Flat roofs, terraces and balconies

7.1.16Accessible thresholds and upstands

Accessible thresholds shall be protected by adequate weatherproofing and drainage.

Generally, where a flat roof or terrace abuts a wall the waterproofing layer should extend up the wall to form a minimum 150mm upstand measured from the balcony/terrace drainage layer of the roof/terrace. The waterproofing material forming an upstand should link directly under a cavity tray to ensure the cavity fully drains. Weep holes should be provided in masonry walls at 1m maximum spacings to assist drainage.

Where there is a door threshold or a window sill less than 150mmm above the balcony/terrace drainage layer and there is a requirement for level access, or the fenestration design involves windows at or close to internal floor level and the paving/decking level , then the following upstand and accessible threshold design requirements, as shown in the diagrams below, should apply.

Designs which continue the waterproofing layer horizontally through/under the outer leaf of a cavity wall and form an upstand against the inner leaf within the cavity are not acceptable because:

  • materials used for the waterproofing layer are generally unsuitable to perform as a DPC supporting masonry loads
  • the cavity must fully drain to avoid water retention and associated problems through prolonged saturation of the wall material, frost action and water turning stagnant
  • water draining from the waterproofing layer must not be directed into a cavity wall
  • future inspection, repair and maintenance of hidden upstands cannot be carried out without significant disruption to the construction.

Accessible thresholds should:

  • be in accordance with the design - specific fire, thermal and acoustic precautions may be required
  • have a maximum 15mm upstand at the threshold (measured at the door position); 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 the ramps and sills should be 15 degrees
  • have a minimum 45mm projecting sill to shed rainwater away from the interface with the waterproofing layer with a drip feature a minimum 30mm away from the face of the upstand.
  • have a 75mm minimum balcony upstand below the underside of the projecting sill, measured from the balcony/ terrace drainage layer.

Uninsulated balcony deck

Insulated terrace deck

Accessible threshold

Waterproofing layers should:

  • be laid without forming ponding and associated stagnant water
  • have a finished fall of a minimum of 1:80 to rainwater outlet(s)
  • be subject to specific third-party assessment where falls are zero degrees with no back falls or ponding
  • 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 at least the capacity of the outlet
  • at least one outlet chute and hopper (Chute should be sized to serve at least twice the discharge capacities to allow for partial blockage without causing flooding into the building)
  • two outlets connected to independent downpipes such that if one downpipe becomes blocked the other outlet(s) can still cope with the discharge, or
  • setting the balcony kerb a minimum 25mm below the door threshold to discharge safely without causing any adverse effect to the construction below.

Outlets beneath decking or paving should be clearly identified and accessible for maintenance. To ensure adequate drainage:

  • 10mm gaps should be provided between decking/paving units and perimeter upstands/thresholds
  • 4-6mm gaps should be provided between individual units of decking or paving
  • spacers and supports which raise the decking or paving should not obstruct the flow of rainwater to outlet(s).

A splash barrier around the perimeters 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 by extending the waterproofing layer to form an upstand with cover flashing and cavity tray. Note: impervious masonry units with porous bed joints are unsuitable within this zone.