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close this bookA Guide to the Development of on-site Sanitation (WHO; 1992; 246 pages)
View the documentPreface
open this folder and view contentsPart I. Foundations of sanitary practice
close this folderPart II. Detailed design, construction, operation and maintenance
open this folder and view contentsChapter 5. Technical factors affecting excreta disposal
open this folder and view contentsChapter 6. Operation and maintenance of on-site sanitation
close this folderChapter 7. Components and construction of latrines
View the documentPits
View the documentLatrine floors
View the documentSlabs
View the documentFootrests and squat holes
View the documentSeats for latrines
View the documentWater seals and pans
View the documentVent pipes
View the documentSuperstructure
open this folder and view contentsChapter. 8 Design examples
open this folder and view contentsPart III. Planning and development of on-site sanitation projects
View the documentReferences
View the documentSelected further reading
View the documentGlossary of terms used in this book
View the documentAnnex 1. Reuse of excreta
View the documentAnnex 2. Sullage
View the documentAnnex 3. Reviewers
View the documentSelected WHO publications of related interest
View the documentBack Cover
 

Slabs

Requirements

A latrine slab serves two main purposes, as a support and as a seal. It has to support the weight of the person using the latrine and, possibly, the weight of the superstructure. It also seals the pit, with the exception of the squat hole and, where required, the vent-pipe hole. This facilitates control of flies and smells and reduces the likelihood of rodents and surface water entering the pit. Where the slab has been made in sections (for ease of placing and emptying) or has a removable cover, the joints should be sealed with a weak mortar such as a lime or mud mortar.

To support the weight of a person over a latrine pit the suspended slab has to act structurally in the manner of a bridge. Where seats are provided, the extra weight has to be allowed for when designing the slab. Depending on the design of the slab, the materials may have to be able to resist forces in tension as well as in compression (Fig. 7.9). The materials needed to carry the tensile forces are often more expensive than those commonly used in low-cost buildings. The slab is often the most expensive individual component that has to be paid for by the user. It is therefore important to ensure that it is carefully designed to serve its purpose with a minimum of costly material.


Fig. 7.9. Tension and compression forces in a slab

 

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The slab normally rests on a foundation or on the top of the pit lining (see Fig. 7.8). This ensures that the weight of the slab and the weight of the person using it are spread evenly on the soil. Particular care must be taken where the slab also has to carry part of the weight of the superstructure. If the ground is weak, the foundation prevents subsidence or collapse of the ground underneath the load. Any gaps between the slab and the pit lining should be sealed with earth or a weak mortar to prevent ingress of water. This seal also prevents small animals and insects getting into and out of the pit.

Where a pit is excavated to a larger diameter than planned, precast slabs are occasionally supported on timber poles. This practice is not advisable as the heavy load on the poles is likely to lead to early failure. However, small slabs (approximately 500 mm square), designed to provide a hygienic squat hole for existing latrines at minimum cost, will not overload a timber support (Fig. 7.10).


Fig. 7.10. Small slabs for upgrading timber and earth structures

 

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The latrine slab should feel secure and should not deflect noticeably under the weight of a person using the latrine. It needs to be as clean and attractive as possible so that people feel comfortable using the latrine. There is then much less chance of the latrine being misused or fouled.

Offset pits used with pour-flush latrines require a cover slab to prevent entry of flies and rodents and to ensure safety, particularly of children. With the omission of a squat hole, the structural requirements are the same as for a latrine slab.

Shapes of direct pit slabs

The shape and size of the pit are the first factors to be considered when designing a supported slab. Latrine pits can be round, square or rectangular and it is usual to find that a particular shape becomes the accepted design for a particular area.

Borehole latrines have a small span and therefore require very simple slabs. The shape will depend on the users' needs for a clean hygienic area with correctly spaced footrests, rather than being controlled by the size of the hole to be covered. Larger, hand-dug pits 1-1.5 m in width require a shape designed to span and seal the pit. An exception to this is where the span of the slab is reduced by corbelling the top of the lining (Fig. 7.11). This decreases the amount of material required in the slab and thus reduces the cost.


