeable, hinged frame (for products of different shapes and thicknesses), whereby the vibrating mechanism is either powered by a 12 volt car battery or hand-powered. (A variety of models, depending on different user requirements and desired output rates are available from the Intermediate Technology Workshops, United Kingdom).

· Setting moulds: these are part of the pantile production kit, and are generally made of impact-resistant pvc, with rib markings (for accurate positioning of the tile edge) and supporting frame for stacking.

· Other equipment: same as for sheets.


FIGURE

Moulding and curing

· The wet mix is trowelled onto the polythene interface sheet on the screeding machine and, under vibration, smoothed with a trowel to the same level as the surrounding steel frame. At a predetermined spot at the top end of the tile, a matchbox-size nib is formed and a wire loop pushed into it (required for fixing to the roof).

· The steel frame is lifted off the screeding surface and the polythene sheet slowly pulled over the pvc setting mould, ensuring correct positioning of the tile edge to achieve uniform curvature.

· The mould with the fresh tile is then placed on a stack of moulds for initial setting and curing (24 hours), after which the tiles can be demoulded and cured for 2 weeks in wafer tanks or in an airtight container with vapour saturated air (vapour curing).


FIGURE

Pantiles and Roman tiles

Two types of tiles are common:

· The pantile : is of a sinouscourve like shape and can easily be placed on a slightly uneven roof.

· The Roman tile: gives a neater roof surface but requires an even roof structure.

Production of FC/MC Ridge Tiles

· Materials and equipment: same as for tiles, but with a different steel frame and setting moulds.

· Moulding and curing: same as for tiles, but with nibs and wire loops fitted after the tile is placed on the setting mould.

Installation of FC/MC Roofing

The FC/MC tiles are laid on timber laths (spaced at 40 cm centres) in the same way as clay roof materials. Slight inaccuracies do not cause major problems especially in the case of pantile. The tiles are fixed with wire loops, nailed or tied onto the timber laths.


FIGURE

Durable thatch with stiff-stem grasses

KEYWORDS:

Special properties

Excellent thermal and sound insulation

Economical aspects

Low cost

Stability

Good, depends on material and workmanship

Skills required

Special training and experience

Equipment required

Locally made thatching tools

Resistance to earthquake

Very good

Resistance to hurricane

Depends on fixing and roof structure detailing

Resistance to rain

Medium to good

Resistance to insects

Low

Climatic suitability

All zones where material is available

Stage of experience

Traditional

SHORT DESCRIPTION:

· Thatch is the most commonly used roof covering in the world, although it is barely recognized by construction experts. In India, for example, some 40 million houses are thatched. Almost any vegetable material, from the bark of trees to finely-tapering water reeds, can be used, though grasses, reeds and palms are most common.

· Traditional types of thatch have short durability and performance, but in certain regions (N.W. Europe, Southern Africa, Japan) skilled workmanship produces good quality functional roofing, with life expectancies between 25 and 70 years.

· Thatch uses renewable, local materials requiring minimal or zero artificial energy input in production, and costing less than most other types of roofing. Their application is labour-intensive - an important advantage in terms of employment generation. At the end of their useful life, thatching materials can be composted or compacted for use as fuel.

· The main drawback is its combustibility, but this is significantly reduced through good quality workmanship and common-sense precautions. Thatch is also susceptible to biological decay and weathering.

· The best thatching materials are stiff-stem grasses and reeds of 1 to 2 metres length and up to 10 mm diameter at the cut end. They should be straight (no bends at nodes), tapering and preferably hollow stemmed, as solid culms tend to dry out slowly and thus rot quickly.

Materials: Harvesting and Processing

· Thatch may come from three different sources: first from naturally occurring indigenous vegetation, secondly as a byproduct of food or cash-crop agriculture, and thirdly through the cultivation of a plant grown specifically for thatching.

· Water reed is most durable, but cereal straw (mainly wheat, but also rye, barley and rice) is more widely available. The less artificial fertilizer is used, the less susceptible they are to fungal attack.

· Harvesting is best done by hand, as modern combine harvesters break the straw. The mature (fully grown, dried) stem is cut about 5 cm above the ground.

· To facilitate tight and even thatching, the straw should be combed (with a hand-held rake) to remove dead leaves and other debris, then bundled and stored in a dry place. (The labour involved in combing the straw will be amply repaid, as it lasts more than twice as long as uncombed straw.)

