Question answered
Do liquid flat roof coatings work?
The short answer
Yes, liquid flat roof coatings work when they go on a sound, dry roof of the right shape. They are brushed or rolled on wet and cure into a continuous membrane, which suits small roofs and awkward details. They cannot cure a rotten deck, wet insulation or standing water, and a poor application leaves thin patches that fail early.
A liquid-applied coating can work well on the right roof and badly on the wrong one. The deciding factors are the condition of what lies beneath, the shape of the roof and who applies it.
How a liquid system goes on
A liquid roof, sometimes sold as liquid rubber, arrives as a resin that is poured, brushed or rolled across a prepared surface. It cures in place into a single skin with no laps and no welded joints. Most systems have a primer, one or more coats of the waterproofing layer and, in many, a reinforcing fleece pressed into the wet coat around corners and joints.
The attraction is in the detail. A membrane that arrives in rolls has to be cut, folded and glued around every pipe, rooflight upstand and awkward corner. A liquid goes wherever a brush goes. That is why it is often chosen for small roofs, porches, bay tops and roofs full of penetrations.
Products differ in chemistry, thickness and what they will stick to, so there is no single answer. The right system is the one whose own data sheet matches your existing covering, and it has to be applied exactly to that sheet, coat by coat, with the drying times it asks for between them.
Where liquid coatings suit
The strongest case is a roof that is basically sound but tired. If the old covering is well stuck down, the deck is dry and the insulation has not soaked up water, a coating can renew the waterproofing without stripping the roof. That saves waste and disruption, and it avoids the labour of removing the old layers.
Complicated shapes are the second case. Dormer cheeks, a bay with several corners, a flat roof around a chimney or a roof with a run of vents all give a membrane in sheets plenty of places to go wrong. A liquid joins itself at every junction, which is a real advantage when there are dozens of them.
Small repairs are the third. A liquid patch on a localised failure can be worked into an existing covering, provided the cause has been found first. A failed corner patched without understanding why it failed will fail again, whatever goes on top of it, so the diagnosis comes before the product.
Where the limits are
A coating is only a skin. It cannot hold a sagging deck level, dry out a wet roof or stop water pooling where the fall is poor. IKO, citing the 2018 edition of BS 6229 (since revised), recommends a finished minimum fall of 1:80 on a flat roof, designed at 1:40; a coating laid over a roof that holds puddles will sit under water for days after rain.
Condition decides the rest. If an old felt roof is blistered and soaked through, our page on blisters on a felt roof explains why coating over the top simply seals the damp in. Rot in the boards, loose edges and failed flashings all have to be mended first.
Thickness is the quiet weakness. A system applied thinly looks identical to one applied properly on the day, and the difference shows only years later. Ask for the coverage the manufacturer specifies and for photographs taken as each coat goes on, so you can see the work rather than take it on trust.
Weather, preparation and cure
Liquids are fussy about the day. The surface has to be clean, dry and clear of frost, and rain landing on a wet coat can ruin it. Cold slows the cure and may stop it altogether, which is why these jobs are best planned around settled weather. Our page on replacing a flat roof in winter covers what the cold does to resins and adhesives alike.
Preparation is most of the labour. Moss and dirt are cleaned off, loose material is cut back, joints are reinforced and the primer is chosen to suit the old surface. Skip this stage and the coating can peel from its base, however good the product itself.
Foot traffic needs thought as well. Some finishes take occasional access for maintenance, but a roof used as a terrace needs a surface built for it, and that is a separate design decision made before a coating is chosen.
Choosing between a liquid, a sheet and a new roof
There is no league table of materials. Liquids, single-ply rubber, fibreglass and felt each suit certain roofs, and the flat roof materials page compares them side by side. A coating is the better choice when the roof is sound and the details are tricky. A new covering is the better choice when the structure underneath is failing.
Be wary of a coating offered without anyone looking inside the roof. A proper survey checks the deck from below, tests for damp where it is suspected and looks at the outlets and falls. Photographs from your own roof guide the advice, and we recommend only what the cause calls for.
Guarantee wording needs reading with care. A manufacturer warranty on a product is not a promise about how long your roof will last, and it depends on the applicator following the specification. Our flat roofing page gives the wider picture.
Liquid roofs: further points
Can liquid rubber go straight over old felt?
Sometimes. The felt must be dry, firmly stuck down and free of blisters and splits, and the primer has to suit the bitumen. Loose or wet felt is cut out and made good first. The survey tells you which case you have.
Is a coating a permanent fix for a leaking flat roof?
Not by itself. If the leak comes from a failed upstand, a blocked outlet or a rotten board, the coating hides the symptom until the cause defeats it again. The cause is found and repaired, and the coating then renews the waterproofing.
Tell us about your roof
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