Fig. 7.11. Minimizing the span of the slab

 

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Slabs may be precast or constructed in situ, which means that the slabs are built over the pit, exactly where they are to be used. In a large agency-assisted programme, slabs are often manufactured at a convenient construction site away from the latrines and then brought to the site and laid across the pits. Where slabs are to be moved, weight and shape are both significant factors.

The shape of the slab is also determined by the type of latrine. Water-seal latrines, aqua-privies, ventilated pits and pits sealed with hole covers all have different requirements. For example, the need for an extra hole close to the edge of the slab for a vent pipe makes the unreinforced dome slab unsuitable for ventilated latrines.

The slab normally overlaps the supporting pit lining or foundation by at least 100 mm on all sides to ensure that the load is adequately transferred. This overlap may have to be extended to 200 mm where the pit is unlined and the slab is resting directly on the soil (see Fig. 7.8).

Cement-based slabs and components

In most countries, concrete or cement-based slabs provide the most durable and economic method of covering latrine pits. There are many different ways of using cement. Its ability to bind with other materials and provide a clean watertight surface make it the obvious choice for the majority of programmes.

Concrete is a mixture of cement, sand, gravel and water. When set, it forms a hard dense material which is extremely strong in compression but weak in tension. Cast as a simple flat slab across a pit, its own weight and the weight of any person on it forces the concrete to deflect downwards in the centre. As the load increases, small tension cracks form on the underside of the beam. With heavy loads, these cracks may extend upwards through the concrete until the slab breaks. To prevent this happening, steel bars or other reinforcement may be placed in the concrete on the lower side of the slab to carry the tension load and prevent the cracks spreading.

Unreinforced concrete

Small slabs, such as those required for borehole latrines or to provide a hygienic platform for the squatting area of timber-supported slabs (see Fig. 7.10), do not need any reinforcement. Where an unreinforced span of greater than 0.5 m is required, the slab should be cast in the form of a "flat" arch. The weight of the load is then directed through the arch to the supporting area on the ground. The underside of the concrete remains in compression and no reinforcement is required. Using this principle, a shallow circular dome or arch can be constructed to cover a latrine pit. The dome is strong enough to support itself and the people using it without any expensive steel reinforcement. A slab using this principle has been developed by a team in Mozambique (Fig. 7.12) and has proved to be economical and popular. The slabs are about 40 mm thick and rise 100 mm in the centre to give the arch effect (International Development Research Centre, 1983).


Fig. 7.12. Dimensions of domed slab without reinforcement

 

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Although such domed slabs fall away from the centre, a small inward slope about 100 mm wide immediately around the squat hole is incorporated to direct any waste into the pit. These slabs have been used most effectively in areas with sandy soil which quickly absorbs any surplus wash water.

The concrete slab is given the shape of a dome by mounding up earth to the required profile of the underside of the slab. The earth is compacted and smoothed. It may then be covered with plastic sheeting or old cement bags, or coated in old engine oil to break any bond between the earth and the fresh concrete. A circular iron strip made from an oil drum is used as the edge former or mould. The concrete around the centre hole is made slightly thinner so that a slope towards the hole can be made. Each slab has to be allowed to harden undisturbed for several days after casting.

To save space in the casting yard, up to five slabs may be cast on top of each other, using a lower, previously cast slab as a former for the next slab. Particular attention has to be given to the concrete mix of a thin unreinforced slab. A maximum aggregate size of 10 mm and slightly more cement than usual is required. The recommended mix is one part by volume of cement to two parts of sand and one and a half parts of 6-10 mm aggregate.

An unreinforced slab may also be produced in a rectangular mould with a flat upper surface and a dome on the underside (Fig. 7.13). As an unreinforced dome slab cannot accept a second hole close to one edge for a vent pipe, flies, smells and cockroaches are prevented from leaving the pit by providing a tight-fitting cover over the squat hole. This is cast directly in the squat hole so that it fits exactly. A layer of cement bag paper may be used to prevent the fresh concrete sticking to the old.


