Austenitic stainless steel to BS 1449: Part 2 or BS 970: Part 1, grade 316, or BS EN 10088-2, grade 1.4401. Terne-coated
Capital cost 拢57/尘2
Net present value for 60-year life 拢57/尘2
Service life 60 years
Austenitic stainless steel to BS 1449: Part 2 or BS 970: Part 1, grade 304, or BS EN 10088-2, grade 1.4301. Terne-coated, minimum 15 micron thickness
Capital cost 拢52/尘2
Net present value for 60-year life 拢52/尘2
Service life 60 years
Austenitic stainless steel to BS 1449: Part 2 or BS 970: Part 1, grade 316, or BS EN 10088-2, grade 1.4401
Capital cost 拢56/尘2
Net present value for 60-year life 拢59/尘2
Service life 50 years
Austenitic stainless steel to BS 1449: Part 2 or BS 970: Part 1, grade 304 or BS EN 10088-2, grade 1.4301. Terne-coated, less than 15 micron thickness (Not suitable for use in exposed marine or aggressive industrial atmospheres)
Capital cost 拢54/尘2
Net present value for 60-year life 拢62/m2
Service life 35 years
Lead sheet
Lead sheet Code 5, 6 or 7 to BS 1178
Capital cost 拢59/尘2
Net present value for 60-year life 拢59/尘2
Service life 60 years
Zinc sheet
Zinc sheet to BS EN 988, minimum thickness of 0.6 mm with roll-joints or standing-seams
Capital cost 拢45/尘2
Net present value for 60-year life 拢52/尘2
Service life 35 years
Copper sheet
Annealed copper sheet to BS EN 1172, minimum thickness 0.5 mm
Capital cost 拢51/尘2
Net present value for 60-year life 拢51/尘2
Service life 60 years
Factors included in whole-life cost over 60 years
Initial supply and installation; removal and replacement at end of service life. No maintenance is required and no costs have been included. Costs are discounted to net present values at a discount rate of 6%.
Factors affecting durability
Choice of material
When correctly detailed and specified, fully supported metal roofing can last a considerable time. There are many examples of lead and copper roofs in excess of 100 years old. 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.
Surface patination or coating
On exposure to the atmosphere, lead, zinc and copper develop a protective surface known as a patina. This can be prevented from forming or be damaged by rainwater containing corrosive agents or acids from organic growth. The formation of patina on copper can take up to 15 years, depending on location and environment. Artificial patination can be achieved. Terne coating (lead and/or tin mixture) to stainless steel is available, but it needs to be applied at thicknesses of 15 microns or more to be effective as protection.
Decking and underlay
Underlay selection depends on the substrate used. The underlay may be required to serve one or more of the following purposes:
- Provide an even surface for the covering
- Protect against underside corrosion
- Make sure that the metal covering to move independently of the deck.
Plain-edge timber decking is preferable to plywood, as it allows the roof to breath.
Exposure
Deterioration is greatest in marine and polluted environments. Copper and zinc are vulnerable in marine environments. Refer to manufacturers鈥� tables and guarantees for terne coating lives.
Maintenance
Fully supported metal roofs are largely maintenance-free. Washing stainless steel is advisable to avoid unsightly corrosive deposits.
Detailing
Fixings must be at the correct centres, so that the material can expand and contract and to avoid stress fractures. Metal fixings must be compatible with cladding to prevent bimetallic corrosion.
Condensation control
Ventilation to the underside of the metal covering is essential to avoid corrosion. If using a cold roof, good ventilation is necessary between the metal covering and the insulation.
Durability tips
- Check that the sheet is thick enough to allow maintenance access, if necessary.
- Make lead as thick as economically possible. Use 0.8 mm thick zinc.
- Provide ventilation of the space below the metal sheeting, even in a warm roof.
Modes of failure
Corrosion
Zinc is more susceptible to corrosion than other metals, particularly in polluted or industrial environments. Trapped moisture on the underside of lead roofs can cause corrosion, as patination is prevented. All non-ferrous roofing materials except copper are vulnerable to corrosion. This is because copper does not rely solely on patina to prevent corrosion. Zinc and copper coverings can suffer surface pitting.
Incompatibility with other materials
Without a barrier, contaminants can leach up from timber decks. Run-off from certain timber preservatives can be a problem with zinc.
Water ingress
Correct detailing of joints, laps, abutments and rolls is essential to stop water ingress. Moisture entering through joints and trapped moisture are particular problems with fully supported metal roofs. The use of a warm roof construction can help overcome this.
Movement
Creep, cracking and splitting caused by thermal movement can be a problem, as can failure of laps, rolls and joints if not properly detailed.
Mechanical damage
Adequate fixing and lap detailing are essential to prevent wind uplift.
Profiled metal cladding
Further to the previous Materials life costings (20 August), we would like to clarify that a 15-20 micron terne coating is available that gives a durable coating to 304 grade stainless steel. Terne coatings that are 1-3 micron thick are decorative only and should be used on 316 grade stainless steel for maximum durability. BS 6582 describes only hot-dip terne coating on mild steel with a minimum thickness of 6 microns. Terne coating can describe tin/lead or tin/zinc alloys, or tin coatings deposited by both hot-dip or electrolytic processes.
Further information
The Housing Association Property Mutual鈥檚 Component Life Manual, written by Construction Audit, the technical audit arm of 好色先生TV Performance Group, provides insured lifespan assessments for more than 500 building components. It is updated twice a year 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 拢175, or on CD-ROM, on 01264-332 424.
The BPG 好色先生TV Fabric Component Life Manual and BPG 好色先生TV Service Component Life Manual will be available from E&FN Spon in loose-leaf and in CD-ROM formats later this year. BS Handbook HB 10141: 1997: 好色先生TVs 鈥� Service Life Planning: Part 1: General Principles is available from the British Standards Institution on 0181-996 9000. For further information, contact Gary Moss at BPG on 0171-240 8070.
