Few ideas in commercial roofing are as tempting as a coating. A roof that leaks in several places, a budget that does not stretch to stripping and replacing, a business that cannot stop trading: a liquid coating applied over the existing surface seems to answer all of it. Sometimes it does. A commercial roof coating applied to a sound but tired roof is a sensible, cost-effective repair. Applied to a roof with a wet deck or soaked insulation, it is a cosmetic layer over a problem. The difference lies in what we find before we open a tin.
What a liquid coating actually is
A roof coating is a liquid that is rolled, brushed or sprayed over the existing roof surface and cures into a continuous protective film. Several chemistries are in use. Acrylic coatings are water-based, easy to apply and reflective, and suit lighter duty. Polyurethane coatings are tougher and more flexible, good for roofs that see foot traffic. Silicone coatings shed water well. Bituminous coatings are traditional and cheap, though often shorter-lived. Reinforced systems add a polyester fleece or mat, which turns the coating into a true membrane.
A coating is not the same as a full liquid-applied roofing system. A coating refreshes and seals a surface that is still doing most of the work. A full system builds a new waterproof layer with reinforcement. Our comparison of commercial flat roof systems places liquid systems alongside single-ply and felt, and this page looks closely at coatings over an existing covering.
Where coatings work well
Coatings suit roofs whose structure and insulation are sound but whose surface has weathered. On the right roof they are quick, they cause little disruption and, because the old covering stays put, there is no stripped waste to take away.
Profiled metal sheets
Steel and aluminium sheeting is probably the most common candidate. Old steel roofs weather, the factory finish wears thin and rust spreads from cut edges and fixings. If the corrosion is surface-level, the sheets can be cleaned back, treated with a rust converter or primer, the laps and fixings sealed, and the whole roof coated. This stops water getting in at the laps and gives the sheets a new protective skin. Cut edge corrosion, where rust starts at the sheared edge of a sheet, needs careful treatment first, and our guide to cut edge corrosion on metal roofs explains the process. Where the metal has rusted through, coating cannot replace missing steel, and those sheets need renewing.
Built-up felt and bitumen roofs
An old felt roof with a sound structure underneath but a crazed or blistered surface can often take a reinforced coating. The surface is cleaned, loose material removed, blisters cut and re-bonded, and a primer applied before the coating goes on. It extends the life of the roof and seals the fine cracks that let water in. If the felt is saturated or the insulation under it is wet, the coating will trap the moisture, so we test before proceeding.
Single-ply and other membranes
Coatings can refresh a tired single-ply roof or seal a failing seam, subject to compatibility. Not every coating bonds to every membrane, and some old materials need a specific primer. Applying the wrong product leads to delamination, where the coating peels away in sheets within a year or two. We check the existing membrane type and the manufacturer's compatibility guidance before choosing.
Awkward details and small areas
Around rooflights, plant bases, pipe penetrations and parapet junctions, a liquid is easier to work than a rigid or sheet material, because it flows into the shape. Even where the main roof is being re-covered with another system, liquids are often used for the details.
Where coatings do not work
The honest answer is that there are several roofs where we would advise against a coating. The commonest is a roof with wet insulation. Water trapped in a flat roof build-up will keep spreading, and a coating over the top seals it in, so the roof deteriorates unseen. Test cores or moisture surveys reveal this before coating starts.
A failed deck is another. If the boards or metal deck are rotten, rusted or sagging, the roof needs repair underneath. A coating also cannot cure a roof that has lost its falls and ponds heavily, since standing water attacks most coatings over time. The roof may need tapered insulation or local re-levelling first.
Roofs with widespread splits, major movement or poor underlying details are poor candidates too. If every upstand and flashing has failed, a coating over the top only covers the symptoms.
Asbestos cement sheets are a separate matter and not something to treat as a routine coating job. Any work on them, including surface treatment, raises questions about disturbing the material, so read our guide to asbestos cement roofs on commercial buildings before anyone proposes it.