Fig. 7.13. Semi-domed slab (Plan)

 

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Fig. 7.13. Semi-domed slab (Section)

 

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Bricks can be used to form an unreinforced arch across a rectangular pit (Fig. 7.14) using a rough framework of bamboo, reeds or forest poles which is left in the pit. The space above the arch is levelled with river sand and topped with a 20-mm cement-sand screed sloping towards the centre. This technique requires very little cement and no steel. However, these structures have to be built by skilled masons and there is no opportunity for precasting. Emptying of the pit can only be carried out through the squat hole.


Fig. 7.14. Arched brickwork lining and support (Section)

 

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Fig. 7.14. Arched brickwork lining and support (View from below)

 

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Reinforced concrete

Because of the weakness of concrete in tension it is often reinforced with other materials. Most commonly it is strengthened by the inclusion of steel bars. Details of the reinforcing steel required for common sizes of slab are shown in Table 7.1. Mild steel bars, 6 mm in diameter spaced at intervals of 150 mm, or 8 mm in diameter spaced at intervals of 250 mm in each direction, are normally sufficient for 80-mm thick slabs of up to 1.5 m in span. This span distance is measured at the point of minimum span, that is, the shortest distance between two points which fully support the slab. Where used correctly, reinforcement in a concrete slab will support at least six adults on a 1.5-m span slab. For the small spans illustrated, extra steel is not required for trimming around the pit opening.

Table 7.1. Spacing of steel reinforcement bars for concrete slabsa

Slab thickness
(mm)

Steel bar diameter
(mm)

Spacing of steel bars (mm) for minimum slab span of:

   

1 m

1.25 m

1.5 m

1.75 m

2 m

65

6

150

150

125

75

50

 

8

250

250

200

150

125

80

6

150

150

150

125

75

 

8

250

250

250

200

150

 

a The steel bars should be fixed on the lower side of the slab, with 12-mm cover or thickness of concrete beneath each bar. Steel to be laid at above spacings in both directions. Size and spacing of steel calculated for grade 20 concrete and mild steel reinforcement, with characteristic yield stress of 210 N/mm2, or high-yield mesh, yield stress 485 N/mm2.

The reinforcing steel is laid in both directions, that is, with one layer of bars perpendicular to the second layer (Fig. 7.15). Where the slab is rectangular, the bars parallel to the direction of the minimum span should be beneath the bars in the direction of the longer span. For the bars specified, a characteristic yield strength for the steel of 210 N/mm2 is assumed. Care is required to ensure that the steel is of the required quality.


Fig. 7.15. Reinforced concrete rectangular slab (for details of reinforcement see Table 7.1) (Plan)

 

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Fig. 7.15. Reinforced concrete rectangular slab (for details of reinforcement see Table 7.1) (Section)

 

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When individual bars are used, some may be omitted by mistake. One way of avoiding this is to use steel mesh, which consists of smaller-diameter bars welded together. This can be cut to the required shape but there is likely to be wastage of the off-cuts that have to be discarded. A mesh with 7-mm bars at 200-mm centres, with a cross-sectional steel area of 193 mm2/m (yield stress 485 N/mm2) is normally sufficient.

Care must be taken when reinforcing concrete with steel to ensure that the steel is completely surrounded by the concrete. There should be at least 12 mm of concrete under the steel bars and at the ends of all bars. This protects the steel from the corrosive effect of gases and moisture in the pit. When concrete is placed in a mould or former it has to be compacted by manual or mechanical vibration to remove any air bubbles and to ensure the durability of the completed slab. Simple wooden or steel moulds can be reused many times to give the required shape to the wet concrete if they are coated with a suitable release agent. There are many proprietary agents, but used engine oil painted on to the mould effectively prevents the concrete from sticking. Alternatively, plastic sheeting or empty cement bags may be used to prevent bonding. These materials may also be used between the ground and the underside of the slab. The squat hole is formed using a shaped wooden mould with a bevelled edge. A vent pipe opening may be created with an offcut of plastic pipe which is removed a few hours after casting so that it can be reused many times.

