Case 3

Good Practices

Drainage of flat roof

A roof will not be able to serve its function, resulting in ponding of water unless means for rainwater flow are provided. The main components of rainwater goods are rainwater gutter and rainwater downpipes.

The removal of rainwater can be effected in 3 ways:

1) by providing at the eaves or low points of flat roofs, gutters of either metal, asbestoes cement or plastic. From outlets in the bottom of the gutter, the water will pass through rainwater pipes into the stormwater chains at the foot of the wall.

2) By forming falls to flat roofs and inserting at the lowest point a rainwater outlet which is securely sealed to the weathering membrane. The outlet will discharge into vertical rainwater pipes that in turn discharge into the stormwater drain at ground floor level.

3) By collection through parapet walls into hopper heads

Rainwater Downpipes

Downpipes convey rainwater from the roof gutters to underground drains, often through a back entry rainwater gully at the ground level. The size of the rainwater pipe should be at least the size of the outlet from gutter. A downpipe which serves more than one gutter have an area at least as large as the combined areas of the outlets.

General installation recommendations for corners, upstands, drainage points and pipes are as follows:

1) Lay horizontal roof surfaces first, but do not adhere with 30cms of any obstacles; this will ensure that the sheet can be folded back to enable work around the obstacles to be carried out.

2) All membrane sheeting joins should face drainage points as far as possible so that water does not collect between the sheet and lap tape in joint. This applies to vertical or near vertical flashing as well as other areas.

3) Work slowly and methodically on upstands, as these are the points on a roof where leakage may occur.

4) Whenever possible work from obstacles towards the edge of the roof. This will facilitate laying of sheeting near the obstacle.