Metal roof coating
Can a liquid coating save a metal roof?
The short answer
Sometimes. A liquid coating can extend the life of a metal roof whose sheets are still sound, because it seals the surface, the laps and the fixings. It cannot rebuild steel that has rusted through, and it will not hold on a badly prepared surface. A survey decides whether coating, partial replacement or new sheets is the right answer.
When coating suits an ageing steel roof on a unit or barn, what the preparation involves, and the signs that the sheets need replacing instead.
What a coating does and does not do
A liquid coating is a waterproof layer applied wet across the whole roof and left to cure into a continuous skin. On a metal roof it covers the face of the sheets, bridges the laps and seals over the fixings, so many of the places that let water in are closed in one operation. It can also reflect heat and give the roof a uniform finish.
What it cannot do is put back metal that has been lost. A coating over thin or perforated steel only hides the problem, and the sheet below keeps corroding. It also cannot support weight, so a roof that is unsafe to walk on is not made safe by painting it. The coating protects the sheet it sits on. It does not turn a failing sheet into a healthy one.
Treat a coating as a way to extend a sound roof, not as a patch for one that has given up, and expect a surveyor to tell you which kind you have.
Is the roof worth coating?
The survey answers that, with photographs of the rust, the laps and the fixings. Roofs that suit coating are structurally sound, with sheets that are fixed securely, have only surface rust or light cut-edge corrosion, and leak mainly at the laps and fixings. Roofs that do not suit it have holes, large areas of deep rust, sheets that flex or crack underfoot, or a failed substructure.
Check the sheet type too. If the roof is asbestos cement, the rules and the risks are different, and the asbestos cement industrial roof guide in our commercial section should be read before anyone considers working on it. HSE says AC roofs are always fragile.
A realistic judgement weighs the remaining life against the cost. Where plenty of service life remains and the coating is priced well below replacement, coating is sensible. If the coating would buy only a short reprieve, the money goes further on new sheets. The survey should say plainly which side your roof falls on.
Preparation decides how long it lasts
Most coating failures are preparation failures. The roof has to be cleaned of dirt, algae and loose paint, usually by pressure washing, and left to dry. Rust is removed back to sound metal and treated, and bare steel gets a primer suited to the coating system.
Then comes the structural work. Loose and corroded fixings are replaced, laps are re-sealed or taped, stitching screws go in where laps have opened, and damaged sheets are cut out and renewed. Reinforcing tape or fabric covers the laps and fixings where the system calls for it. Only after all that does the coating go on.
Skipping this stage is why some coated roofs peel within a season or two. If a quote is for coating alone with no mention of cleaning, rust treatment, fixings and laps, ask what is being left out. Our guide to why metal roofs leak at the laps explains why the laps need dealing with first.
Coating, over-roofing or new sheets
Coating is one of three routes. New sheets replace the covering altogether, which is the lasting answer when the steel is badly corroded. Over-roofing fixes a new weatherproof layer over the old one, which keeps the old sheets in place and can avoid disturbing the building, though it adds weight and needs the existing structure to take it.
Each route has a different disruption profile. Coating often goes on while the occupier carries on trading, since the work stays outside and nothing is stripped off. Replacement means a longer job and more exposure while sheets are off. Our guides on over-roofing and replacing steel roof sheets set out the other two routes.
Whichever is chosen, a warranty from the coating maker is a statement about the product on paper, not a promise of how long the roof will last. Ask what conditions it carries and who is allowed to apply the system.
What drives the price and what to ask
Coating is usually priced below replacement, but the figure depends on the roof in front of us. Area, access, the amount of rust and the number of fixings to renew all push it up or down, as does how many sheets need cutting out first. The system chosen matters as well, since thicker coatings with reinforcement cost more than a single thin coat.
Questions worth asking any contractor are simple. What preparation is included? How are the laps and fixings treated? What is the coating, its thickness and its data sheet? What does the warranty require? And how will you know when it has been applied to the right depth?
Our commercial roofing survey starts with photographs of the sheets, the laps and the rust, and ends with a clear recommendation. When coating is the right call, the report recommends it, and when new sheets are the better answer, it recommends those. The price comes afterwards in writing as a single total.
Related questions
Coating questions
Can a coating be applied in winter?
Most systems need a dry, clean surface and set within the temperature range on the data sheet, so winter days can be wasted. Our guide on commercial roof work in winter covers the limits, and the maker's sheet governs.
Does coating stop condensation under a metal roof?
Not by itself. A coating changes the outside surface, while condensation forms on the cold underside. If the building sweats, the cause is moisture and ventilation, which needs its own answer.