An alternative way of using steel for reinforcement is to precast a ferrocement slab. The method of construction is described under construction of linings. A flat ferrocement slab is strong enough to carry the imposed load but is too flexible for the users' comfort. In order to ensure adequate stiffness, the ferrocement may be shaped as a dome or may be cast with ribs on the soffit (Fig. 7.16). Four layers of mesh are normally required for a slab with a 1-m span. It is necessary to ensure that the cement mortar has been adequately pressed through all the layers of wire mesh and compacted to a dense material if it is to have adequate strength.


Fig. 7.16. Ferrocement slab (Section)

 

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Fig. 7.16. Ferrocement slab (Plan)

 

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Steel reinforcement is used in various ways in different countries reflecting differences in price and availability. Because of the relatively high cost of steel, many techniques have been investigated in the search for cheaper alternatives. One approach is to reinforce concrete with small unconnected fibres with a low modulus of elasticity. These are either natural fibres, such as sisal, jute, coir, Manila hemp or kenaf, or man-made fibres such as fibrillated polypropylene. The fibres are chopped and added to the cement mix. Use of these low-modulus fibres does not reinforce the concrete in the conventional sense of carrying the tensile load, but is particularly beneficial in ensuring adequate curing of the concrete without the formation of minute shrinkage cracks (Parry, 1985). The resultant "unreinforced" concrete attains a much higher tensile strength than would otherwise be possible. Slabs made from fibre-reinforced cement should normally be given the shape of an arch or dome to minimize tensile forces in the soffit.

Slabs have also been reinforced with barbed wire, fencing wire, scrap steel from cars and broken machinery, redundant universal beams and almost anything that is available. Although a saving is made on reinforcement, these methods usually lead to a much greater use of concrete in order to cover the larger sections of steel and therefore are rarely economical.

Bamboo has a high strength-to-weight ratio and in certain parts of the world is widely available. Because of the low cost, bamboo strips have been used as an alternative to steel bars but it is important to ensure that the bamboo strips in a slab are completely covered by the concrete so that water and vapours cannot rot the bamboo. The strips should initially be treated with preservative. One recommended method (UNCHS, undated) is for the bamboo to be dipped in white lead and 10% varnish to inhibit water absorption from the freshly placed concrete. Even where treated, there is some doubt as to the long-term durability of bamboo as reinforcement.

Where cement is relatively expensive, a technique known as reinforced brickwork can be utilized, in which part of the concrete is replaced by whole or half bricks, leaving steel reinforced concrete ribs to support the bricks (Fig. 7.17). The whole slab requires a cement skimming over the surface to make it impervious to fouling by the users.


Fig. 7.17. Reinforced brickwork slab

 

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Concrete mixes

Different concrete design mixes (that is, combinations of cement, sand, aggregate and water) are suitable for use in various circumstances. The concrete mix that is most often used is 1:2:4 (one unit by volume of cement with two units by volume of sand and four units by volume of aggregate). The sand should be clean and hard and may be sized by sieving through ordinary mosquito netting. Coarse aggregate comprises graded stones 6-18 mm in size and should be free of fine dust. This mix results in a finished volume of concrete which is approximately 70% of the total volume of the individual dry materials.

The cement, sand and coarse aggregate have to be mixed with a specific amount of water to give the optimum strength for the amount of cement used. For concrete mixed and placed by hand, there should normally be a water: cement ratio of 0.55 by weight, i.e., the weight of water is approximately half the weight of cement. Cement weighs 1400 kg/m3 and water 1000 kg/m3; a 50-kg bag of cement thus has a volume of 0.035m3. A 1:2:4 concrete mix using one 50-kg bag of cement therefore requires 0.070m3 of clean sand, 0.140m3 of aggregate and 0.027 m3 of water, which results in 0.17 m3 of finished concrete.

The volume of water is applicable where the aggregate and sand are "saturated, surface dry". In hot dry climates, the small pores in the aggregate, as well as the surface, are likely to be "oven dry" rather than saturated. To use the specified amount of water would then lead to an extremely stiff, unworkable concrete. The aggregate should therefore be thoroughly wetted with water before mixing begins. The correct water: cement ratio results in a relatively stiff but workable material which produces a skim of water on the surface of the concrete as it is worked flat with a trowel. When the mix has too much water, the strength is reduced considerably. An increase of only 50% in the water content decreases the finished concrete strength by half, which is the equivalent of wasting half the cement in the bag.

