Metal self-supporting roofing is becoming ever more popular, and not just in the commercial and industrial sectors. Peter Mayer of Building Performance Group considers the options and costs
The benefits of lightweight, profiled metal roof claddings include the ability to span long distances and to incorporate unusual roof shapes such as curves. This is available at a reasonable cost compared with other materials if the total cost of the whole roof assembly is considered. Profiled metal cladding can be single skin or double skin, with profiled metal liner, insulation and outer skin assembled on site or with composite panels factory made with a rigid foam core.
Material and manufacturing requirements for self–supporting roof coverings are given in the European standard BS EN 508 Parts 1, 2 and 3, which are for steel, aluminium and stainless steel respectively. Stainless steel tends to be a “special”, as it is rarely specified. BS EN 506 applies to copper or zinc self–supporting roof coverings. These come in the form of shingles or tiles rather than sheets; sheet copper and zinc tend to be laid as fully supported roof coverings.
Typically, self–supporting roof coverings are based on aluminium or steel profiles. The key durability issues include:
“Through fixing” relies on penetration of the metal cladding with a fastener; weatherproofing is then provided by a plastic cap, sealing washer and sealant. Through fixing can be a successful method where appropriate materials are specified. However, the weathering materials have lives of 20 to 30 years and there are increased risks of corrosion
and leaking. Standing seam or concealed fixing methods minimise these risks by avoiding penetration through the weathering surface of the profiled metal sheets.
The type of coatings will influence the repainting and replacement
cycles. The common options are listed in the costs table below. Additionally, the following factors result in longer intervals between repainting:
- The use of light colours rather than dark
- Roof pitches of greater than 10°
- Aspects other than south-facing
- Rural or urban environments rather than coastal or industrial environments
- Regular inspection, cleaning and maintenance.
Aluminium profiled sheets
Coatings on aluminium roof coverings are largely decorative as roofing grades of aluminium alloy are intrinsically durable. When aluminium is exposed to the atmosphere a protective layer of aluminium oxide forms and protects the underlying surface. Where corrosion of the surface occurs it mostly takes place in the first few months with little change over periods of 20–80 years. Surface corrosion may take place in aggressive environments where there are high concentrations of chlorides or sulphates. Additional protection may be required in such environments.
Steel profiled sheets
The durability of the steel base material is largely a function of the decorative coating and anti-corrosion coating. If repainting is not carried out, the decorative coating will deteriorate and expose the anti-corrosion coating. Once the coating fails, the life of the steel base material will be short.
Two anti-corrosion options are generally available: hot-dip zinc or an aluminium/zinc alloy coating. Their resistance to corrosion is a response to the concentration of sulphates in the environment, the thickness of the coating and its adherence to the base metal. Aluminium/zinc alloys are said to have good corrosion resistance.
Profiled sheets invariably have to be cut to fit, but one of the most common failures with steel profiled sheet roofing is the corrosion of cut ends. Suitable protective treatment applied to cut edges should prevent premature failure.
Whole life performance and cost issues
Guidance for design and installation can be obtained from BS 5427-1, the Metal Cladding and Roofing Manufacturers Association and the National Federation of Roofing Contractors, as well as manufacturer’s instructions.
- Ensure adequate provision for movement and the provision of a vapour control layer, breather membrane and ventilation to built-up systems
- The side and end laps’ sealants should be the non-setting, gap-filling type, or pre-formed tape. These should be compatible with the metal and its coating, and ultraviolet resistant if exposed to sunlight
- Take measures to avoid bimetallic corrosion – see BS PD 6484
- If regular access is required, provide special walkways to prevent damage to the cladding
- Abnormally long sheets may require police notification and an escort during transport to site. This may incur additional transport costs. Forming sheets on site may be an alternative.
Building Performance Group specialises in whole-life performance using software tools to determine best value options based on lifecycle costs, pay back and cost–benefits analysis.
BLP Construction Durability Database at www.componentlife.com has durability information for building components. There is free access for registered social landlords.
Contact Peter Mayer at email@example.com or 020-7583 9502.