Advice

A guide to recoating

Cover or Repaint?

Most metal roofing and cladding sheets are manufactured from steel, zinc alloy coated, and finished with a factory applied protective coating. It is this coating and not the sheet that usually needs replacing. Some of the many, often unseen, disadvantages of simply over sheeting an existing metal roof are discussed below: 

Physical disadvantages

Added weight – The first consideration must be whether or not the building structure will bear the weight of another layer of recommended gauge steel sheets and supports. Calculations must factor in snow loading.

More holes – ‘Through fix’ installations that utilise the existing purlins must necessarily involve drilling further holes in the existing sheet, risking moisture penetration and air leakage leading to condensation.

Condensation – Where the old roof is retained as a liner, a new vapour protection layer must be added if it cannot be sealed. The VCL should be located above what will become the internal sheet but is difficult to seal when punctured with fixings for the support brackets for the new skin, risking ‘cold bridging’.

Other materials’ performance – A vast array of support and ancillary fixings, sealants and thermal performance materials must match the design life guarantee of the chosen over-sheet system.

Site disruption – A strictly controlled programme involving large and heavy deliveries, storage, movement of men and materials and handling must be carefully planned and safely managed.

Environmental disadvantages

Future maintenance – Like the original sheet, the over-sheet will itself need recoating as it reaches its normal expected repaint schedule. The same factors affecting performance will prevail on the factory coating of an over-sheet option. Cut edge corrosion will almost inevitably occur during its lifetime.

Sustainability – Steel sheet production requires large resources of finite raw materials and energy. Having a steel roof already in place requires only recoating to protect it indefinitely.

Life cycle costs – Over-sheeting means double roof sheet disposal at building end of life.

Summary

Planned repainting will extend profiled metal sheet life indefinitely, maintaining the strength and value of the building without disturbance. Over-sheeting by comparison is heavy, increases the risk of leaks & condensation and leaves the same coating failure dilemma further down the line. In essence, the roof protection is now provided by a sheet with characteristics and weaknesses possibly worse then that being over clad, with an unnecessary layer of steel effectively doubling sustainable planned maintenance and life cycle costs.