NHBC Standards 2011

7.1 Flat roofs and balconies


7.1 - M1
All materials shall:
(a) meet the Technical Requirements
(b) take account of the design

Materials that comply with the design and the guidance below will be acceptable for flat roofs and balconies.

Materials for flat roofs and balconies should comply with all relevant standards, including those listed below. Where no standard exists, Technical Requirement R3 applies (see Chapter 1.1 'Introduction to the Standards and Technical Requirements').

References to British Standards and Codes of Practice include those made under the Construction Products Directive (89/106/EEC) and, in particular, appropriate European Technical Specifications approved by a European Committee for Standardisation (CEN).

Appendix 7.1-A lists the materials suitable for:

  • waterproofing, including flashings
  • structure, deck and decking
  • thermal insulation
  • vapour control layer
  • preservative treatment.


7.1 – M2
Waterproofing materials shall be of adequate durability and resist the passage of moisture to the inside of the building


Only high performance reinforced bitumen membranes should be used, i.e. those including polyester reinforcement (e.g. Type 5U, 5B/180, 5E/250 to BS 747).

Type 5 reinforced bitumen membranes are colour coded blue for identification.


Examples of single layer materials include PVC (polyvinyl chloride) and TPO (thermoplastic polyolefine)

These materials soften with increasing temperatures. Laps are welded, using either hot air or a specific solvent to melt and fuse them. These materials should be assessed in accordance with Technical Requirement R3.


These materials are applied as a liquid, often laid in two or more coats. Most include a reinforcement scrim to distribute the tensile stresses.

  • cold-applied flexible polyurethane systems
  • hot melt rubberised bitumen systems. These are applied hot in two 3mm coats, with reinforcement between, and with a substantial reinforced bitumen membrane protection sheet. They should only be used in buried applications, such as inverted roofs, green roofs, podium areas, etc.

These materials should be assessed in accordance with Technical Requirement R3.


To BS 6925 type 988 T25, 20mm thick on the flat, laid on black sheathing felt.

For green roofs, three coats horizontally (30mm total thickness) and two coats vertically (20mm total thickness) are required.

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

Appendix 7.1-B gives suitable surface treatments for the waterproofing.


Materials should be in accordance with the following:
BS EN 501 'Specification for fully supported roofing products of zinc sheet'
BS EN 504 'Specification for fully supported roofing products of copper sheet'
BS EN 506 'Specification for self supporting products of copper or zinc sheet'
BS EN 12588 'Lead and lead alloys. Rolled lead sheet for building purposes'.



7.1 – M3
Structural timbers shall be of the appropriate grade and size to support the imposed loads

Items to be taken into account include:

  • appropriate load/span tables published by TRADA in support of Building Regulations and associated documents
  • structural softwood for internal use should be dry graded to BS 4978 and marked 'DRY' or 'KD'.

I-joists and metal web joists should be specified in accordance with the manufacturer’s recommendations. They should not used in situations where any part of the joist is exposed to external conditions.



7.1 – M4
Profiled metal roof decks shall be of the quality, type and dimensions required by the design

Profiled metal roof decks should be:

  • galvanised steel to BS EN 10147 and used in compliance with BS EN 1993-1-3, or
  • aluminium to BS EN 485-2.


7.1 – M5
Green roofs and proprietary roofing systems shall be suitable for their intended use

Green roofs, both intensive or extensive, should be a complete system from the membrane manufacturer and not individual components or materials.

The waterproofing for an intensive roof should be of reinforced bitumen membrane (RBM) or mastic asphalt.

The complete green roof should be installed by a contractor trained and approved by the membrane manufacturer.

Proprietary roofing systems, which do not meet with the principles given in Appendix 7.1-A, should be assessed in accordance with Technical Requirement R3.