The short answer
A liquid coating makes sense when the roof beneath is dry, firmly fixed and mostly sound, because it renews the weathering surface for much less disruption than a strip-off. It is the wrong choice over wet insulation, failed seams, rotten decking or standing water. Coating a roof in that state hides the fault for a season or two. A survey with moisture checks decides which route applies.
How a roof coating differs from replacing the covering, the checks that decide between them, and the warning signs of a coat applied over hidden damage.
What a coating actually does
A roof coating is a liquid that is rolled or sprayed onto an existing covering and cures to a continuous, unbroken skin. Depending on the product it may be acrylic, polyurethane, bitumen-based or silicone. It does three jobs: it seals small defects, it restores the waterproofing and reflectivity of an ageing surface, and it protects the covering from the weather for a further period.
What it cannot do is repair the structure beneath. A coating is only as strong as the surface it bonds to, so the roof must be clean, free of loose material and firmly attached. It does not add insulation, it will not remove water that is already trapped in the roof, and it will not stop a deck from moving. Treat it as renewal of the outer skin, not as a new roof.
For a large industrial roof, that can still be a sensible saving, because the building keeps operating and no waste is stripped off. The question is always whether the roof underneath has enough life left to justify the coat. If it has, a coating is a fair option and we will say so.
Checks before anyone should coat a roof
Our survey looks at the points where coatings most often fail. Is the existing covering stuck down to the substrate, or does it lift and ripple underfoot? Are the laps and seams open? Is the insulation dry? Moisture surveys, using a probe or a thermal image taken at the right time of day, show wet areas that look perfectly normal from above. A coat laid over damp insulation can trap vapour, which pushes the new skin into blisters within months.
We also check the details that take most of the wear: upstands, rooflight kerbs, gutters, outlets and penetrations. These are often where a coating fails first, so they may need patching with reinforcing fabric before the main coat goes on.
Ponding matters too. Standing water for more than a day or two after rain is a sign that the falls are wrong or the deck has dipped. Some coatings tolerate it, many do not. Our guide to commercial roof condition surveys sets out how a full survey is organised and what the findings should include.
Signs that replacement is the honest answer
Replacement is usually the better route when more than local patches of the insulation are wet, when the deck is corroded or soft, when the seams are failing across the whole roof, or when the roof has been recovered more than once already. Another layer adds weight and traps old problems. Published figures show roughly where a roof sits in its life: Less gives felt as 10 to 20 years and glass-reinforced plastic as 25 to 40, and GreenSpec gives EPDM as 20 to 30. Those are typical ranges, not promises.
It also leans towards replacement if you want better thermal performance. A coating does not insulate, whereas a new build-up can include new insulation and corrected falls. If you are planning solar panels or plant on the roof, put the new covering down first, with the loading in mind.
None of this means a coating is a poor product. It means each roof needs the right answer, and we would rather tell you a coating will not last than apply one and have you call us back next winter.
Comparing the two and checking warranties
When you compare a coating quote with a replacement quote, compare what you are being offered, not just the totals. A coating normally buys a defined period of protection on a sound roof. A replacement resets the clock and lets the build-up be corrected. Ask each contractor to say what condition the roof has to be in for the coating to be worth doing, and what they found when they tested it.
Check what any warranty actually covers, because many only cover the product, not the workmanship, and some are void if the roof had wet insulation. Our notes on the commercial flat roof warranty explain what to look for in the small print. Ask whether VAT and any hired access equipment sit inside the price.
For the membrane route, our guide to single-ply roofing membranes sets out the main replacement systems. After the survey you receive a single written total for the option that fits your roof, with photos of what turned up. The rest of our work is listed on the commercial roofing page, and we can start with a look at the roof you already have.
More on this in Commercial Roofing in Crawley & Gatwick
Two coating queries
Can a roof be coated more than once?
Sometimes, provided each coat bonds well and the roof beneath stays dry, though every layer adds weight and makes it harder to trace the next fault in the roof. At some point the surface shows blistering, cracking or poor adhesion that cannot be fixed with another coat. A survey at that stage will usually point to replacement.
Does a coating make a leaking roof watertight?
Only if the leak comes from a small surface defect that the coating can seal. Many leaks on commercial roofs start at a detail such as an outlet, a rooflight kerb or a lap, and follow a path under the covering, so the entry point can be some distance from the stain. Finding the entry point comes before any coating.