· The bundles should measure 55 cm in circumference at the binding, which is tied about 30 cm from the cut end. Once bundled the straw is ready for thatching.


FIGURE

Roof Structure

· Almost any shape of roof with a minimum pitch of 45° can be thatched. Thatch will mould itself to any curve except a convex-shaped roof.

· Pole timbers and split battens may be used, and simple configurations work best, that is, valleys and other changes of roof pitch are not recommended.

· The structure should be capable of supporting up to 40 kg/m2, which is the weight of the heaviest material - reed.

· A tilting board, 35 mm thicker than subsequent battens, fixed along all the eaves and barges at eave level, is essential to force the first course into tension, making the rest of the thatch more tightly compacted.

Thatching Method

· Roof-work tools: pen-knife for opening bundles and cutting ties; leggatt (thatcher's mallet) for beating the thatch upwards to tighten the thatch coat; trimming knives for tidying completed work.

· Grass is sorted: short grass for eaves, gable edges and top course; long grass for rest of roof.

· Thatching begins at a right-hand verge (unless the thatcher is left-handed) and can be worked in vertical lanes (more common) or horizontal sections.

· The first course of thatch performs the same function as the foundations of a wall, and as it has the greatest vulnerability to wind damage, it needs to be very secure.

· Thatch is placed in horizontal layers, approx. 20 cm thick, secured by stitching, layer by layer, at approx. half-way between cut end and ear. Layers overlap as tiling, so fixings are covered and protected. Total thatch thickness is 30 cm. After fixing, the grass is wedged tightly into the ties with a leggatt. The compacted surface forms a pitch, identical to that of the rafters, and exposes only 2 - 3 cm of each stem. A slight lip should be left at the top of each course end will be driven back with the next course to form a neat and invisible junction.

· The ridge is the most vulnerable part of the roof and can be made of a variety of very durable materials, eg half-round burnt clay tiles, sheet metal, ferrocement, but they are expensive and detract from the appearance of the roof. More appealing and cheaper is a flexible grass wrapped over the apex, covering the upper course fixings and held with horizontal stitching.

· Material requirements are approx. 10 bundles grass per m2 of roof area; tough local string or steel wire for fixing ties. Experienced workers should fix 10 to 20 m2 per day.


FIGURE

Rainwater Collection

· Thatch roofs are generally not suitable for rainwater collection, unless a wide gutter - 30 cm minimum - is provided. A method called " tile substitution", developed and tested by Nicolas Hall, makes collection at eaves more efficient.

· Burnt clay tiles are substituted for thatch on the first course, producing a hard, straight cave.

· By doing so, the eaves are significantly strengthened (increasing the life of the roof); only a 10 cm gutter is needed (cheap, easily available, easily fixed); and the fire risk is considerably reduced.

· The main drawback of collecting rainwater from thatch roofs is that debris will first be washed off, contaminating the water. Hence, methods should be employed to discard the first flush of debris laden water.


Using split bamboo guttering with palm thatch

Durability

· A competently-laid grass thatch might last up to 40 years or more, though a grass ridge will need renewal every 8 - 10 years.

·Thatch is combustible and common-sense is the best protection against fire: avoidance of high building densities (urban areas); avoidance of open fires near thatched buildings; avoidance of chimneys, or careful design and construction only at the ridge, well insulated, regularly swept; protection of all electrical fitting in the roof space. In addition, the underside of thatch can be protected by fixing an incombustible board ceiling to the rafters.

· Chemical treatments to reduce the risk of fire, organic decay and weathering are possible, but none are cheap, permanent or of good value, and prohibit rainwater collection.

Further information: Bibl. 12.02, 12.03 and 23.11 or contact Nicolas Hall, 48a Hormead Road, London W9, U.K.

Bamboo roof structure

KEYWORDS:

Special properties

High strenght, flexibility, great variety of forms

Economical aspects

Low costs

Stability

Good

Skills required

Traditional bamboo craftmanship

Equipment required

Tools for cutting, splitting, tying bamboo

Resistance to earthquake

Very good

Resistance to hurricane

Good

Resistance to rain

Depends on protective measures

Resistance to insects

Low

Climatic suitability

Warm humid climates

Stage of experience

Experimental

SHORT DESCRIPTION:

The main advantages of using bamboo for roof constructions are:

· It is a traditional technology, which is well understood by local artisans. No special tools are required.

