School buildings in developing countries
Practical Action
Figure 8: Shed roof.
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
Walls of building
Figure 9: Triangular roof trusses
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