Roofing materials vary hugely – different products are suitable for different applications. Richard Teale of NBS explains how they should be specified
1. Watertightness
Most roofing materials are waterproof but they must be assembled correctly to form a watertight construction. Roofing membranes – single-ply and bitumen sheets, asphalt and liquid coatings – should be applied with sealed laps or as layers that provide continuity and watertightness, even on flat or low-pitch substrates. Discontinuous roofing units – slates, tiles and metal sheets – should be laid at steeper pitches and with overlaps and/or interlocks to achieve watertightness.

2. Weathertightness
Weathertightness is also an essential construction consideration – a measure of the resistance of the roof to water, snow, wind and dust. Bear in mind it does not follow that watertight constructions are also weathertight. For example, a watertight slate or tile roof may be intentionally open to wind action to ventilate a cold roof construction. Where ventilation is required, specify proprietary products wherever possible, and ensure that purpose-made ventilators have adequate cloaking or baffles. Remember, snow is blown upwards.

3. Integrity
This term is much used for security and fitness for roofing. Build-up and attachment of roof constructions will be fully described in prescriptive specifications, or assigned in part or whole to the contractor in performance specifications. Base all integrity calculations on actual site conditions – degree of exposure, wind loadings to BS 6399-2, permanent and temporary loadings, roof pitch, atmospheric pollution, thermal movement and so on.

Contact product manufacturers and trade associations for comprehensive guidance and, in many cases, project-based fixing information.

4. Thermal insulation
Improving thermal regulations requires not only greater thicknesses of insulation but also periodic re-evaluation of the actual physics of roof construction. Last year's updating of Building Regulations Part L and Scottish Technical Standards Part J aimed to advance the conservation of fuel and power through carefully integrated strategies, setting out whole building and elemental objectives. Those applicable to specification of thermal insulation for roofing are:

  • Reduced U-values promoting the use of thicker and lower conductivity insulation products. Thicker insulation, where mechanically fixed, will require longer and stronger fasteners.

  • Reduced thermal bridging at penetrations and interfaces achieved by improved continuity of insulation.

    The ODPM 2001 paper: Limiting thermal bridging and air leakage: Robust construction details for dwellings and similar buildings is a helpful guidance document promoting good practice in domestic construction.

    5. Airtightness
    Part L also requires reduction of "unwanted air leakage", reinforcing an established practice in roofing construction of creating a continuous air and vapour barrier on the warm side of the thermal insulation. The regulations introduce air-leakage testing for all new buildings, but accept that smaller buildings (including dwellings) of less than 1000 m2 can comply, as long as it is certified that "appropriate design details and building techniques have been used".

    The specifier should consider airtightness in terms of the whole building and just the roofing element; include whole-building testing requirements in the general conditions section of a specification (as NBS section A33).

    To ensure airtightness, consider roofing product, performance and execution requirements in the relevant work sections of a specification.

    6. Vapour control
    If water vapour condenses within a roof construction, serious damage to the structure, fixings and insulation could result. Water vapour entry into all roof constructions should be restricted by specifying continuity of all internal linings or ceilings, usually with provision of additional vapour control layers over areas of high humidity.

    Where a degree of water vapour transmission is tolerated, such as in traditional pitched cold-roof construction for dwellings, it must be removed by ventilation via proprietary products or purpose-made openings. The recently revised BS 5250 Code of practice for control of condensation in buildings gives guidance relevant to most types of roof construction.

    7. Fire protection
    Building Regulations Approved Document Part B and Scottish Technical Standards Part D require roof coverings to provide adequate protection against the spread of fire over their external surface. The testing standard is BS 476-3 Fire tests on building materials and structures. Specifiers should be aware that the substrate and any applied surfacing will affect test results.

    The 2002 amendments to Part B do not propose replacement of testing standard BS 476-3 with a harmonised European standard.

    • Postscript

    NBS - Publisher of National Building Specifications. www.nbsservices.co.uk

    Roofing materials