How a coating job runs on site
On a coating job, the order of work is fixed and each step has to be finished before the next begins. First comes cleaning: pressure washing removes dirt, algae and chalky residue, and any moss is treated. Then repairs: loose or lifted laps are re-fixed, rusted fixings replaced, blisters cut out, splits patched and holes closed. Only after that does the primer go on, chosen to suit the substrate, because a primer that works on steel will not necessarily key onto old bitumen.
Details are done before the main field. Seams, laps, upstands, rooflight surrounds and outlets are sealed or reinforced with a fleece set in wet coating, and left to cure. The main coat follows, applied at the thickness the manufacturer specifies, often in two passes with the second laid at right angles to the first so thin spots are covered. We check wet film thickness as we go, rather than guess from the quantity used.
Weather decides the programme. Rain on a wet coating can wash it out, and cold or damp air stops it curing, so a job is stopped and restarted if the forecast turns. For a trading business that is manageable: the work is usually done in sections, the roof is made watertight at the end of each day, and nobody inside has to move out. The surface is not walked on until it has cured, after which a walkway route can be marked out for maintenance staff.
The checks we make before recommending a coating
We start with the free survey, photographing your roof. Then we look at the things that decide whether a coating will last.
- The substrate: its type, its age and whether it is firmly bonded to what is beneath it.
- The moisture content of the insulation, found by core samples or a moisture survey where the roof justifies it.
- The falls and the outlets, to see whether the roof drains.
- The condition of every upstand, flashing and penetration.
- Foot traffic, plant and any maintenance routes that will need extra wear protection.
Surface preparation is where the lasting result is won or lost. The roof has to be cleaned, often by pressure washing, with moss and algae treated, loose material removed and repairs made before any primer goes on. Coatings also need dry weather and temperatures within the manufacturer's range to cure properly, which limits when the work can be done. Cut corners at this stage and the coating fails early, however good the product.
Cost and lifespan expectations
Coating costs depend on the substrate, the amount of preparation, the system and how easy the roof is to reach. We do not publish a price list, and the only commercial benchmark we hold is a low-confidence informal figure for a full strip and replace, and it is a typical UK range for comparison only. A coating over a sound roof will cost a fraction of replacement, which is its appeal, but it only holds that value if it lasts. Your fixed quote follows the survey.
Lifespan varies with product, thickness, preparation and exposure. A well-applied reinforced coating on a sound roof can postpone replacement for a long time, while a thin coat over poor preparation may fail within a few seasons. The manufacturer's guarantee is only as good as the application, and we are happy to explain what any guarantee covers before you commit. Our own workmanship guarantee is written down and runs for ten years.
Looking after a coated roof
A coating is not a one-off. Like any waterproof layer, it wears under sun, frost, foot traffic and debris, and it lasts longest when somebody looks at it regularly. A visit after the autumn leaf fall and another in spring is enough for most roofs: clear the outlets and gutters, remove debris from the surface, check the laps and details, and touch in any damage before it spreads. Spotting a scuffed patch early means a small local repair, where leaving it means water reaching the substrate beneath.
Keep a record. Photographs of the roof when the coating went on, the product used and the date make it far easier to plan the next recoat, and they matter if the building is sold or the lease changes hands. When plant engineers or other trades go on the roof, make sure they know the route to use and that nothing sharp or hot is dragged across the surface.
Coating, repair, overlay or replace
A coating is one of four options, and the others deserve fair consideration. Local repair suits a roof with a few identified faults. A coating suits one with a sound structure and a worn surface. An overlay, in which a new membrane goes over the old, suits a sound roof where a new waterproof layer is wanted. Replacement suits one with failed insulation, a failed deck or general decay. Our guide on repairing, overlaying or replacing a commercial roof compares the four, and the planned approach in our guide to commercial roof maintenance keeps a coated roof in good order.
We would rather tell you a coating will do than sell you a new roof you do not need, and we would rather tell you it will not do than watch you pay for a layer that peels. The survey photographs show you your roof, the report explains our recommendation, and the fixed written quote follows. Site cleared and waste taken away as standard. For the wider picture, see commercial roofing in Basingstoke, or ring 01256 213956 and we will arrange the survey.
