Austenitic stainless steel to BS 1449: Part 2; or BS 970: Part 1, grade 316; or BS EN 10088-2, rade 1.4401. Terne-coated to BS 6582
Capital cost £23/m2
Net present value for 60-year life £23/m2
Service life 60 years
Austenitic stainless steel to BS 1449: Part 2; or BS 970: Part 1, grade 316; or BS EN 10088-2, rade 1.4401
Capital cost £18/m2
Net present value for 60-year life £18/m2
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 to BS 6582. Not suitable for use in exposed marine or aggressive industrial atmospheres
Capital cost £22/m2
Net present value for 60-year life £24/m2
Service life 35 years
Ferritic stainless steel to BS 1449: Part 2; or BS 970: Part 1, grade 430 or 434; or BS EN 10088-2, grade 1.4000 or 1.4223. Factory-applied organic coating, 25-50 microns nominal thickness. For use with manufacturer's approval in mild atmospheres only*
Capital cost £16/m2
Net present value for 60-year life £22/m2
Service life 50 years
Mild steel
Hot-dip galvanised steel to BS EN 10142; or BS EN 10147, minimum 275 g/m2 zinc coating weight. PVC/plastisol coating, 200 microns nominal thickness*
Capital cost £13/m2
Net present value for 60-year life £18/m2
Service life 35 years
Hot-dip aluminium-zinc coated steel to BS EN 10215, minimum 150 g/m2 zinc coating weight. Acrylic/polyester/PVF2/PVDF coating, 25-50 microns nominal thickness*
Capital cost £15/m2
Net present value for 60-year life £20/m2
Service life 30 years
Hot-dip galvanised steel to BS EN 10142 or BS EN 10147, minimum 275 g/m2 zinc coating weight. Acrylic/polyester/PVF2/PVDF coating, 25-50 microns nominal thickness*
Capital cost £15/m2
Net present value for 60-year life £20/m2
Service life 30 years
Aluminium
Aluminium to BS 1474 (extrusions); or BS EN 485 (fabrications/sheet). Aluminium-zinc alloy anodic coating to both faces. British Board of Agrément or other third-party certified
Capital cost £16/m2
Net present value for 60-year life £17/m2
Service life 40 years
Aluminium to BS 1474 (extrusions); or BS EN 485 (fabrications/sheet). Acrylic/polyester/PVF2/PVDF coating, 25-50 microns nominal thickness*
Capital cost £15/m2
Net present value for 60-year life £20/m2
Service life 40 years
Aluminium to BS 1474 (extrusions); or BS EN 485 (fabrications/sheet). Mill-finished
Capital cost £14/m2
Net present value for 60-year life £16/m2
Service life 30 years
Factors included in whole-life cost over 60 years
Re-coating of applied finishes at year 20 and every 10 years thereafter (British Steel has a guarantee system); recoat only where marked with *. Edge sealants may require attention, but this will entail dismantling the roof, and therefore has not been included in the whole-life cost. Costs are discounted to net present values discount rate of 6%.
Factors affecting durability
Cladding material
Stainless steel and aluminium are more corrosion-resistant than mild steel, and so are less dependent on factory-applied coatings. In marine or aggressive industrial environments, grade 1.4401 austenitic stainless steel to BS EN 10088-2 should be specified. Mild steel should have a minimum 275 g/m2 galvanising or 150 g/m2 aluminium-zinc coating.
Condensation control
Interstitial condensation can be a major problem with metal claddings and can result in rapid corrosion and loss of thermal insulation performance (a consequence of the insulation getting wet). Ensure that adequate provisions have been made to prevent this, such as the use of vapour control layers or ventilated cavities.
Fixings
The correct specification and detailing of fixings are essential to the long-term performance of metal claddings. Ensure adequate provision has been made for thermal movement, or fixing holes can become elongated and fixings pulled out. Metal fixings must be compatible with the cladding material to prevent bimetallic corrosion; use austenitic stainless steel fixings with aluminium, but isolate the fixings in marine or heavily polluted areas. Protect plated-steel fixing heads from corrosion with push-on plastic caps or specify integral plastic heads.
Exposure
Deterioration of cladding materials and surface coatings is greatest in marine and polluted environments. The lives of polymeric coatings can vary greatly, depending on the location and orientation of the building, and the coating colour. Refer to the manufacturers' tables and guarantee schemes for typical coating lives.
Maintenance
Insitu recoating of metal claddings is specialised and costly, but some proprietary systems carry a guarantee of 10 years and the prolonged cladding life will often justify the cost. Coating lives are affected by the frequency of cleaning to remove dirt build-up. BS 5427 recommends washing every three months in polluted or marine environments, and every six months in other areas. Where present, side- and end-lap seals may also need attention during the life of the cladding.
Surface coatings
These serve both as decoration and protection against corrosion. PVC and plastisol coatings have the greatest build thickness and offer the best protection, but darker colours absorb more ultraviolet radiation and are likely to deteriorate more quickly than lighter ones. Anodised aluminium and Terne-coated stainless steel are available, but these finishes are primarily for decoration.
Modes of failure
Corrosion of base metal
Mild steel claddings are most susceptible to corrosion, but aluminium and stainless steel can also corrode rapidly if inadequately specified or protected. The cut ends and fixing holes of mild steel cladding have little or no surface protection and often corrode rapidly. Site-applied treatments can help, but these are difficult to apply. A better solution is to avoid cut ends in the design.
Breakdown of surface coatings
Over time, surface coatings will deteriorate under the influence of ultraviolet radiation, thermal and moisture cycles, atmospheric pollutants and wind-blown abrasive particles. Chalking, fading and crazing of finishes are largely aesthetic defects. However, the cracking, blistering or peeling of finishes will probably be unacceptable, and can lead to rapid corrosion of the base metal. The life expectancy of the coating will be influenced by a number of factors, such as its specification and colour (darker colours absorb more ultraviolet radiation and deteriorate faster), the orientation and exposure of the building, and the importance placed on its aesthetic appearance. Manufacturers' guarantees typically range from 15 to 25 years.
Delamination
Some older factory-made composite panels, in which metal sheet is bonded to a rigid insulant core, suffered delamination as a result of the high surface temperatures reached in the outer metal layer. However, recent experience shows that this problem has, on the whole, been addressed by manufacturers.
Durability tips
- Where regular maintenance access to the roof is required, provide walkways to prevent impact and abrasive damage to the cladding.
- Use barriers to protect the cladding from vehicular or human impact at lower levels.
- Specify lighter coloured surface finishes to prolong the coating life.
- Avoid exposed cut ends by specifying full-length cladding sheets. If this is not possible, specify site-applied treatment of cut ends to protect against corrosion.
Further information
The Housing Association Property Mutual's Component Life Manual, written by Construction Audit, the technical audit arm of Building Performance Group, provides insured life-span assessments for more than 500 building components. The Component Life Manual 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 £175, or on CD-ROM.
The BPG Building Fabric Component Life Manual and BPG Building Service Component Life Manual will be available from E&FN Spon, in loose-leaf and CD-ROM formats, later this year.
BS Handbook HB 10141: 1997: Buildings and 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.