· The large-scale utilization of bamboo has no disastrous environmental consequences (as in the case of timber), on account of its quick replacement within 4 or 5 years.

· The physical properties of bamboo make it an ideal construction material for seismic areas.

· Compared with most other building materials, bamboo is cheap to buy, process and maintain.

There are, however, drawbacks that need to be overcome, for example:

· limited durability, mainly on account of excessive wetting and drying, insect and fungal attack, physical impact, and wear and tear;

· limited social acceptability, as a result of the limited durability of bamboo.

Further information: Bibl. 13.05, 13.06, 13.07.

Barrel Vault (Bibl. 13.05)

· This construction system was developed at the Research Laboratory for Experimental Construction, Kassel College of Technology, Federal Republic of Germany, headed by Prof. Gernot Minke.

· It demonstrates an unusual use of bamboo, in which the construction obtains its stability by compressive forces, acting perpendicularly to the bamboo's axis.

· On the principle of masonry barrel vaults, full-section bamboo culms are laid horizontally, one on top of the other following a curve, defined by an inverted catenary. (This is a curve formed by hanging a uniform chain freely between two points. The tensile forces induced by gravitation run along the line connecting the points of contact of each chain link. Since the curve remains stable when reversing the direction of forces, an inverted catenary is the ideal shape of a barrel vault.)

· Split bamboo strips of equal length are hung such that their ends are exactly the same distance apart as the ultimate roof span. The full-section bamboo culms are laid horizontally forming an inverted vault. Split bamboo strips are then laid on the inside, exactly opposite the outer ones. Holes are drilled through the split and whole bamboo and fixed by bolts or rivets.

· The whole structure is then turned over and fixed on the top of the walls, which preferably should have a timber or concrete ring beam, onto which the roof is connected.

· The roof should be covered with a waterproof membrane for rain protection. This can be covered by a suitable local thatching material, or more appropriately by a 10 cm layer of soil on which grass can grow. For initial reinforcement (to prevent slipping) the soil should be held down by a strong net (as used for fishing). The dense structure of the grass roofs will give the soil cover its ultimate stability.


FIGURE

Small Geodesic Dome (Bibl. 13.05)

· This construction system was also developed and tested by Prof. Minke and his team.

· The supporting structure of the dome is made up of approx. 1.5 m long pieces of full-section bamboo culms, connected in a series of triangles, making it rigid. The lengths of the bamboo members are determined by a geometrical design, which requires fairly accurate cutting to achieve a uniform shape. However, the simple connection system allows for adjustments during assembly. For a tighter fit at the connecting points, at which in alternate succession six and five members meet, the bamboo ends are bevelled (slanted).

· In the example described, the span of the dome was 5 m, a size that is easy to prefabricate and transport manually with 5 people.

· Sand filled tin cans served as footings, providing simple adjustment to differing loads. These were placed in foundations made of old steels drums, which were filled with building rubble and lean concrete.

· A strong waterproof membrane is needed to cover the dome, on which several roofing materials may be used, eg palm leaf or soft stem grass thatch, or wooden shingles on lathing. The structure erected at the Kassel College of Technology had a grass roof.


Connection detail

Grid Shell on a Square Base (Bibl. 13.05)

· The aim of this project, carried out by the Aachen Technical College, Federal Republic of Germany, was to develop a low-cost, earthquake resistant roof structure for developing countries, using only local materials and tools. The result was a bamboo grid shell, which is prefabricated on a flat surface and later lifted in the centre to give it its ultimate shape.

· The bamboo cane used had an average diameter of 30 mm and length of approximately 4 m. For the required length 7.2 m, each grid bar required the joining of two canes. Tests showed that the strongest joints were obtained by inserting thinner bamboo pieces in the cavities at the connecting ends and fixing them by means of short dowels.

· With these lengthened bars, a grid is laid out on the ground forming grid sectors of 50 x 50 cm. Each cross point had a dowel connection which was tied with string to prevent slipping, but to allow a scissor-like movement. After lifting the centre of the grid to the required height, l m cane pieces are placed approximately diagonally to the rhombic grid sections, in the direction of slope, and firmly tied to the grid, giving it stability.

