Case 1

Good Practices

Drainage of flat roof

The primary functions of the roof is the provision of shelter from natural elements like rain. Rainwater which collects on the roof’ surface will drain away quickly if proper drainage is designed for. If ponding often occurs, the roof will not be able serve its functions well. A roof will not be able to serve its function, and cause 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. Providing at the eaves or low points of flat roofs, gutters of either metal, asbestos 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. Forming falls to flat roofs and inserting at the lowest point a rainwater outlet which is securely sealed to the weathering membrane. Pitch on flat roofs can be created by varying the thickness of roofing layers to create a gradient to direct surface roofwater towards drainage outlets. 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 Gutter

Gutters come in half round or moulded in section and together with its connection accessories are made in a variety of materials such as cast iron, asbestos cement, PVC-U, pressed steel and wrought copper.

The flow into the gutter depends on the area of surface being drained and gradient of the surface. BRE Defect Action Sheet 56 recommends that gutters should be fixed with their center line vertically below the edge of the roof covering and close beneath it so that they are efficient in collecting rainwater that falls off the roof.

Gutters should be laid with a slight fall of about 1 in 350 towards the nearest outlet to allow for possible structural movement. Gutters should also be able to discharge any overflow in excess of the design capacity, clear of the building.

(Ivor H. Seeley Building Technology Fifth Edition)

A problem usually face by gutters is the accumulation of leaves at outlets, leading to choking and overflowing. Regular cleaning of gutters is necessary to ensure smooth flow of rainwater into the downpipes.

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

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.

Overflow Scupper

The rate at which rain is displaced from the roof deck through rainwater drainage pipework may not be sufficient to prevent the accumulation of heavy volumes of rainwater. This problem is especially relevant in event of heavy rainstorms. Overflow Scuppers can alleviate this problem if the roof drain is located next to the parapet wall.

Overflow scuppers are through holes incorporated in the parapet walls and are set aprroximated 2 – 4 in. above the roof drain. In event of heavy rainstorm, or when the roof drain inlets are clogged, rainwater can thus be directed through the overflow scuppers and discharged via the sides of the building.