Flat roofs, terraces and balconies

7.1.4Drainage

Drainage system

A drainage system of outlets and downpipes should be:

Where a downpipe discharges above ground level, or above a drainage gully, the downpipe should be fitted with a shoe.

A design approach for free draining balconies can be accepted if rainwater will always be routed away from the building to drain via a suitably formed soffit drainage tray that directs water to run outwards over a continuous formed perimeter edge. But where this drainage design is installed for stacked balconies on multi-storey elevations - the rainwater will tend to cascade down from these edge drainage slots as water runs off each balcony.

Under typical wind driven rainstorm conditions the water runoff volumes from these balcony catchment surfaces will progressively increase in a downward direction until the full volume of water reaches ground level. This can adversely affect locations such as main entrances to apartment buildings as well as any access doorways and private gardens of individual homes situated at ground level.

Therefore, in addition to the free draining balcony design there is a requirement for design of ground level drainage to effectively prevent ponding or flooding of water. This drainage design is required along all building perimeter locations where thresholds, access locations and other places of regular use will exist.

Rainwater outlets should:

Where a flat roof, balcony or terrace has an upstand on all sides, drainage should consist of a minimum of two outlets connected to separate downpipes, or one outlet plus an overflow. The overflow should be:

Falls

The finished roof, balcony or terrace should have effective drainage to the outlet(s) without creating back falls or ponding on the waterproofing layer and WFRL where fitted.

For medium to large, and complex roof layouts and roofs where the ‘design fall’, as shown in Table 2, is not used for the design, a detailed analysis should be undertaken to establish overall and local deflection, under load and long-term creep, and direction of falls. Allowances should be included for workmanship and construction tolerances particularly with steel frame erection tolerances. The effects of localised loadings from features such as planters, service equipment, etc. should be included in the design.

The design should show how ponding will be avoided, e.g. by placing outlets at points of maximum deflection, by use of a screed, use of tapered insulation, or firrings, to take out the deflection and form falls to outlets.

Where falls are formed by use of screeds follow the guidance in clause 7.1.8.

Where tapered insulation is used:

Firring pieces should be:

Table 1: Size of firring pieces used to from falls

Where a detailed analysis is not undertaken the roof should be designed with a ‘design fall’ that caters for initial and long-term deflection to ensure the ‘finished fall’ provides effective drain- age without back falls or ponding.

Table 2: Suggested ‘Design’ falls for various roof types

Zero fall roofs

Zero falls are not accepted for roofs with exposed waterproofing layers, such roofs should be designed to the falls shown in Table 2.

Deck Survey

Prior to laying the waterproofing layer(s) a site survey of the deck should be carried out by the deck erector and any back falls should be addressed. Depending on the deck material this may be achieved by applying localised screed, to remove the depression and create falls to outlets, the adjusted areas should be resurveyed to ensure no back falls remain, or by providing additional rainwater outlets at the point(s) of maximum deflection. A formal handover procedure should be undertaken between the deck erector and the waterproofing contractor.