· The edges of the grid form a square of 6 x 6 m, corresponding to the wall dimensions. A vertical bamboo piece is embedded in each corner of the walls and a kind of bamboo ring tie beam is fixed to them. This in turn holds the grid shell roof in place. The roof is covered by a waterproof membrane and a suitable local thatching material, other than stiff-stem grass. A possible alternative to thatch is a ferrocement cover, which would remain in place even if the bamboo grid shell should cease to support it.


Bamboo joint with thinner piece inserted in cavities; Corner detail with ring tie beam (a. top view, b. section)

Irregularly Shaped Grid Shells (Bibl. 13.05)

· In order to construct spatially curved load-bearing structures using relatively thin bars, the same principle of inverting catenary lines, as described under "Barrel Vault", is applied. The shape of such grid shells is, therefore, not designed, but determined by using suspended models (eg with chain nets). Several such structures using split bamboo have been developed and erected on a joint project of the Institute of Lightweight Structures, Stuttgart, Federal Republic of Germany, and the School of Architecture, Ahmedabad, India.

· Corresponding to the chain net, the grid is assembled on the ground and tied at each cross point. For irregular base plans, each bar will have a different length, which is measured off the suspended model. Since the split bamboo gets more twisted, the steeper the slope of the grid shell, dowel joints cannot be used, while rope tie joints maintain a harmonious curvature of the structure.


Suspended chain net model

Bamboo Trusses (Bibl. 13.06, 13.07)

· In many regions, bamboo is traditionally used for truss constructions, but often use more bamboo than necessary and are not always structurally sound.

· A research project, conducted by Dr. Jules Janssen of the Eindhoven University of Technology, Netherlands, developed and tested four types of bamboo joints and an improved truss design.

· Joint 1: plywood on both sides of the bamboo and held by steel bolts.

· Joint 2: the diagonal member rests against pins inserted through the upper member, whereby the pins support both the purlin and the diagonal member. An intermediate layer (a kind of washer) considerably improves the strength.

· Joint 3: two "horns" at the end of the diagonal fit into two holes in the upper member. (Disadvantage: requires craftsmanship, time and excludes prefabrication).

· Joint 4: bamboo pin passing through three bamboo members, the outer two being parallel.

· The improved bamboo truss, built with joint 2 and a free span of 8 m, was tested in the laboratory by placing it on the floor and simulating vertical roof loads, by a system of hydraulic jacks acting horizontally.

Pole timber roof structures

KEYWORDS:

Special properties

Cheaper and stronger than sawn timber

Economical aspects

Low to medium costs

Stability

Good

Skills required

Carpentry skills

Equipment required

Carpentry tools

Resistance to earthquake

Very good

Resistance to hurricane

Good

Resistance to rain

Depends on protective measures

Resistance to insects

Low

Climatic suitability

All climates

Stage of experience

Partly, traditional, partly experimental

SHORT DESCRIPTION:

· Unprocessed roundwood is cheaper and more easily available than sawn timber, and is mainly used for frame structures, ie skeleton wall and roof structures, trusses and the like.

· The advantages of using pole timber from young trees (5 - 7 years old) as compared to those of using sawn timber are numerous. The main ones are:

· The cost and wastage of sawing are eliminated.

· 100 % of the harvested timber's strength is utilized, while the immense original strength of large tree trunks is forfeited by sub-division or lost in the sawing wastes.

· A timber pole is stronger than sawn timber of equal cross-sectional area, because the fibres flow smoothly around natural defects and do not end as sloping grain at cut surfaces.

· Poles have large tension growth stresses around their perimeters and this assists in increasing the strength of the compression face of a pole in bending.

· Sawn timber is a product of trees that have grown for several decades. Since their replacement takes so long, excessive felling can cause serious environmental problems.

· Hence, from the points of view of economy, strength characteristics and environmental acceptability, the use of pole timber (eg from mangrove swamps, thinnings from eucalyptus or softwood plantations, etc.) can be far more appropriate for a range of building constructions than the use of sawn timber.

Scrap Metal Plate Connections (Bibl. 00.39)

· This simple and cheap technique, developed at the Intermediate Technology Workshop in Cradley Heath, U.K., uses thin sheet metal, cut to the required size and shape, which is wrapped around the joints and firmly nailed onto the timber.

