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Guidance - Flat Roof - Drainage

Flat Roof - Drainage


A correctly build flat roof will drain the rainwater quickly and effectively into the gutters without allowing water to collect in depressions and ‘pond’ on the surface of the roof leading to the build-up of silt deposits on the roof and stresses in the membrane when the water freezes (a small amount of ponding is evitable and it should not reduce the performance of the roof covering).

Ponding will be reduced by ensuring the roof is provided with a decent fall allowing the rainwater to be drained effectively towards the outlets and gutters at the edge of the roof.

Design Falls

When specifying a fall for a flat roof, the designer should take account of:

  • Any potential deflection of the structural members and decking under dead and imposed loads (a particular problem with timber and metal deck roofs).
  • Possible inaccuracies in construction.
  • Type of weatherproofing material used.

Because the desired fall may be difficult to achieve in practice, the designer may need to adopt a design fall considerably steeper (often twice) than the recommended finished fall.

The building regulations encourage a minimum fall of 1:40.

Flat Roof Slpoe

Minimum finished fall required according to BS6229 is 1:80.

Recommendations for specific materials are as follows:

  • Aluminium 1:60
  • Copper 1:60
  • Zinc 1:60
  • Lead sheet 1:80
  • Built-up bitumen sheet 1:80
  • Mastic asphalt 1:80
  • Single ply membranes 1:80
  • Liquid waterproofing systems (hot- or cold-applied) 1:80
  • Green roofs should have a fall of not less than 1:60 and be built in accordance with manufacturer’s details and British Board of Agrément certification.

The desired fall may be created in one of the following ways:

  • Sloping decks

    The fall of the roof may be created in the structure itself by laying the supporting beams, or joists at a slope (giving a sloping soffit) or by installing tapered beams with horizontal soffits.

  • Firrings

    It is normal practice for the joists of a flat roof to be set level thus creating a perfectly level ceiling. The required fall is then formed using strips of tapered shaped timbers known as firrings fixed along the tops of the joists before the deck is laid. These firings should be the same width and length of the joists. The firrings may also provide the fall by each one decreasing in thickness along the slope of the roof.

    Flat Roof Firrings

    The firrings can be fixed at right angles to the joists instead of along the length of the joists, this will provide a better level of ventilation. However, when fixing in this manner it is essential that the firrings are of suitable structural strength to span between joists and should not be less than the following sizes.

    Size of Firrings
    Distance between Joists Width of Firrings Depth of Firrings
    400 / 450 mm 38 mm 38 mm
    600 mm 38 mm50 mm
  • Pre-formed insulation boards

    Proprietary preformed tapered insulation boards can provide drainage falls to a warm decked roof (manufacturers recommend falls not less than 1:60). The boards should be laid onto a vapour control layer and be covered with the waterproofing system.

  • Concrete and Screeds

    Falls on a flat roof with a concrete deck are either provided within the structural concrete itself or created within the screed which is laid over the concrete deck (as for in-situ cast concrete slab).

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  • Gutters and downpipes should be adequately sized to deal with storm conditions in accordance with BS EN 12056-3. Flat roofs should be designed to drain the roof to one or two edges towards gutters and outlets.
  • There should be a smooth transition into the gutters which can often be lined using the roofing membrane to achieve a completely uniform finish (check the manufacturer's details).
  • Internal gutters can be used but conventional eaves gutters are preferable.
  • If unavoidable, internal outlets should be fitted with leaf and gravel guards.
  • Welted drips should extend to the middle of the rainwater gutter, and there should be a minimum of 50mm turndown.
  • To ensure the rainwater is directed towards the gutters, the sides of the roof surface (apart from the gutter edge) require a minimum of 50mm upstand verge provided using triangular pieces of timber called ‘tilt fillets’ nailed to the edges of the decking, not to the wall.
  • All gutters and downpipes should be accessible for maintenance.

Weathering Details

Flat Roof Flashing


  • At the point where the roof deck meets the external wall or at parapets, careful detailing will be required to prevent water ingress.
  • To avoid a sharp bend where the membrane is folded up the wall, which could cause a split, a small strip of timber ‘angle fillet’ a minimum 75mm x 75mm is fitted under the felt and fixed firmly to the deck.
  • The waterproofing membrane must be taken up the wall at least 150mm (providing an upstand) above the roofing surface.


  • The membrane upstand should be protected by a cover flashing to give a good watertight joint.
  • Lead should be cut into the horizontal mortar joint and overlap the top edge of roofing material not less than 50mm, finishing no closer than 75mm above the roof surface.

Cavity Tray

  • If the wall is a cavity wall, a cavity tray must be fitted, stepping downwards at least 150mm and weep holes should be provided.

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