The short answer
A commercial roof can be coated instead of replaced when the existing covering is sound, the insulation is dry and the deck is in good order. A liquid coating seals and renews the surface, which is useful on a tired but intact roof. It will not cure trapped water, rotten decks or failed details, so a survey has to confirm the roof is a suitable base before any coating is specified.
What a roof coating actually is
Within our commercial roofing work, a liquid roof coating is a waterproof layer applied over an existing roof and left to cure into a continuous skin. It is sprayed, rolled or brushed on, usually over a primer, and often with a reinforcing fleece pressed into the wet layer at joints, laps and details.
Several chemistries are used, including acrylic, polyurethane, silicone and bitumen-based products. They differ in how they cope with standing water, ultraviolet light, foot traffic and temperature, and the right one for your roof is set by the surface beneath. The product is chosen for the substrate, not the other way round.
The attraction is clear. The old roof stays in place, so there is no strip-out, less waste and less disruption inside the building. On a large industrial or retail roof that can mean a shorter programme and a lower cost than a full replacement, because there is no new covering or insulation to buy and lay.
The catch is equally clear. A coating performs only as well as the surface it grips. It renews a surface and does not replace a failed structure beneath it.
Roofs that coat well
Coatings give the best result on roofs where the problem is the surface rather than the build-up.
- Profiled metal sheet that is weathered but not perforated. Surface corrosion can be treated, primed and coated, which stops it spreading.
- Fibre-cement and similar sheets that are stable and not asbestos-bearing, or are handled by the proper route. Anything that may contain asbestos needs specialist advice first, covered in our note on asbestos on industrial roofs.
- Built-up felt or bitumen that is intact, adhered and dry, with minor cracking and crazing from age.
- Single-ply membranes with sound seams and a surface that has simply weathered.
In each case the existing roof must be clean, firm and dry. A roof like this might otherwise be near the end of its useful life, and a coating can give it a fresh surface and a second period of service. How long depends on the product, the substrate, the workmanship and how the roof is used, and any figure comes from the manufacturer's data for that product, not from a general rule.
Roofs that should not be coated
This is where an honest view matters, because a coating applied to the wrong roof only hides the fault for a season.
Wet insulation. If water has got into the insulation under a membrane, coating over the top traps it. The roof will blister, and the damp keeps spreading. Moisture surveys, including core samples, help find this before work starts.
A tired deck. A coating cannot stiffen a soft or rotten deck, or strengthen steel sheets that have corroded through. If people cannot walk the roof safely, the covering is not the first problem. Our page on fragile roof safety explains the risks.
Failed details. Leaks often start at upstands, outlets, gutter linings and rooflight kerbs. A coating over a split flashing or a blocked outlet will not stop water finding the same path.
Ponding. Standing water on a roof with poor falls shortens the life of many coatings. Some products tolerate it better than others, but the underlying fall problem remains.
Where the roof is past coating, the sensible conversation is about repair or replacement. Our guide to commercial roof lifespan helps frame when that point arrives.
What the survey checks before we recommend a coating
Because the existing roof becomes the foundation for the new surface, the survey looks at the old roof in more depth than a straight replacement quote would.
We check adhesion, by looking at whether the covering is stuck down or lifting. We look for blisters, splits and open laps. The outlets and gutters get a look too, where debris and standing water show where the drainage is working. We check the roof's access and its edges, and we compare what we see on the surface with what the building looks like from inside, for stains at the ceilings and rooflights.
On a single-ply or felt roof we may recommend tests for trapped moisture. On a metal roof we check the fixings, laps and the cut edges of each sheet, where corrosion usually begins.
The outcome is one of three recommendations: coat as it stands, repair first and then coat, or replace. We give our reasoning with photographs of what we found, so that you can judge for yourself and compare with any other quote.
Coating, overlay or replacement
A coating is one of three ways to avoid a full strip. The others are an overlay, where a new membrane or sheet goes over the old, and a patch repair.
An overlay adds a new waterproof layer with its own life, but it adds weight and buries the old roof's problems, including any wet insulation. Our page on commercial roof overlay covers when that is sensible.
Replacement is the cleanest answer when the build-up is failing, because it lets you correct the insulation, the falls and the details at the same time. Replacement is also when energy performance and Building Regulations can come into the picture, so ask us what applies to your building.
The decision often comes down to remaining life and money. A coating costs less now, but if it only delays a replacement by a few years you may have paid twice. The timing of a re-roof is covered in how long a commercial re-roof takes.
Leased buildings and managed property
Coating decisions often involve more than one party. In a let building, the lease decides who pays for repair and who pays for improvement, and a coating may count as either. Our page on roof work on student lets and HMOs touches on the same landlord and tenant questions on a smaller scale.
Where several tenants share a roof, it helps to have a written scope that the freeholder or managing agent can circulate, so that cost and access are agreed before work begins. We provide a written specification after the survey, with a fixed price, and the coating product, the preparation and the details are named so everybody can see what is being bought.
Commercial roofing is priced on survey, since the system, the access and the substrate all change the total. The written price is fixed after we have been up and looked.