To check that the calculated amount of water is correct, a trial mix may be prepared and a slump test carried out. In this test, the concrete mix is compacted into a slump cone (Fig. 7.18), which is similar to an upturned bucket 300 mm high with the base removed. When the cone is removed, the concrete will slump, i.e., reduce in height; the maximum slump, for concrete that is to be reinforced, should be about 100 mm, and less for unreinforced concrete.


Fig. 7.18. Checking the water content of concrete with a slump cone (A)

 

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Fig. 7.18. Checking the water content of concrete with a slump cone (B)

 

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Caring for concrete

After it has been cast, concrete must be cured. It should be covered with either wet sand, straw, cement bags, jute sacks, plastic or palm leaves to keep the concrete moist and as cool as possible. The chemical reaction which causes the cement particles to bind is dependent upon the amount of water present. If the moisture has been sucked out from the surface of the concrete by the heat of the sun, the chemical reaction cannot take place and the surface of the slab will not be durable. In hot dry climates the concrete and its covering need to be watered twice a day for seven days after casting. If the concrete is not cured, it will have only 60% of its ultimate design strength; if cured for three days, it will attain only 80%, but if kept damp for seven days will reach almost 100% (Reynolds & Steedman, 1974).

A good guide for field workers is: "Make the concrete mixture as dry as you can; and then keep the cast concrete as wet as you can."

The most effective way of checking the strength of a slab is to test load it seven days after casting. As, normally, only one person at a time will use the latrine, to test load the slab with five or six people gives an adequate and convincing factor of safety. The slab should be supported at its edges by four or five bricks placed on flat ground, and the people should stand on the slab, avoiding areas directly over the bricks. Testing the strength of precast slabs by throwing them off the back of the delivery truck at the site, on the understanding that those that do not break are adequate, is not recommended.

The final concrete surface should be clean, dense and free of blemishes. The surface will absorb urine unless it is sealed effectively with, for example, proprietary sealant, alkali-resistant gloss paint, bitumastic paint, or two coats of a 25% solution of silicate of soda (Khanna, 1985).

A screed (a thin layer of cement mortar) is sometimes applied to a flat slab after casting to create the desired slope towards the squat hole. However, unless the screed is applied before the concrete has completely set there is a danger of its flaking off in use. Wherever possible the required slope should be cast in the original concrete, a dense surface being obtained by trowelling with a steel float as the concrete begins to set. Alternatively the slab may be cast upside down on plastic sheeting to ensure a good finish.

Footrests are normally cast separately, after the concrete of the slab has hardened. The area where the rests will be cast is roughened when the slab surface is being given its final trowelling. Formers for the footrests can be made out of any available material such as tin or wood, but the individual formers should be connected together and to fixed points on the edge of the slab to ensure that the rests are always cast in the same position (Fig. 7.19).


Fig. 7.19. Formwork for the casting of footrests (Plan)

 

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Fig. 7.19. Formwork for the casting of footrests (Section)

 

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Weights of concrete slabs

If cement-based slabs are to be moved, weight is an important consideration. For example, a 65-mm-thick circular concrete slab, 1.5m in diameter, weighs approximately 275 kg, while an 80-mm thick slab weighs 340 kg. A rectangular slab, 65 mm thick, designed to cover a pit of size 2.2 m × 1.1 m would weigh 360 kg, unless made in sections (Fig. 7.20). Circular slabs are not normally made in sections. When whole, a round slab can be moved by two or three people, rolling it on its edge (Fig. 7.21). This is particularly useful in the management of the construction yard and can sometimes even be used to transfer the slab to the household site without a vehicle.


Fig. 7.20. Rectangular slab in two sections (Plan)

 

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Fig. 7.20. Rectangular slab in two sections (Section)

 

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Fig. 7.21. Circular slab for ease of transport

 

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Concrete for other components

Concrete for floors of latrines that are not directly above the pits is cast in situ. A slightly weaker concrete mix of 1:3:6 may be used but the curing requirements remain as described above. The outlet pipe and pan should be carefully laid to the desired level before the concrete is cast.