· The most suitable application of this method is in the prefabrication of pole timber trusses. To ensure uniform dimensions, the trusses are made with the help of a template laid on the ground and held in place by wooden or steel pegs. The poles are placed as accurately as possible on the template, then cut to size and joined together as described above.


Joint detail

Steel Flitch Plate Connections (Bibl. 14.10)

· The nailed flitch plate connection, developed at the Building Research Establishment, Garston, U.K., consists of mild steel sheets inserted into longitudinal saw cuts in the timber poles and connected to them by nails driven through the timber and the steel at right angles to the plate.

· Mild steel sheets up to 1 mm thickness can be easily penetrated by normal round wire steel nails without pre-drilling. Thicker sheets require drilling or the use of hard steel nails. Tests have shown that for most applications and timber species two 1 mm plates provide sufficient strength of the connections. (Considerations of cost suggest that it is better to increase the number of 1 mm plates rather than their thickness.) Stronger timbers may require flitch plates of larger areas to achieve appropriate design stresses.

· The ability of the nailed flitch plate connection to sustain loads after initial failure is a characteristic which could prove valuable in areas where buildings may be subjected to earthquakes and high winds.


FIGURE

Timber Jointing with Dowels (Bibl. 14.02)

· Nails and toothed plate connectors are quite often impossible to use on harder timber species. When used on softwoods, they tend to loosen when the timber shrinks.

· A more appropriate alternative, developed at the University of Nairobi, Kenya, is the use of dowels, which are fitted into predrilled holes. Where structural considerations permit, these are preferably wooden dowels, as they are cheaper and do not corrode. They should, however, be prevented from slipping out by means of nails or pegs, inserted at different angles.

· Alternatively, holes can be drilled into the ends of the wooden dowels, into which hardwood wedges can be fitted to keep the dowel in place. Thus the hole into which the dowel is inserted can be slightly oversized to facilitate and speed up work.

· Where strong connections are vital, steel bolts and nuts are most suitable, but also very expensive, costing three to four times that of the mild steel rods from which they are made. Using the rods straight away as dowels is cheaper and equally effective. To prevent them from slipping out of the timber, 10- 12mm deep holes should be drilled into the ends of the dowels, as described above in the case of wooden dowels. With a cross saw cut, the end pieces can be bent back like flower petals, holding down a steel washer.


FIGURE

Space Frame Connections (Bibl. 23.10)

· A method of using short length, local pole timber to construct space frames for large covered areas (such as meeting halls, workshops, markets, etc.) was developed in Sweden by Habitropic. The system is based on special space frame connectors, comprising a cros-scomponent of welded steel, and tail end connectors with screws, washers and nuts.

· The poles are all cut to the same length, say 1.5 m, and cut lengthwise at both ends with a saw. Holes for bolts are drilled at each end, the steel tail-end connectors inserted in the saw cut and fixed with bolt, washer and nut. After prefabricating all the required pores, they are assembled on the ground, directly below their final position and lifted into place by a pulley system.

· With pole thicknesses of 5 - 6 cm the weight per m2 is 20 kg, and the consumption of material per m2 is approximately 3.5 poles and 1.1 space frame connectors.


FIGURE

Hogan Roof Construction (Bibl. 23.16)

· The North American Navajo Indians traditionally build their homes (hogans) with this simple method. A hogan is usually an octagonal house covered by several layers of timber poles, which are laid across the corners of the layer below, thus reducing the void with each new layer. The same system can be used to cover triangular, square or other polygonal structures, without the need for supports other than at the periphery of the roof.

· A well designed roof with accurately cut and assembled poles should in theory be stable with only a few bolt or dowel connections at certain strategic points. However, it is advisable to fix each pole firmly to the one below to avoid excessive lateral movement, especially in earthquake or hurricane prone regions.

· Traditionally, the hogan roof is covered with earth to provide a high thermal capacity, which is advantageous in climates with large diurnal temperature fluctuations. Lighter roofs with low thermal capacity are also possible by merely constructing a framework and bridging the gaps with a waterproof membrane and light roof cover (eg wooden lathing and shingles, mats, thatch).


