Roof coating or liquid-applied membrane: which does the roof need?
Updated 23 October 2026 · SEO Dons Editorial
A proposal that says "coat the roof" can mean two quite different things. Sometimes it is a thin protective or reflective finish over a roof that still keeps water out. Sometimes it is a reinforced waterproofing system that takes over the waterproofing job from a failing covering. This guide explains how to tell which one you are being offered, what certification to ask for and where each can and cannot help. Solar reflective claims are covered in the cool roof guide, and the choice between resin types in the PMMA, polyurethane and GRP comparison.
What is the difference between a roof coating and a liquid-applied membrane?
The trade uses the words loosely, so the label tells you little. The US Environmental Protection Agency lists "liquid-applied roof coatings with light or cool colors" and "fluid-applied membranes (used for waterproofing and protection of bitumen or metal roofs)" as separate low-slope product types. The Liquid Roofing and Waterproofing Association (LRWA), by contrast, describes its design and specification competence framework as covering liquid-applied membrane systems, including roofing, terraces, podiums, metal roof coatings, balconies and walkways.
For this page, use these working definitions, which are not taken from a standard. A coating is a thin liquid finish applied to protect or reflect and relies on the roof beneath still being watertight. A membrane is a reinforced, certificated system that is the waterproofing. The practical test is what the document says: is it a system assessed for your substrate, or a product sold for general protection?
What does reinforcement change?
The LRWA says specifying a liquid-applied membrane with reinforcement must be the default solution. It says reinforcement layers typically use a polyester fleece or glass fibre mat, and that it has observed manufacturers introduce liquid systems to the UK market that lack a reinforcement layer. The exceptions it gives are mostly extremely fast-curing systems approved solely for concrete, typically in new build, while membranes on profiled metal or asbestos cement sheets may need partial reinforcement for areas prone to movement. The LRWA's list of generic types also notes that unreinforced polyurethane is very tough in its own right and that reinforcement may be added selectively, so the rule is a default, not an absolute.
Bitumen emulsions and solutions show where the line blurs. The LRWA says these systems may also require primers, reinforcements and solar reflective or protective finishes. Whether a bitumen product is a coating or a membrane therefore depends on what the full system includes.
What certification should you ask for?
The LRWA says most liquid membranes are covered by European Assessment Document (EAD) 030350-00-0402, previously ETAG 005, for liquid applied roof waterproofing kits. It adds that systems not covered by the EAD are still usable if they hold a valid Agrément certificate for the intended use, and recommends specifying only systems with current third-party accreditation such as BBA, BDA or ETA certificates. Kiwa, a certification body, summarises the EAD as classifying kits by useful life, with Category W2 for ten years and W3 for 25 years, alongside climate, payload, slope and temperature categories. A category describes the assessed use, not a promise.
A manufacturer's page should give the certificate number, which you can then check against the certificate itself. For example, Sika lists BBA Certificate No. 14/5158 and ETA-14/0331 for its Decothane Ultra. The BBA says it is important to read a certificate in full to confirm the parameters of use. The guide to BBA certificates shows how, and the LRWA's online Liquid Roofing Finder uses third-party accreditation to show which substrates a system is approved for.
What can each do on felt, asphalt and metal roofs?
| Existing roof | What the sources say about preparation | Where to read more |
|---|---|---|
| Bituminous felt | LRWA: most member manufacturers have products that can be laid onto bituminous surfaces; preparation would involve cutting and re-bedding blisters, power washing and cleaning, and possibly a primer | Felt roof repair |
| Single-ply | LRWA: an adhesion test to ascertain whether, and what type of, primer is required | EPDM repair |
| Mastic asphalt | One manufacturer requires cleaning, sealing major cracks and an assessment for moisture and air entrapment | Mastic asphalt guide |
| Profiled metal or fibre cement | LRWA: many membranes can overlay profiled metal or fibre cement sheets; it publishes separate guidance notes for each | Cut edge corrosion, asbestos cement roofs |
Neither route fixes wet insulation or falls that do not drain. The LRWA publishes a guidance note on drying existing roof substrates before liquid systems are installed, and names BS 6229 as the main British Standard for flat roofs. See why flat roofs pond for the falls position. Where the sheets are asbestos cement, which the LRWA names as a substrate, identify the material before any preparation (see fibre cement or asbestos roof sheets).
How do the two options compare on the factors that matter?
| Factor | Surface coating | Reinforced liquid membrane |
|---|---|---|
| Substrate condition | Needs a sound, clean, bonded surface; ask for an adhesion trial | Same preparation, plus reinforcement that helps with movement, and a certificate naming the substrate |
| Wet insulation | Cannot fix it; moisture must be assessed first | Cannot fix it; the build-up must be dry or replaced first |
| Expected life | Ask for the manufacturer's stated figure and its evidence | Ask which working-life category the ETA assigns, and what the certificate says |
| Guarantee | Ask who issues it, the term, what it covers and its conditions | LRWA says its manufacturer members supply systems with a minimum 10-year guarantee; ask the same questions |
A guarantee is a finite contractual promise by a manufacturer or insurer and is separate from the roof's service life. The guarantees page sets out what to ask for. If the core sample shows the build-up is sound and dry, the commercial flat roof refurbishment page explains how coating, overlay and strip-and-recover routes compare. If it is wet, a liquid system of either kind is not the answer. The liquid-applied roofing page covers systems that are assessed as membranes.
What should you ask before accepting a coating proposal?
- Does the manufacturer describe it as a coating or as a reinforced membrane system?
- Which BBA, BDA or ETA certificate applies, and does it name my existing covering?
- What do the cores and moisture survey show, and who has seen them?
- What adhesion test is proposed, and what happens if it fails?
- Who issues the guarantee, for how long, covering materials and workmanship, and on what maintenance conditions?
- How will upstands, outlets and penetrations be treated, since details are where roofs fail first? See flat roof details that fail.
You can send roof details through the form on the refurbishment page to compare routes for your own roof.
Sources
- Liquid Roofing and Waterproofing Association, Generic types, Importance of reinforcement, FAQs, About, Overlay systems, Technical downloads and competence framework news, checked 22 October 2026
- US Environmental Protection Agency, Using cool roofs to reduce heat islands (last updated 18 June 2026), checked 22 October 2026
- Kiwa, Test of liquid applied roof waterproofing kits according to EAD 030350-00-0402, checked 22 October 2026
- BBA, Why choose a BBA certified construction product, checked 22 October 2026
- Sika, Decothane Ultra (facts about that product only), checked 22 October 2026
Common questions
How many years does a roof coating last?
There is no single figure. Ask the manufacturer for its stated term and the evidence behind it, and for a membrane check the working-life category on the ETA or the BBA certificate. A guarantee term is a separate, contractual thing and is not the same as service life.
Can you coat over a roof that is already leaking?
Only if the build-up beneath is dry and the covering is bonded, which a core sample and moisture survey establish. Sealing over wet insulation does not dry it, and the LRWA publishes guidance on drying existing substrates before liquid systems go on.
Does the chemistry (polyurethane, acrylic, bitumen) tell you which is better?
Not on its own. The LRWA lists many generic types, and what matters is the certificate for your substrate, whether the system is reinforced and how the details are treated. The PMMA, polyurethane and GRP comparison covers the resin choice for membranes.
Is a coating the same as a cool roof?
No. A reflective finish is one property a coating or membrane can have, and it does not make a failing roof watertight. The cool roof guide covers what reflectance does and does not change.