Coating a metal roof is one of the few maintenance products that genuinely does what it says, on the right roof, and one of the most oversold on the wrong one. The distinction is measurable rather than a matter of opinion: it depends on whether the sheet is still sound in the field, and a survey answers that in an afternoon.
How commercial metal roofing and steel roof cladding corrode
A coated steel sheet is protected by its factory finish, and that finish is cut through at the ends of every sheet when it is made to length. Water sits at those cut edges, particularly where sheets overlap at end laps, and corrosion starts there and works inward. Fasteners are the second front, because their washers perish and the hole around them then holds water. The middle of a panel is usually the last thing to go, which is why a roof can look terrible from the gutter and be sound across its slopes.
What a steel roof coating system does
The cut edges and laps are cleaned back to sound metal, treated and encapsulated, and the fasteners are replaced or sealed. Then a coating is applied across the sheet to restore a continuous finish. Done on a sound roof by an approved applicator it carries a guarantee and buys years, sometimes a decade or more. Done over a roof with perforations, or over a surface that was not properly prepared, it hides the problem for a season and peels.
Metal roof refurbishment cost: what contractors publish
Duratite gives its complete GacoFlex coating system on industrial roofs from £40 per square metre fully installed, surveyed and warranted. Anglia Decor gives overcladding at £15 to £60 per square metre, depending on the new material and the condition of the existing roof. Nationwide Industrial Roofing gives £90 to £180 per square metre to strip and replace a commercial roof. That gap is why a survey finding the sheet sound is worth paying for first.
When refurbishment is not the answer, including on a metal flat roof
If the sheet has perforated in the field, if the insulation below is saturated, or if the building's thermal performance is the reason for the work, coating does not address any of it and an overlay or a replacement does. On a metal flat roof, ponding is the additional question, because a coating over standing water fails at the pond. Ask for a moisture survey and a fastener pull test before agreeing to a coating on anything but a young roof.
Questions people ask about metal roof coating
How long does a metal roof coating last?
System guarantees are commonly in the ten to twenty year range, conditional on an approved applicator and on the roof being suitable at the time. The condition is the important half of that sentence.
Can a metal roof be coated in winter?
Most systems need a dry surface and a minimum temperature, so the working season is limited. An applicator willing to spray in any conditions is one to avoid.
Is an overlay better than a coating?
It is a bigger intervention that also lets you add insulation, so it costs more and does more. On a sound sheet where thermal performance is not the issue, a coating is the cheaper and proportionate answer.