Flat roofs, terraces and balconies

7.1.4Drainage

Flat roofs, balconies and terraces shall have adequate and effective rainwater drainage to a suitable outfall. Issues to be considered include:

  1. drainage system
  2. falls
  3. zero fall roofs
  4. deck survey.

Drainage system

A drainage system of outlets and downpipes should be:

  • provided to all flat roof, balcony and terrace areas.
  • of sufficient size to accommodate normal rainfall and sized to cope with concentrated flows including the accumulative effect of one roof draining on to another.
  • fixed in accordance with the design and supported and jointed in accordance with the manufacturer's recommendations.
  • installed ensuring any gutters, including box gutters, are provided with stop ends where applicable, and with sufficient falls to provide effective drainage to outlets/ downpipes.
  • provided with overflow(s) to avoid flooding in the event of one or more drainage outlets becoming blocked.

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:

  • be of the size and number required to deal with the expected rainfall intensity in accordance with BS EN 12056-3.
  • be positioned to provide effective drainage to all areas of the roof.
  • be recessed to facilitate the free flow of water without forming ponding at the junction with the waterproofing layer.
  • be accessible for maintenance.
  • be insulated to avoid surface condensation on the outlet and downpipe if passing through habitable areas.

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:

  • Provided through parapet walls or perimeter upstands
  • Sized for effective flow rate and positioned to prevent water from entering the building, particularly in relation to door thresholds and low window sills
  • of higher capacity than the combined capacity of the other outlet(s).
  • positioned to discharge safely away from the building.
  • be visible when in operation.

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:

  • Drainage should be designed by the insulation manufacturer, with ‘design’ falls of no less than 1:60
  • Installation should comply with the design and manufacturer’s recommendations
  • The sequence of installation should ensure that boards are waterproofed, and the roof sealed at the end of each day, or before the arrival of inclement weather.
  • It should be installed directly onto the air and vapour control layer, with the primary waterproofing layer above
  • Changes in the direction of falls should be formed with mitred joints
  • Successive roof layers should be installed with a minimum of delay, to avoid trapping water during construction.

Firring pieces should be:

  • used to form falls, unless the design specifies a sloping joist or ceiling
  • of the size given in Table 1 where installed across the joists
  • adequately fixed to the joists in accordance with the design.

Table 1: Size of firring pieces used to from falls

Joist centres (mm)Minimum width (mm)Minimum depth (mm)
400 or 4503838
6003850

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

Type of roofDesign fallMinimum finished fall
Membrane and liquid applied waterproofing1:40 ³1:80
Profiled metal roof system (self-supporting)5.5° (1:10) ¹4° (1:14) ²
Profiled metal roof (fully supported)6.5° (1:9)5° (1:12) ⁴
Standing seam roof system (self-supporting)2.5° (1:23) ¹1° (1:60) ⁴
Flat sheet hard metal roof (fully supported)1:40 ³5° (1:12) ⁴
Lead roof (fully supported)1:40 ³1:80
Green/biodiverse roof1:40 ³1:80 (at drainage level)
Blue roof1:40 ³1:80 (see also Zero fall roofs)
Tapered insulation1:40 ³1:80

Notes

1 The supporting structure should be designed at a pitch of 1.5° more than the minimum pitch for the sheeting, or the designed roof slope, to allow for tolerances
and onsite variations, unless justified by a detailed structural analysis of the main frame and secondary steelwork to account for deflection/settlement.

2 Based on through fixings. For lesser pitches, including the apex of any curved roof which approaches flat, proprietary secret fixed types of profiled sheeting
should be used in accordance with the system manufacturer’s instructions.

3 ‘design fall’ to take account of deflection and construction tolerance for the supporting deck to ensure at least minimum finished fall on completed roof. ‘Design
fall’ usually taken as twice the ‘finished fall’ unless a detailed deflection/settlement of the deck is carried out.

4 Check with manufacturer for alternative pitch recommendations.

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.