Flat roofs and balconies


Flat roofs and balconies shall have adequate and effective rainwater drainage to a suitable outfall. Issues to be taken into account include:

  1. falls
  2. outlets.

The principles for drainage given in Chapter 7.2 ‘Pitched roofs’ are applicable to flat roofs and balconies.

Rainwater disposal from roofs and balconies 6m2 or less in area should be considered. Where run-off may cause damage or staining to a façade, or damage to landscaping, then rainwater gutters and downpipes should be provided. The cumulative effect of water discharging from multiple balconies in vertical alignment should be taken into account.

Open slatted balcony decking should drain away from the home.


Flat roofs and balconies should:

  • be designed with a fall no shallower than 1:40 to ensure a finished fall of no less than 1:80, unless a detailed analysis which includes overall and local deflection is used as justification
  • have a minimum finished fall of 1:80 (green roofs 1:60), unless it has a metal sheet covering
  • account for deflection in the structural design where falls are achieved using screeds (particularly on large roofs).

Where decking or paving is installed above the waterproofing and is less than 150mm below the sill, it should be of a type and design that prevents a build-up of standing water.

Where tapered insulation is used:

  • drainage should be designed by the insulation manufacturer, with falls of no less than 1:60
  • construction 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 vapour control layer, with the primary waterproofing above
  • cross falls should be formed with mitred joints
  • successive roof layers should be installed with a minimum of delay, to avoid trapping water during construction.

Metal sheet roofs

Flat roofs with metal sheet roof coverings should be designed with a fall of no less than 1:30 to ensure a finished fall of no less than 1:60.

Concrete roofs

Concrete roofs can be finished with sand/cement screed topping set to achieve the falls. Screed finishes should be:

  • free from ridges and indentations
  • finished with a wooden float to provide a smooth, even surface for the vapour control layer and waterproof finish
  • installed by specialist contractors where a lightweight finish is used, and have a topping of 1:6 (cement:sand), 13mm thick
  • to the minimum thickness in Table 7 where a cement/sand screed, 1:4 (cement:sand) is used
  • suitably dry and primed to receive the waterproofing system.

Table 7: Minimum screed thicknesses

Location of screedNominal thickness (mm)
Bonded monolithically to in-situ or precast concrete 40 (25 minimum.)
Unbonded (on separating layer) 70 (50 minimum.)

Timber roofs

Firring pieces should be:

  • used to form falls, unless the design specifies a sloping joist or ceiling
  • of the sizes given in Table 8 where installed across the joists.

Table 8: Size of firring pieces used to form cross falls

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

Rainwater outlets

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 recessed to facilitate the free flow of water
  • be accessible for maintenance.

Where a flat roof or balcony has an upstand on all sides, drainage should consist of either two outlets or one outlet plus an overflow. The overflow should be:

  • provided through parapet walls or perimeter upstands
  • sized and positioned to prevent water from entering the building
  • of higher capacity than the combined capacity of the other outlet(s).