Cover slabs for offset pits and floors and cover slabs of septic tanks are normally also made of concrete. Walls of septic tanks are usually constructed from concrete blocks or cement-plastered fired bricks. The requirements for good quality concrete are identical to those for components discussed previously.

Other materials for slabs

Wood

The simplest slabs in rural areas are made from rough poles and tree branches laid closely together over the pit. A timber slab is always liable to deterioration because of fungal decay owing to the moist gases rising from the pit and also because of the threat from termites and boring insects in tropical climates. Durable timbers such as the heartwood of some tropical hardwoods are normally too expensive for use in latrines but, where available, may be expected to last satisfactorily for several years.

A thick layer of earth or mud is often spread over the poles or branches to bind them together and create a smooth surface (Fig. 7.22). In many places, people are skilled at making mud floors which are almost as hard as cement and quite smooth. They need not be rough or unsanitary. There are various methods of improving the mud with local materials, such as mixing the soil with a liquor obtained by soaking animal dung overnight. In some areas the mud is mixed with charcoal or other small aggregate, or with cow dung and then smeared with ashes. Alternatively, the mud from ant-hills has been found to make a hard, practically waterproof surface (Denyer, 1978). If the surface is not kept in good condition, however, there is a danger of hookworm larvae penetrating the feet of users.


Fig. 7.22. Timber and earth slab (Plan)

 

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Fig. 7.22. Timber and earth slab (Section)

 

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The life of a rough timber slab can be extended by using a mixture of soil and cement to plaster and protect the wood. Alternatively, a thin cement mortar screed can be laid over the surface of the earth to protect against hookworm and to improve hygiene. However, it is usually more cost-effective to use the cement to provide a permanent concrete slab which can be transferred to a new pit when the first is filled. Where more than half a bag of cement is needed to stabilize the earth, a concrete slab is likely to be a cheaper alternative.

In an area where timber is abundant, hewn or sawn logs supporting a platform of wooden planks make a floor that is preferable to the mud and pole version (Fig. 7.23). The surface can be kept clean, and signs of imminent collapse are normally apparent to the adult user. The durability of timbers may be improved by some form of treatment. The effectiveness of these treatments depends upon the amount of preservative that the timber can be made to absorb, which is a function of the permeability of the timber and the process used. Suitable preservatives include ordinary tar, tar-oils such as creosote, water-based preservatives such as copper/chrome/arsenic, and specialized organic solvents (Tack, 1979). Each type of preservative has its own characteristics and particular uses. Where treated timber is not available and the cost of using preservatives on a small scale is high, other more durable alternatives may be cheaper in the long run.


Fig. 7.23. Sawn timber slab (Plan)

 

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Fig. 7.23. Sawn timber slab (Section)

 

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Simple timber slabs are often considered to be unsuitable for sanitation projects, since people are less likely to use the latrine if they are afraid that the slab may collapse under them. However, the danger of collapse is usually less than the dangers associated with not having any appropriate system of sanitation. If no other materials are available at reasonable cost, a rough pole slab that has to be renewed every few years is to be preferred to no latrine at all.

Scrap iron and steel

In urban areas where sanitation is most urgently required, supplies of even the cheapest materials, such as rough poles, are usually limited and relatively expensive. The simplest alternative used by householders on an informal basis is to lay parts of discarded vehicles or any other scrap materials across the pit opening to provide support, with flattened containers, oil drums or galvanized iron roofing sheet to make a surface. Such materials do not seal the pit but they enable the user to excrete into a relatively safe hole rather than at the side of the street. However, where there are significant dangers, especially for children, these methods cannot be recommended.

Miscellaneous materials

Slabs have been made in a variety of other materials. Glass-reinforced plastics, polyvinyl chloride (PVC), ceramics and glass fibre have all been used to meet particular needs and situations. Plastic floors tend to flex under the weight of the user unless they are deeply ribbed. Some of these materials can also be used to give a special surface finish to concrete slabs.

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