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Use of pre-finished steel compared with aluminium for standing seam roofing

28 July 2010 No Comment

Standing seam roof cladding is often specified where low pitch or curved roofing is required. The construction generally offers a more aesthetic, less industrial profile, with no through fixings, which could be vulnerable in low pitch applications.

Low pitch roof cladding 

Standing seam profiles are mainly specified in aluminium, however pre-finished steel can offer a number of significant benefits.

Where would you use pre-finished steel in preference to aluminium?

Pre-finished steel can provide enhanced durability and aesthetic performance in certain environments. Generally mill finish aluminium is tolerant of pH values between 4.0 and 8.5 at room temperatures, forming a film of hydrated aluminium oxide on the surface.

A coastal environment can cause pitting to the aluminium surface with the severity of the pitting being dependant on the aggressiveness of the surrounding environment. Therefore it is often recommended that painted aluminium be specified. The most common paint employed to coat aluminium is PVDF, which typically has a guarantee of 15 years.

Colorcoat HPS200 Ultra® pre-finished steel has a 200 microns coating thickness providing a more effective barrier against the permeation of salt polluted moisture than thinner coatings such as PVDF (27 microns thick). The excellent corrosion resistance and robustness of Colorcoat HPS200 Ultra® is of particular advantage where a potentially aggressive atmosphere can readily attack steel substrate exposed at cut edges or due to physical damage.

Where a coloured roof is required Colorcoat HPS200 Ultra® provides a Confidex® guarantee of up to 30 years on roof cladding in coastal environments and 40 years in inland environments

How do the material physical properties affect the roof performance and construction considerations

Pre-finished steel generally has a higher yield strength than aluminium, with increased resistance to point loading and damage from pedestrian activities on the roof and also offers better resistance to deflection under load. To compensate for the reduced mechanical properties, a thicker aluminium gauge must be used.

The coefficient of thermal expansion for aluminium is approximately twice that of steel. Careful design is required to ensure that aluminium cladding can expand and contract freely with changes in ambient temperatures, without affecting the weather-tightness of the building.

The thermal conductivity of aluminium is approximately 5 times that of steel. When designing building details particular care must be taken to minimise cold bridging through the building envelope, which will cause increased heat loss and potential condensation risk.

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