Metal roof coverings are many and varied – and so are the accompanying costs. In the first of Specifier’s Lifetime costs series, the Building Performance Group offers a guide to lifespan, whole-life costings and the durability of metal sheet roofing
Factors affecting durability
1: Choice of material
When correctly detailed and specified, fully supported metal roofing can be expected to last a considerable time; there are numerous examples of lead and copper roofs well in excess of 100 years old. Stainless steel is a more modern roofing material; results from accelerated ageing tests and extrapolations from performance on existing buildings suggest a likely service life of at least 100 years. Lead and copper have a significantly greater corrosion resistance than zinc, which can corrode rapidly in industrial, polluted or marine environments. Generally, the thicker the material, the longer it will last. But remember, it is important that thicknesses are constant throughout the sheet to avoid local stresses.
2: Surface patination or coating
On exposure to the atmosphere lead, zinc and copper develop a natural protective surface known as patina. The formation of patina can be retarded and existing patina damaged by acid rain or acids from organic growth. The formation of the familiar green patina on copper may be delayed as long as 15 years depending on location and environment. However, artificial patination can be achieved on site.
Stainless steel of appropriate grade does not require a protective surface coating. However, terne coating (a lead and tin alloy; also available as a lead-free coating) is commonly applied to provide a traditional finish similar to lead. Unless applied at thicknesses of 15 microns or more, terne coatings will be little more than decorative.
3: Decking and underlay
Selection of underlay will depend on the type of substrate used. The underlay may be required to serve one or more of the following purposes: to provide an even surface for the covering; to protect against underside corrosion; or to enable the metal covering to move independently from the deck.
Plain-edge timber decking is preferable to plywood, since it allows the roof to breathe. For shallow roof pitches, less than 10°, ensure that plain-edged boarding is not warped to avoid localised ponding.
If the decking under lead has a moisture content greater than 16%, consider protective pre-treatment to the underside of lead sheeting.
4: Exposure and environment
For a given exposure category, the roof covering should be designed to withstand the expected wind loads. Pitch of roof; density of fixings and pull-out value; width and thickness (dead weight) of sheeting are the factors that influence the integrity of the covering. There may be a case to enhance these requirements to take account of perceived future increased wind loads resulting from climate change.
Deterioration of fully supported metal roofing is greatest in marine and polluted environments. Marine environments may exist up to 15 miles inland from the sea or estuaries, depending on the wind conditions.
Copper and zinc can corrode rapidly in marine environments – refer to manufacturers’ tables.
Fully supported metal roofs are largely maintenance-free. Periodic washing down of stainless steel roofs is advisable to help to prevent the build-up of unsightly corrosive deposits.
Correct detailing of metal roofing is essential. Key detailing issues include adequate provision for movement and also the correct specification and detailing of fixings. It is essential to provide fixings at the correct centres so that material can expand and contract freely to accommodate the stresses induced by thermal and wind movement. Correct bay lengths and widths in relation to the sheet thickness should be specified to avoid metal fatigue. Metal fixings and adjacent metal features must be compatible with the cladding material to prevent bimetallic corrosion – for example, run-off from aluminium to lead must be avoided.
Overworking or excessive hammering of copper may cause cracks. Stainless steel joints that are soldered should be strengthened with rivets if there is a risk of stresses developing. Where leadwork is bossed to form corners or edges stretching, thinning or splitting of the lead should be avoided. Zinc with an artificial patina or coated with a lacquer should be handled with care so as not to damage the protective surface.
8: Condensation control
Ventilation to the underside of metal covering is essential to avoid corrosion. Condensation may cause gradual corrosion of the underside of lead, especially where aggressive chemicals are involved. Generally warm roofs are preferred to cold roofs. If using a cold roof, good ventilation is necessary between the metal covering-underlay-boarding structure and the insulation; this roof construction is described as a “ventilated warm roof”.
9: Durability tips
- Where regular maintenance access to the roof is required, check that the sheet is of a thickness to compensate for increased wear.
- Increase the thickness of lead to the maximum possible.
- Where possible, use zinc of 0.8 mm thickness.
- Copper of 0.6 mm to 0.7 mm thickness is preferred.
- Use stainless steel of 0.5 mm thickness for very exposed situations.
- Use terne-coated stainless steel where theft of lead may be a risk.
- Provide ventilation of the space below the metal sheeting-underlay-boarding structure even in a warm roof situation.
- Reliance on inaccessible short-life sealants should be avoided at junctions or laps.
- There should be annual inspection of roofs, especially where there are complicated junctions.
- Spillage of masonry-cleaning chemicals, such as hydrochloric acid or sodium hypochlorite should be washed off immediately.
Modes of failure
Fully supported metal roofing can suffer problems from creep, cracking and splitting due to thermal movement, failure of laps rolls and joints if not properly detailed.
Zinc is more susceptible to corrosion than other metal roofing materials, particularly in polluted or industrial environments. Trapped moisture on the underside of lead roof coverings can cause significant corrosion as surface patination is prevented from taking place. Underside corrosion can occur with all non-ferrous roofing materials except copper, as it does not rely solely on patination to prevent corrosion. Zinc and copper coverings can also be affected by surface pitting.
Incompatibility with other materials
In some situations, it may be necessary to provide a barrier to prevent contaminants leaching upwards from timber decks. For example, zinc and lead are affected by organic acids released from damp timber, especially oak, Douglas fir and western red cedar. Run-off from certain timber preservatives can also be a problem with zinc.
Correct detailing of joints, laps, abutments and rolls is essential to prevent water ingress. Moisture entering through joints (thermal pumping) and trapped moisture are particular problems with fully supported metal roofs. The use of a warm roof construction can help overcome the “thermal pumping” effect.
Adequate fixing and lap detailing is essential to prevent wind uplift. Roof edges and roof areas in the lee of obstructions such as parapets or rooflights are particularly vulnerable to wind suction.
The Housing Association Property Mutual's Component Life Manual, written by Construction Audit Limited, the technical audit arm of Building Performance Group, provides insured lifespan assessments for more than 500 building components. It is regularly updated to reflect industry feedback and changes to standards and codes of practice. Published by E&FN Spon, it is available in loose-leaf format, price £175, or on CD-ROM, on 01264-33 24 24. Two companion durability manuals are available: the BPG Building Fabric Component Life Manual from E & FN Spon, and the BLP Building Services Component Life Manual from Blackwell Science. BPG are currently lead partners in a PII project researching the longevity of building services plant. For further information, contact Alan Swabey (costing research) or Peter Mayer (technical research) at Building Performance Group on 020-7240 8070 or 020-7204 2021.