FIGURE

Bamboo and wood shingles

KEYWORDS:

Special properties

Attractive, durable roof cover with replaceable elements

Economical aspects

Low to medium costs

Stability

Good

Skills required

Traditional craftmanship

Equipment required

Bamboo cutting tools, shingle knife, hammer

Resistance to earthquake

Good

Resistance to hurricane

Depends on fixing

Resistance to rain

Good

Resistance to insects

Low

Climatic suitability

Warm humid and highland zones

Stage of experience

Widely used

SHORT DESCRIPTION:

· Shingles are used to cover pitched roofs (and quite often walls) on a supporting grid of bamboo or wooden laths. The appearance is typically a fish-scale structure, but some types of bamboo shingles rather resemble Spanish tiles.

· Appropriate lengths of bamboo culms or timber logs are cut and the shingles are split off these vertically, whereby bamboo culms are split into quarter or half sections, and wood shingles are flat tiles cut with a special knife and hammer.

· For fixing bamboo shingles, pre-drilled holes are needed for nailing or tying with a tough string. Quarter-cut bamboo shingles can also be made with splints which are hooked onto the lathing.

· Timber shingles are nailed onto the battens, whereby the curvature of the shingles after drying must be taken into consideration.

· The minimum roof pitch for shingles is 45°. Pressure impregnated timber and bamboo can have lower pitches, but are no/recommended: higher costs; chemicals are gradually washed out and become ineffective; rainwater cannot be collected from the roof.

Further information: "The Shingle Roofing Manual" (available from the Forest Products Research Centre, Box 1358, Boroko, Papua New Guinea); Bibl. 00.19, 23.24.


Bamboo Shingles with Splint or String Fixing (Bibl. 23.24)


Bamboo Shingles as Spanish Tiles (Bibl. 23.24)


Wood Shingles (Bibl. 23.24)

Corrugated metal sheet roofiing

KEYWORDS:

Special properties

Light roofs, quick assembly

Economical aspects

Medium costs

Stability

Low to medium

Skills required

Average construction skills

Equipment required

Carpentry tools

Resistance to earthquake

Very good

Resistance to hurricane

Low

Resistance to rain

Good, but extremely loud

Resistance to insects

Very good

Climatic suitability

Warm humid climates

Stage of experience

Widely used in almost all countries

SHORT DESCRIPTION:

· The metal sheets are either galvanized iron or aluminium, whereby gi is susceptible to rapid corrosion if the zinc coating is not sufficiently thick (a common problem with cheaper varieties). Aluminium is lighter, more durable and reflects heat more efficiently, but is more expensive and produced with an extremely high energy input.

· The corrugations make the thin sheets stiff enough to span between two purlins without sagging. Thus large areas can be roofed with a minimum of supporting construction, making the roof light (good in earthquake zones) and cheaper (less timber or steel framework).

· Thin gauge sheets are often too weak to walk on, can be dented, punctured or torn off by strong winds.

· Major problems of metal sheet roofing are the immense heat transmission to the interior (less severe with aluminium) during sunshine, and water condensation on the underside when the roof cools down at night; unbearable noise caused by heavy rains; havoc caused by whirling sheets that are ripped off in tropical windstorms; poor fire resistance.

· Many of these problems can be alleviated with good design, material qualities and workmanship.

Further information: Bibl. 00.55, 23.17, 25.06.

Construction of Corrugated Metal Sheet Roofing

· Such roofing should tee avoided in areas of intense solar radiation and rapid temperature changes, to avoid hot indoor climate and condensation problems.

· In most cases it is advisable to construct a suspended ceiling (of a light reflective material), providing a ventilated air space which removes the accumulated heat before it can reach the interior.

· The air space also reduces the noise problem during rains. In addition, shorter distances between purling, as well as felt or rubber washers at the suspension points, rigid bolt connections and thicker gauged sheets help to reduce sound transmission.

· Similarly, thicker sheets, rigidly fixed hook bolts with large metal washers (underlaid with felt or rubber to avoid bimetallic corrosion) and avoidance of overhangs, are measures to prevent damage by strong winds.

· A fire-resistant suspended ceiling and other common-sense fire precautions can eliminate the fire risk completely.


Overlaps of roofing sheets must take into consideration the main direction of wind; Rafters should be firmly held by a fastening strap or reinforcing bar, which is embedded in the concrete or masonry. (Bibl. 25.06); A ridge ventilator can help to improve indoor climate and also reduce internal pressure and thus decrease the total roof uplift. (Bibl. 25.06)

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