Flat roofs and balconies

7.1.8Waterproofing and surface treatments

Flat roofs, and balconies forming roofs, shall adequately resist the passage of water to the inside of the building. Issues to be taken into account include:

  1. installation of waterproofing
  2. waterproofing systems
  3. surface treatments.

Installation of waterproofing

Prior to the waterproofing being installed:

  • the structure and receiving surface should be checked and approved by the waterproofing contractor
  • the manufacturer’s recommendations for preparation, including priming upstands, roof outlets, etc. should be followed to achieve a satisfactory bond with the waterproofing
  • the surface should be even and dry and nails should be punched below the surface
  • the manufacturer’s recommendations for conditioning, and unrolling in advance of laying, should be followed
  • concrete and screed surfaces should be adequately dry.

Environmental conditions should be suitable for installing waterproofing. Issues to be taken into account include the following:

  • Sheet membranes should not be installed or handled when the air temperature is 5°C or less (unless otherwise agreed with the manufacturer).
  • Self-adhesive reinforced bitumen membranes should not be installed below 10°C, and the manufacturer’s recommendations should be followed.
  • Membranes should not be installed on damp or frosted surfaces or when any rain, sleet or snow is falling.

Waterproofing should be:

  • installed in accordance with the design and the manufacturer’s recommendations
  • installed by a specialist roofing contractor approved by the manufacturer, where a proprietary system is used
  • installed by the same contractor who installs the vapour control layer, insulation and surface finish
  • checked by the contractor to ensure that the deck and insulation boards are waterproofed at the end of each day, and before inclement weather
  • installed so that membrane laps near outlets do not impede drainage
  • installed so that successive layers do not trap water.

Inverted roofs should:

  • not be used for slopes greater than 10°
  • be designed to support the loads, particularly from ballast needed to retain the insulation material
  • be ballasted to the depth specified in the design
  • be ballasted using paving slabs or minimum 19mm diameter rounded pebbles.

Waterproofing systems

Reinforced bitumen membrane

Reinforced bitumen membrane should be high performance and reinforced with polyester reinforcement, e.g. type 5U, 5B/180, 5B/250 to BS 8747 (type 5 reinforced bitumen membranes are colour-coded blue for identification).

Table 3: Reinforced bitumen membrane used in warm roof construction

Table 4: Reinforced bitumen membrane for an inverted warm roof

Torching onto insulation boards, except rockwool or perlite, is not acceptable.

Elastomeric (i.e. SBS polymer-modified) bitumen membranes offer increased extensibility and flexibility, especially at low temperatures, and can provide a longer service life.

Mastic asphalt

Mastic asphalt should be:

  • to BS 6925, type 988 T25, 20mm thick on the flat and installed on black sheathing felt
  • (for green roofs) 3 x 10mm layers on horizontal surfaces and 2 x 10mm layers on vertical surfaces, for green roofs.

Polymer modified asphalt should be assessed in accordance with Technical Requirement R3.

Thermoplastic single-ply membranes

Thermoplastic single-ply membranes, including materials such as PVC (polyvinyl chloride) and TPO (thermoplastic polyolefine) should be:

  • either bonded to the insulation, mechanically fixed to the deck or loose laid, and sealed and ballasted in accordance with the manufacturer’s recommendations
  • welded at laps using either hot air or a specific solvent
  • assessed in accordance with Technical Requirement R3.

Surface treatments

Surface treatments should be in accordance with Table 5.

Table 5: Surface treatments for flat roofs


1 Loose surface finishes should be prevented from being removed by weather and discharged into gutters and drain pipes. Chippings should be a minimum of 12.5mm limestone or white spar, not pea gravel.
2 Cement/sand blinding should be installed on two layers of waterproof building paper or two layers of 1000 gauge polyethylene separating membrane.
Slabs should be kept back 75mm at perimeters and a 25mm movement gap incorporated for every 9m2 of paving.
3 Timber decking systems should only use compatible preservative treatments. The undersides of the bearers should have large, smooth contact areas, with no sharp edges or corners.