With a relatively small school based on one, two or a small number of classrooms, the scale of construction and skill requirements are not greatly different from house construction - except for the case, already noted, that classrooms are generally significantly larger than domestic rooms. A local small building contractor, who can also be assisted to a large extent by local people, can then build the school. In some cases schools have been built entirely by local communities, sometimes assisted by an NGO or government agency.
Plans would need to be prepared for even a one-roomed school. This would need to be done by an architect, draughtsperson or someone who has already been involved in a school building project. However, if plans from another school building project are available, from, for example, an NGO or a government department, it might be possible to use these, if they are considered suitable, and this saves on the cost and time needed to prepare plans.
Once the plans have been prepared the quantities of materials and amount of labour required would need to be estimated, and the costs calculated. It would also be useful to prepare a schedule of construction activities and an estimate of how long each activity would take. Someone who has been involved with a school building project before could most usefully prepare these estimates. During construction ordering and use of materials, spending and progress on building can be monitored against the estimates to assess whether the project is ahead of, within or behind target. If problems occur, e.g. the project is costing more than expected, more materials are being used than expected or it is taking much longer than planned, an investigation would need to be carried out into why these problems are occurring and what can be done to remedy them.
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Once the site of the school has been chosen the boundaries of the outer walls of the school would need to be marked out on the ground, according to the plan, and the area within the boundaries cleared of topsoil and, levelled or, on sloping ground, terraced. Surveying tools would need to be used to ensure that the site is level and that the angles between the walls are the ones required, especially if the shape of the building is more complex and does not use only right angles, though experienced builders might be able to lay out simple buildings by eye and hand. Once the ground has been cleared and levelled, intermediate walls between the classrooms and other rooms can also be marked out on the site.
Unless raft or slab foundations - which are generally quite expensive options and not usually used for low cost buildings, are used, foundations would need to be provided underneath all structural walls. Note, however, that raft or piled foundations would be likely to be needed if building on expansive soils such as black cotton soil to avoid excessive settlement of the foundations. Structural walls are those which support substantially the load of upper storeys, roof, or other imposed loads on a building. For simple, small, rectangular buildings they are generally the external walls, though for larger and more complex buildings this would also include some of the internal walls, which together with the external walls would form the shell or structural form of the building.
Rectangular ditches for the foundations are dug to a depth until solid rock or soil with a good bearing capacity is reached. For one to three storey buildings foundation depths are typically 0.8 to 1.2 metres. Exceptionally, if beyond this depth the ground is still not providing a firm base, the ditch can be dug deeper then backfilled with brick and stone rubble before levelling. Note, however, that in cold climates the top of the footing of the foundation needs to lie below the frost line. If ground frost of below - 15°C might be expected the depth of the ditch would need to be greater than 1.2 metres, rising to 1.6 metres for ground frost of - 30°C.
The footing of the foundation is usually made of a relatively rich concrete mix, for example of one part cement to three of sand to six of river or lake bed gravel or crushed rock, which needs to be well-compacted. The width of the footing would be determined by local ground conditions, so the dimensions for other well-constructed buildings in the area would be a useful guide. Conventionally 45 or 60 cm are used for relatively small buildings. The height or thickness of the footing can be taken as the same as the width of the foundation wall above the footing. The foundation wall needs to be as wide, or slightly wider, as the wall above. It is unlikely that foundation wall widths below 20 cm could be considered completely safe, and even for relatively simple buildings widths as high as 40 or even 45 cm have sometimes been used. Again, experience from other buildings in the area would be a useful guide.
The foundation wall needs to be built with a relatively strong, durable and water-resistant material. These materials would include most hard ashlar or rubble stone (most soft stone as well as some types of limestone might not be durable or strong enough), most stronger types of fired clay bricks - generally not field fired bricks, dense concrete blocks or bricks, or cast concrete. If stabilised soil blocks are used these need to be made with a low-shrinkage soil, such as laterite, with extra stabiliser and waterproofer and preferably made in a machine press that can apply higher pressures than manual presses. It is normal practice for the foundation wall to be built to protrude above the ditch that was built for it to at least the level of the internal floor. It is also advisable, unless in very dry climates where the water table is always far below the footing, to cap the foundation wall with a damp proof course. This can simply be a rich concrete mixed with a waterproofing compound.
