Liquid roofing is a waterproofing system that arrives as a liquid, is applied to the roof with a roller or brush, and cures in place into a single continuous waterproof layer with no seams or joints anywhere in it. That is the whole idea in one sentence.
Everything else worth knowing follows from two facts: it is formed on the roof rather than rolled out in sheets, and it goes on cold. This guide explains what that means in practice, and how to tell a certified reinforced system apart from the cheap coating being sold under the same two words.
How is it different from a normal flat roof?
A conventional flat roof is built from sheets, and a liquid roof is formed as one piece. That difference explains almost everything about how the two behave.
Felt, EPDM and single ply all arrive as material in rolls. They are laid out, lapped over each other, and joined by torching, bonding or welding. Around a rainwater outlet, a pipe penetration or the upstand where the roof meets a wall, the sheet has to be cut, folded and dressed, then sealed.
Those cut-and-sealed details are where flat roofs leak. Not usually in the middle of the roof, where the material is continuous and undisturbed, but at the edges and around the things poking through it.
A liquid system does not have that problem, because there is nothing to cut. The same liquid that covers the open field of the roof is taken up the upstand, into the outlet and around the pipe, and it cures as one piece. Reinforcement fabric is embedded into it at those details so they end up thicker and stronger than the flat area, which is the opposite of how a sheet roof is built.
What does cold applied mean?
Cold applied means the system cures chemically at ambient temperature, with no heat, flame or torch involved at any point.
This sounds like a technical detail and it is actually the most practically important thing about it. Torch-on felt requires an open gas flame on your roof. That means a hot works permit, a fire watch, a cooling-down period after the crew leaves, and a genuine fire risk to the building underneath. Insurers have views about it, and on an occupied commercial building or a block of flats those views translate into paperwork and restrictions.
With a cold-applied system none of that applies. There is no flame, so there is no permit and no fire watch. The building below carries on operating. For a warehouse that cannot stop picking, a shop that cannot stop trading or a block where the residents are not going anywhere, this is usually the deciding factor rather than a nice-to-have.
It also means the roof is never opened up. On a strip-and-replace job there is a period, sometimes several days, where your deck is exposed and the weather is a real risk. An overlay never creates that window.
What is the difference between a certified system and a cheap coating?
This is the part the industry does not explain clearly, and it is the reason published prices for liquid roofing run from roughly £40 to £180 per square metre as indicative UK market rates for 2026.
That spread is not a market being irrational. It is two genuinely different products sold under the same name.
At the bottom of the range sits a single-component acrylic coating. It comes in a tub, it goes on in one or two coats, it is essentially a thick elastomeric paint, and it has a short service life. It will make a roof look better and it will stop a minor weep for a while. It is not a waterproofing system in any meaningful sense.
At the top sits a certified, reinforced, multi-coat system: a specified build-up, applied to a specified thickness, with reinforcement fabric embedded at details and often across the whole roof, installed by an applicator the manufacturer has trained and approved, and carrying a European Technical Assessment.
Three questions separate them, and you can ask all three on the phone:
- What is the product called? A certified system has a name and a manufacturer. “A membrane refresh” or “a roof restoration treatment” is not a product name.
- What is its certification number? Ours is ETA-23/0735. That is a number your surveyor, your building control officer or your insurer can look up. A system without one has nothing to look up.
- Is the contractor an approved applicator for it? Manufacturers train and certify applicators, and only approved applicators can have the manufacturer product guarantee issued on a roof. If the answer is vague, the guarantee will be too.
There is a fourth signal worth knowing: trade guidance flags prices under £18 per square metre on a roof more than ten years old as a warning sign. At that rate, on a roof that old, there is not enough money in the job to do the preparation properly.
What is an ETA, and why does it matter?
A European Technical Assessment is an independent, published assessment of how a construction product actually performs, carried out by a designated Technical Assessment Body rather than by the company selling it.
The product is tested against a common European assessment document covering watertightness, resistance to wind uplift, behaviour in fire, durability and so on, and the results are published under a reference number. That is the entire point: it converts a marketing claim into something verifiable by a third party.
For a building owner the practical value is straightforward. If a surveyor asks what is on your roof, “a liquid waterproofing system certified to ETA-23/0735” is an answer that can be checked. “A high-performance roof coating” is not.
It is worth being precise about who does what here, because the two roles get blurred and they should not be. HYDRONYLON LTD manufactures the system and holds the certification for the product. K & M Roofing Solutions is a certified applicator, trained and approved by them to install it. We do not manufacture it and do not claim to. That distinction is also what allows the separate 10-year product guarantee to be issued on your roof at all, because a manufacturer will only stand behind an installation carried out by someone they approved.
Which roofs can it go on?
A liquid system will bond to most existing flat roof surfaces, provided each one passes its own specific check.
- Felt and bituminous roofs are the most common overlay in London. The check is adhesion: blistering and lifted laps can be cut and made good, but a covering that has lost adhesion across the whole roof has to come off.
- Mastic asphalt overlays well once cleaned and primed. The check is crazing and thermal cracking at upstands and outlets.
- Concrete decks are a good substrate, common on London blocks. The check is trapped moisture in the slab.
- GRP and fibreglass are straightforward once abraded and primed. The check is delamination and star cracking around fixings.
- Single ply membranes overlay well and avoid stripping a large commercial roof. The check is primer compatibility and the condition of the seams.
- Profiled and standing seam metal is approved for use on metal substrates and is the standard route for an industrial roof leaking at the laps. The check is corrosion and fixings, both treated first.
The service page on liquid applied roof waterproofing goes through each of these in more detail, including the honest list of situations where liquid roofing is the wrong answer entirely.
Is it the right answer for your roof?
Liquid roofing is the right answer when you have a structurally sound roof with a failed waterproofing layer, and it is the wrong answer when the structure itself has gone.
If the deck has rotted, the insulation underneath is saturated, or the roof ponds because the falls were never right, no waterproofing product fixes that. Those roofs need replacing, and a contractor who offers to seal them anyway is selling you the cheaper job rather than the correct one.
For everything else, and that is most leaking flat roofs in South and West London, an overlay is faster, less disruptive, considerably cheaper and produces a roof with no seams in it. That is worth understanding before you accept a quote to strip a roof that did not need stripping.
If you want the numbers rather than the principles, our flat roof service page sets out how we survey, prepare and apply the system, and what the guarantee covers.