Flat roofs and balconies

7.1.11Accessible thresholds

Accessible thresholds shall be protected by adequate weatherproofing and drainage.

Accessible thresholds should:

  • be in accordance with the design – specific fire, thermal and acoustic precautions may be required
  • have a maximum 15mm upstand (measured at the door position) at the door threshold; additional sloping transition elements, such as a small internal ramp and external sill, may be provided either side of the upstand; the maximum slope on ramps and sills should be 15 degrees
  • have a minimum 45mm projecting sill to shed rainwater away from the interface with the waterproofing layer
  • have a 75mm minimum balcony upstand below the underside of the projecting sill, measured from the balcony drainage layer.


The drainage layer is not necessarily the waterproofing layer (i.e. the top of the insulation of an inverted roof should be considered as the drainage layer).

Waterproofing layers should:

  • prevent ponding and associated stagnant water
  • generally fall a minimum of 1:80 away from the building to the rainwater outlet(s)
  • be subject to specific third-party assessment where falls are zero degrees
  • be designed to ensure that where falls are towards or parallel to the building, blockage of the outlet(s) cannot cause flooding to the building
  • be fully protected from direct trafficking
  • be capable of withstanding point loads from supports to decking or paving
  • be UV resistant or fully protected from daylight.

Drainage arrangements should be effective and have a suitable overflow. The building should not flood where an outlet or downpipe is blocked. This can be achieved by using:

  • at least one outlet and an overflow with the capacity of the outlet
  • at least one outlet chute and hopper (sized to serve both the discharge and overflow capacities)
  • two outlets connected to independent downpipes, or
  • setting the balcony kerb a minimum of 25mm below the door threshold.

Outlets beneath decking or paving should be clearly identifiable and accessible for maintenance.

To ensure adequate drainage:

  • gaps should be provided between decking and paving at balcony perimeters
  • minimum 10mm gaps should be provided between individual units of decking or paving and the threshold sill, perimeter walls and kerbs
  • spacers and supports which raise decking or paving should not obstruct the flow of rainwater to outlet(s).

A splash barrier should be provided:

  • to ensure water does not reach any part of the wall that could be adversely affected by the presence of moisture
  • to a minimum of 150mm above the decking or paving
  • using an impervious wall finish or cladding, or extending the waterproofing layer to form an upstand with cover flashings and cavity trays.