The floor is made by partly filling the space between the foundation walls with pebbles, broken brick, coarse sand, or even broken bottles provided that the glass does not protrude from the floor. This then needs to be levelled. This is covered by a layer of concrete, stabilised soil, or concrete or clay tiles. Timber floors are often used in Western countries, but this is generally quite an expensive option and liable to be attacked by termites where they occur.
EXAMPLE
The above example of a foundation wall is the most common type of foundation, but other types of foundation, e.g. arches, are also possible. Special care needs to be taken on sloping ground to ensure that the trench for the foundation is level. If the slope is steep the trench might need to be dug as a series of steps with the footing covering more than one of the steps.
Some of the materials and types of construction used for walls have been discussed previously. For low-rise buildings the simple masonry wall is by far the most common type of construction. The use of a structural frame with precast infill panels is unlikely to offer any advantage unless a very large school building programme is envisaged with the components manufactured at a central facility. A variety of types of bond is possible for erecting the bricks or blocks. One particular type is the rat-trap bond, originally developed in India and being more economical in the use of bricks and blocks and mortar than most other types of bond. Drawings of the rat-trap bond and other economical types of brick and block bonding construction can be found on: -
http://www.saud.ku.edu/ngore/nilsweb/cinvablocks/kucinva/strength2.html
or http://janathakshan.net/files/rat_trap.pdf
Masonry materials are bonded by a cement-based or lime-based mortar. For the wall to function properly as a structural element it is important that the mortar is not significantly stronger or harder than the material which it bonds and it can even be significantly weaker. A mortar based only on cement and sand can also be troublesome for letting water into the wall and for causing spalling (the breaking off pieces from the surface) of the bricks or blocks. However, a mortar based only on lime needs to be applied with quite a high level of skill for good effect, takes a long time to harden and is best applied in a very thin layer. To overcome these disadvantages of both lime and cement it is common practice to mix lime and cement for a mortar, for example in proportions of one part lime to one of cement to six of sand, or one part cement to two of lime to nine of sand. If a low strength mortar is required, for example if using relatively low strength stabilised soil blocks, soft stone or field fired bricks it can be made from one part lime to two parts pozzolana, such as some types of volcanic ash and ground up fired clay, to nine parts sand. As the mortar material is usually more expensive than the bricks or blocks it is used to bond it is good practice to make the mortar joints as thin as possible, and to only mix sufficient material that can be used within about half and hour, or somewhat longer if using pozzolanic cement.
One of the most difficult aspects of masonry construction is the making of openings for doors and windows. It is important to plan in advance of the construction where these openings would be situated and what their size would be. Before building the wall wooden pieces can be cut out and set up to show where the door would be. The same would need to be done for window openings once the wall has been taken up to the course adjacent to the bottom of the opening. It might sometimes be necessary to cut or break bricks or blocks to fit in the door or window openings, but with planning the positions carefully this can be minimised. It is necessary to cover the top of the opening with a rectangular lintel, arch or corbelling, as mortar joints are extremely weak under bending stress and would give way easily when subject to this stress, as occurs at top of an opening. Steel reinforced concrete is the material normally used to make the rectangular lintel.
Walls are built up to a height of 2.6 to 3 metres above the floor level for each storey. Local building practice or building codes, if available, would determine the height for the top of the wall. It would not be advisable to choose a height below 2.5 metres, even though well above almost everyone's head height, as some people would feel uncomfortable with a low ceiling. A height close to 3 metres would need to be used if electric lights are to hang in the room. The exact height would also be determined by the dimensions of bricks and blocks used in the construction.
