The short answer
Yes, on the right roof. A liquid coating is a jointless layer brushed or sprayed over a clean, sound roof, and it can seal minor leaks and slow weathering of the surface beneath. It cannot repair a roof that has failed. Rotten fixings, rusted-through sheets and wet insulation need fixing or replacing first, otherwise the coating just covers the problem.
A coating can add useful years to a roof that is basically sound. Used on a failing one, it delays the real decision and can make the eventual repair harder.
What a liquid coating is
A liquid roof coating is a waterproof compound applied wet, which cures into a continuous skin. It is rolled, brushed or sprayed over the existing surface, so there are no sheet joints left to leak. Products vary by chemistry, and each has its own requirements for the surface beneath.
Which roofs can take a coating?
Coatings are used over many surfaces, including profiled metal, bitumen felt and some single-ply membranes, with a primer chosen to suit the surface. The manufacturer's data sheet sets the rules for that product, and we follow it. Where a roof is a patchwork of different surfaces, each patch may need its own preparation, and the cost of that preparation is often greater than the cost of the coat.
The key requirement is that the roof is dry, clean and sound. A coating bonds to whatever is there. If what is there is flaking, wet or loose, the coating will lift with it, however good the product is, and the owner is left with a blistered finish and a roof that still leaks. Cleaning is therefore the first stage of the job, with moss, grease, loose paint and standing water dealt with before any primer opens.
Where coatings earn their keep
On a metal roof with surface rust but sound sheets, a coating can stop the rust spreading and seal laps and fixing heads in a single pass. On an old felt or membrane roof with dry insulation beneath, it can seal small splits and blisters without a strip. For an owner who cannot close the building, avoiding a full strip is a real benefit, because the work is done from the outside and the roof is never opened up.
How long will a coating last?
That depends on the product, the preparation and the roof underneath, so we do not put a number on it here. The most reliable guide is the manufacturer's data for the product being used, read alongside the condition of your roof. A thin coat on a poorly prepared surface will not last as long as a thicker coat on a well prepared one.
Our page on coating a rusty metal roof looks at the metal-sheet case in more detail, including what to do about the rust that has already taken hold.
Where a coating only hides the failure
Coatings go wrong when they are asked to do the job of a repair. A sheet with rust through it has lost its thickness, and a skin of coating will not restore it. Fixings that have backed out need re-fixing before they are covered. Wet insulation under a membrane stays wet and keeps rotting the deck, even if the surface looks neat from the yard. A coated roof that still drips inside the building is the clearest sign that something underneath was missed, and by then the coating has to come off before the real repair can start.
Can a coating go over a leak nobody has located?
It can, and that is the trap. If water is still getting in through a hidden path, the leak carries on under the coating and the damage grows out of sight. A moisture survey, or at least a thorough look from inside the unit, comes before any coating decision. Corroded cut edges also need treating, since the coating can bridge them but not stop them; see cut-edge corrosion.
Fibre-cement sheets of unknown age raise the asbestos question before anything else. Read about fibre-cement roof sheets before anyone prepares or coats one.
How we decide between coating and replacing
The first visit covers the roof surface, the underside of the sheets and the floor below any stain, and we photograph what we find and test the doubtful areas. The question is simple: is the roof underneath doing its job apart from the surface? If the answer is yes, a coating is a sensible choice and the report says that. If not, it says that instead, and explains why replacement or overcladding is the better buy.
What does the choice depend on?
The condition of the deck or sheets, the state of the insulation, the number of leaks, the age of the existing covering, the access, and how long the building can be disturbed. Those factors, not the product brochure, decide the price.
You see the photos and receive one written price for whichever route fits. If a full re-roof is likely at some point, insulated roof panels are one alternative worth comparing, and the overview sits at commercial roofing. To get the survey moving, send photos of the roof surface and of any leak stains inside the building.
Coating questions
Can a coating be applied while the unit is trading?
Usually yes, because the work is on the outside and does not open the roof. Coatings need dry weather to cure, and some give off a smell during application, so we agree timings and access with you in advance and plan around the days your site is busiest.
What happens when a coated roof wears out?
It can be recoated if the earlier coating is still bonded and the roof beneath is sound. If the roof has failed underneath, the coating is removed or covered by a new system. Keeping the survey photos from the first coat helps judge the second one.
The full picture: Commercial roofing in Taunton ยท More short answers