Flat Roof Construction Detail – A flat roof is essentially a low pitched roof the angle is usually around 10 degrees or less to the horizontal.
The angle is usually governed by the amount of annual rainfall in the area its planned to be built.
The functions of the roof are to provide a barrier to rain wind snow and dust. Water and boil proof ply can be used on the outer surface to provide seal flat roofs extra attention to junction details is required to prevent water ingression.
Flat roof’s are required to prevent excessive heat loss in winter and to keep the interior of the building cool in summer. Materials used in their construction include softwood boarding chip board plywood and tongue and groove.
Insulation boards are also used to improve their thermal properties.
Boards laid over the decking create a warm deck roof, whereas quilted materials draped over the joists or placed between them make a cold deck.
To accept the movement due to changes in temperature and moisture content flat roofs should integrate a void between the ceiling and decking.
The flat roof structure should provide lateral restraint and stability to adjacent walls and be capable of accommodating all stresses encountered.
The simplest form of roof construction to perform these functions is a timber flat roof covered with an impervious material to prevent rain penetration.
The construction of a timber flat roof follows the same methods as these employed for the construction of timber. They are constructed from timber joists.
Timber flat roofs are usually constructed to fall in one direction towards a gutter or outlet. This can be achieved by adapting the joists to the required fall, in order to achieve this fall a firings must be attached to the top side of the joists.
Weather proof finishes include asphalt, lead copper, zinc, aluminum and built up roofing felt.