In areas where there is a risk of earthquakes special measures for the walls would be needed to increase safety. These would include a ring beam - a beam the sections of which are joined together and extending continuously over the top of the external walls, and bracing or a framework structure within the walls. The elements of the ring beam, bracing or structural framework would need to be made of a tough and flexible material such as timber or reinforced concrete with adequate steel reinforcement. Any locally applicable seismic design and building codes must be strictly adhered to.
Where there could be a termite problem a termite barrier would need to be inserted near the bottom of the wall. One possible option is shown below. This is of concrete and would need to be well-made to ensure that there are no cracks in it which termites can penetrate and enlarge.
An external plaster or render would help to protect the building fabric from weather damage, improve appearance and fill in any holes which could harbour insects and dirt. As with the mortar it is important that the plaster is no stronger or harder than the material to which it is bonded, particularly for relatively weak and friable materials such as earth or soft stone. A plaster made only with Ordinary Portland Cement as the binder would be too hard for most materials, and there is the risk that water penetrates behind it causing the plaster to fall off and sometimes taking pieces of the material to which it is bonded with it. Use of a lime and cement mixture, lime only, or lime-pozzolana for the binder would almost always be preferable to just using cement. Typical mix proportions of plaster mixes are one part lime to two of sand, one part lime to two of pozzolana (such as some types of volcanic ash or finely ground burnt clay) to nine of sand, or one part cement to two of lime to nine of sand. Plaster needs to be well-bonded in to the surface to which it is fixed, so a good key needs to be provided. This can be done by removing some of the mortar from the joints to create grooves (raking), making grooves in the surface or fixing a wire frame tightly to the wall. Plaster is applied in two, but preferably three thin layers. The layers or coats would preferably each be less than one centimetre thick, but certainly so for the final or finishing coat which needs to be the thinnest.
The first coat is applied, scratched to create a key for the second, then allowed to harden for several days before the second is applied. Keeping a cement-based coat moist would help it to harden, while with a lime-based coat allowing the drying to take place only very slowly would help to reduce cracking. The procedure is repeated for the second coat if a third coat is to be added. The finishing coat is made with a richer mixture (containing more binder), and a fine sand so that it can produce a surface of smooth appearance.
Walls can also be plastered or painted internally. A white gypsum or lime-based surface makes a room seem lighter, can help to improve acoustics and creates a smooth surface where insects and dirt have difficulty accumulating. Gypsum is a particularly useful plastering material, but is not recommended for external use, except in dry climates, as it is damaged by water. Note that if walls are to be left unplastered it is important to point the mortar joints for neater appearance and to reduce weather damage. Even if it is decided not to plaster a wall, a thin layer of limewash applied every year can help to reduce rain damage and improves the appearance.
Pitched and flat roofs are the two most common roofing types, though as discussed previously other types, e.g. vaults, domes and corbelling, are also possible. Flat roofs of low cost materials such as compacted earth or a lime based mortar are generally only used in dry climates as in wetter areas they would be easily damaged by rain and let in water. Flat roofs have been used in the some temperate countries where the climate is wetter, typically made of timber decking covered by bitumen felt sheets, but this is a more expensive option, and no guarantee that the roof would not leak. Bitumen-based sheets would also be likely to soften and deteriorate significantly in the hot sun in tropical countries, so the use of expensive additives in the formulation might be needed to reduce this.
Note the two most common types of roof construction of pitched roofs - the gable roof and the hipped roof
There is no particular advantage of one of these roof types over the other, though despite the more complicated wall construction detail required for the gable roof the overall construction of a building with this type of roof would be less complicated than for one with a hipped roof, where trusses of different sizes would be required and these would need to be positioned quite accurately.
The simplest roof is the shed roof, but this is generally restricted to small one storey buildings less than about 4 metres across, otherwise the economy gained in the construction of the roof is lost in having to build one of the walls considerably higher than the other to give the roof sufficient slope. Most schools are usually not as small as this.
