Roof Drainage: Key Considerations When Designing A Drainage System For A Roof

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There are a number of factors that have to be considered when designing an effective drainage system for a roof. The most important of which are;

  • Rainfall intensity
  • Average recurrence interval
  • Catchment area
  • Influence of wind
  • Hail
  • Outlets
  • Serviceability considerations
  • Maintenance

Blocked drain

Rainfall intensity

Rainfall in the RSA is basically derived from frontal coastal systems along the coast which produce steady rainfall over a number of hours and short duration high-intensity thunderstorms in the interior which produces heavy rainfall within a period of minutes. However, high-intensity thunderstorms are known to occur at intervals along the Natal coast plus Port Elizabeth and surrounds. Intensity is measured in millimeters per hour with the greatest intensity occurring during a period of five minutes. Generally, the country is divided into three rainfall regions i.e. summer 200 mm/h, year-round 150 mm/h and winter 100 mm/h.

Average recurrence interval (ARI)

Defines the period, in years, between which the above intensities are likely to be exceeded and the capacity of the drainage system being inadequate. The average ARI for residential buildings is 10-15 years and all others between 25 & 50 years depending on the risk of damage to plant, processes, and stock. For buildings of strategic importance such as hospitals, a period of 100 years is recommended.

Catchment area

The area of roof to be drained by a section of gutter or downpipe, this will include areas of higher level roofs draining onto a lower level. As rain rarely falls vertically an allowance will need to be made for runoff from adjacent walls.

Influence of wind

Rainfall is normally accompanied by winds of varying intensity which can either impede flow or induce surges in gutters and we recommend the provision of a freeboard allowance equal to 10% of the calculated depth for a gutter with a minimum of 25 mm on residential and 40-50 mm on industrial buildings respectively.


The accumulation of hail in gutters and downpipes can considerably reduce the effectiveness of a drainage system often resulting in flooding and we recommend the installation of hailguards in areas prone regular hailstorms. (Please refer to our website for details pertaining to the design of hailguards).


The shape and size of outlets have a major influence on the efficiency of a drainage system. The smoother the transition from horizontal to vertical flow the greater the capacity of flow into the downpipe. The insertion of a conical outlet can improve flow by 10-20% and the insertion of a header box or sump buy double that. Side outlets and spigots are less efficient and also impede flow in a gutter and are not recommend for use outside of residential applications. Flanged outlets to be attached to the outside surface of gutters.


A factor often overlooked when designing drainage systems is the speed of discharge from a roof resulting in the water overshooting the gutter which can have disastrous consequences with box gutters, we recommend a minimum width of 600mm for a roof with a slope length of 16 m inclined at not more than 10°. Alternatively, it will be necessary to install baffle flashings. On slopes of less than 26°, it is of paramount importance that the pans of cladding overhang the gutter by at least 50 mm and the pans of the cladding are bent down to form a drip in order to eliminate the risk of runback up the underside of the cladding.

In order to improve drainage and eliminate ponding, it is essential that gutters are installed with a fall of not less than 1:500 and that the outlet is at the lowest point.

The installation of grids or cages over outlets is not recommended as they or more readily blocked by debris, even a partial buildup of debris can reduce the capacity of an outlet by 50%. Often rainwater is transferred from a higher to lower roof via a downpipe fitted with a spreader. This practice should be confined to areas of no more than 20 m2 to avoid flooding the pans of the cladding profile on the lower roof.


Drainage systems i.e. gutters and downpipes need to be inspected and cleaned of all debris and accumulated silt at not more six monthly intervals. Downpipes need to be flushed to ensure there is no blockage within the pipes. All cleaning to be carried out with non-metallic utensils.