Backfill and waterproofing
If the house is built on a slope that has been excavated and backfill is needed on the outside of the house against the wall, such a wall must be 345 mm (three bricks placed alongside one another) wide. A thick plastic sheeting of 0.25 mm should be built into the wall vertically, to prevent moisture from penetrating the building. This sheet is added in-between the outer and central layer of bricks. Backfilling is done in 150 mm layers and compacted thoroughly.
It is advisable that a structural engineer be appointed to design a retaining wall.
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On completion of all the walls, a carpenter can erect the roof structure, comprising either beams for a flat roof or roof trusses for a pitched roof. Since the roof is an extremely important structural element of a house, it is vital that a competent person do the design. Remember, a roof is subjected to its own weight, which can be considerable in the case of a heavy concrete tile roof, and to wind forces, which can be considerable in the case of a lighter, sheet-metal roof. It is therefore always advisable to have the roof structure properly designed and constructed.
These days it is much easier and safer to make use of the services of one of many prefabricated roof companies that take the exact measurements, create a computer-aided, engineered design, assemble the trusses in the factory, and erect them on site according to specifcation. Local authorities require an engineer’s certificate for the design and erection of a roof structure.
The minimum slope of a fat, sheet-metal roof is 5°, but this may be decreased to 3° if long, singlespan plates are used. The minimum slope of a pitched concrete tile roof is 26° without waterproofing, and 17° with an under-tile waterproofing membrane.
During roof construction, it is the plumber’s job to fit drainage valleys and flashings on walls, chimneys and ventilation pipes that run through the roof. Please note that some local authorities require a roof-structure inspection as well.
Once the roof cover has been completed, the fitting of rainwater gutters and down-pipes as well as fascia boards is optional.
The owner-builder should take note of the fact that, on average, once the roof is completed, about 50% of the building costs should have been spent.