An important feature on the top of the wall is the wall plate, used for fixing the roof trusses of a pitched roof to the top of the wall. This is usually of wood. There are a number of ways of fixing the wall plate to the wall including drilling or forming holes for bolts in the wall, into which the bolt is inserted, then pouring a slurry of cement mortar around it, with holes drilled in the wall plate to match position of the bolts in the wall. Another way is to insert steel rods horizontally into the wall 0.5 to 1 metre below the top of the wall. As the wall is being constructed steel straps are tied to the rods, looped round the wall plate and tied back to the rod. The cost of inserting the wall plate can be reduced by instead of having bolts or straps in the wall about every metre, to put these in about once every two metres, but then also to embed the wall plate in a good cement mortar at the top of the wall.
For small low cost buildings in areas without earthquake risk and on flat ground a wall plate is recommended but not essential if flat roofs are used. Beams are laid across the walls then purlins laid at right angles across the beams and tied to them. Then a woven reed mat, thin timber battens or wire mesh fixed to the top of the purlins. Finally a lime-based concrete or stabilised earth covering is laid in several layers over the top, finished with a thin cement or lime-based screed containing a higher proportion of cement, or a bitumen-based compound to reduce water leakage. The massive construction of such a roof makes it very unlikely that it would move about significantly if no wall plate is used.
For pitched roofs a roof structure needs to be put in place to take the roof covering. Normally this consists of triangular roof trusses placed across the walls, and purlins across the trusses to which the roofing sheets or tiles are nailed or tied. The trusses and purlins are usually of timber, but can also be of steel or reinforced concrete. A simple type of truss suitable for relatively small buildings is shown below
EXAMPLE
Other types of trusses are used for larger spans, and for details on these the reader is referred to the Roof Structure Guide, the Roofing Primer (especially part 1) and Roof Truss Guide produced by SKAT.
Note also an alternative arrangement for a shed type building - in this case the slope of the roof is taken by the truss, so all the walls can then be of the same height.
EXAMPLE
The type of trusses to use, their spacing, the size of their members and the size and spacing of the purlins depends on a number of factors including the type and strength of material used to make them, the type of roof (gable, hipped, shed or other), the size and shape of the building, the weight of the roof covering and the sizes of the tiles or sheets used, and other imposed loads on the roof, e.g. from wind or snow. In some cases standard roof designs can be used, or guidelines exist from local or national building standards. However, where these are not appropriate the design of the roof structure would need to be determined. Details on roof structure design are too lengthy to be given here, but can be found in The Roof Structure Guide, and The Roof Truss Guide, both produced by SKAT, further details on which can be found in the Reference section.
The internal angle of a typical roof truss is taken as 30°. Although for small buildings angles as high as 45° have been used, this is not recommended for school buildings where the truss would be likely to need to have a greater span than for some other types of building such as houses. It is also generally not recommended to have an internal angle of the truss of less than 25°, or water runoff might cause seepage of water underneath the roof covering.
Roof trusses are assembled on the ground according to prepared or available design drawings then lifted into place at the appropriate position on the roof. They are fixed to the wall plate with bolts or steel straps. Until the permanent purlins have been put in place the trusses would need to be kept stable by connecting them together with temporary purlins or ties. When the trusses are all in place the permanent purlins are then laid across them and nailed or tied in place.
The roof covering (usually sheets or tiles) is laid over the purlins, starting from the bottom of the roof. The sheets or tiles are nailed or tied to the purlins. A special type of tile, known as a ridge tile is needed to cover the top of the roof. Fixing this in place can be slightly more complicated than for laying the rest of the tiles or sheets. With corrugated iron sheets making the ridge or crown is relatively straightforward, as this just consists of a flat steel plate bent at an angle the same as the top of the roof, then nailed to the tops of the sheets and the purlin below.
For information on vaulted, domed or corbelled roofs the reader is referred to Bonner & Das (1996) and Joffroy & Guillaud (1994).
