PMMA, polyurethane or GRP: choosing a liquid-applied roof
Updated 23 October 2026 · SEO Dons Editorial
PMMA, polyurethane and GRP are all cold-applied, reinforced liquid roofing chemistries, and they differ mainly in how they cure, how they smell, how they handle details and what the certificate says about your roof. This guide compares them for commercial roofs using the Liquid Roofing and Waterproofing Association (LRWA) and the manufacturers' own published data. The manufacturer figures below describe single named products, so they show the kind of difference to look for, not a ranking. The short answer for the liquid-applied roofing page sits in its FAQ, and this is the deeper comparison. The system page covers PMMA flat roof and other liquid systems in full.
What are the main liquid roofing chemistries?
The LRWA says there are now 10 liquid chemistries available in the UK, all with different qualities. Its generic descriptions of the three in question are:
- PMMA: two-component, solvent-free products that cure by an extremely rapid in-situ polymerisation activated by a peroxide catalyst.
- Polyurethane: single or two component, curing by reaction or moisture, with single-component products usually containing solvents and two-component systems able to be formulated solvent free.
- GRP: a glass fibre reinforced polyester resin cold applied on site, using resins specifically formulated for roofing.
The fourth family often asked about is cold-applied bitumen. The LRWA describes polymer-modified bitumen emulsions and solutions as supplied water based or solvent based. Its product register also lists categories such as hybrid polymer resin and polyurethane modified methacrylate (PUMA), so check what a hybrid name actually contains in the certificate and data sheet.
How do cure time and weather limits compare?
The LRWA says PMMA cures extremely rapidly and that the cure proceeds rapidly even at temperatures around 0°C. One PMMA manufacturer, Triflex, states that its systems are rainproof in 30 minutes, trafficable in 45 minutes and can be overcoated after just 1 hour, and can be applied down to 0°C (-5°C for its ProDetail product). That speed matters when weather windows are short, and it is why PMMA features in the worked example of a PMMA overlay on an office plant deck.
Polyurethane products cure more slowly, and the data differ by product. Sika publishes for its single-component Decothane Ultra an application range of +2°C minimum, rain resistance in about 1 hour and full cure of about 12 to 24 hours at +10°C. Kemper System states for its two-component Kemperol 2K-PUR, at +23°C and 50% relative humidity, a 30-minute workability time, rainproof after 2 hours, walkable after 16 hours and fully cured after 3 days.
For GRP the LRWA describes a glass fibre mat saturated with catalysed polyester resin, so the minimum temperature and working time come from the resin's own data sheet. Ask for those figures for the product and season, and for what the contractor will do if the weather turns before cure.
Which suits occupied buildings, given odour?
All three are cold applied, and the LRWA says many liquid systems are now seen as a safe alternative by insurance companies, main contractors and specifiers (the point developed in the hot works guide). Odour is a separate question. Sika says independent sensory testing by Olfasense confirms reduced odour levels for Decothane Ultra, and Kemper describes Kemperol 2K-PUR as solvent-free and odourless and ideal for sensitive areas such as nurseries, schools, hospitals and retirement homes. Those are claims about named polyurethane products. The sources cited here give no equivalent verified odour figure for PMMA or GRP products, so ask for the safety data sheet and any emissions or odour testing for the specific product, and agree ventilation and air-intake protection with the building manager. The LRWA also publishes an information note on isocyanates (January 2022), worth asking the supplier about if polyurethane is proposed.
How do they handle plant-heavy roofs and details?
The LRWA's default is reinforcement, typically polyester fleece or glass fibre mat, and its detail drawings cover upstands, gutters, movement joints and pipe penetrations. Each family has its detailing route. The LRWA says pre-formed GRP trims can give a fast, reliable and neat solution to finishing details. Sika states that Decothane Ultra is applied in one coat reinforced with its Reemat Premium fleece. Triflex says its systems cure at 0°C for main areas and down to -5°C at details. Whatever the chemistry, a plant deck is judged on how penetrations, kerbs and plinths are dressed, and the LRWA publishes a guidance note on electronic integrity testing of liquid applied membranes.
What about overlays and foot traffic?
For overlays the question is compatibility. The LRWA says most member manufacturers have products that can be laid onto bituminous surfaces, and for single-ply membranes the process would involve an adhesion test to ascertain whether a primer is needed. For asphalt, Sika's instructions require assessment for moisture and air entrapment, and the mastic asphalt guide covers that case. For traffic, the certificate and data sheet state the access category the system was assessed for. Plant maintenance routes and walkways should be specified deliberately, as the GRP roof problems guide and the GRP, felt and single-ply comparison discuss.
How do the options compare at a glance?
| Question | PMMA | Polyurethane | GRP (polyester) |
|---|---|---|---|
| How it cures | Peroxide-catalysed, extremely rapid (LRWA) | Reaction or moisture cure, one or two component (LRWA) | Catalysed polyester with glass fibre mat (LRWA) |
| Temperature and cure data | LRWA: rapid around 0°C; Triflex: rainproof 30 minutes | Sika: +2°C minimum, rain resistant about 1 hour; Kemper: rainproof 2 hours | From the resin's data sheet |
| Odour | Ask for safety data and testing | Sika: verified low odour; Kemper: odourless | Ask for safety data and testing |
| Details | Manufacturer detailing products | Fleece-reinforced detailing | Pre-formed GRP trims (LRWA) |
| On the certificate | Substrate, traffic category, thickness, fire classification, working life | The same checks | The same checks |
What certification and guarantees should you expect?
Choose a system, not a chemistry. The LRWA recommends specifying only systems holding current third-party accreditation such as BBA, BDA or ETA certificates, and says its manufacturer members must supply systems with a minimum 10-year guarantee. Start from the site's overview of liquid-applied roofing, then read the certificate for your substrate, as the coatings versus membranes guide and the guarantees page explain. The LRWA suggests consulting its Product Selector or a manufacturer member to find the best-suited liquid material.
If your roof is a candidate for a liquid overlay, the commercial flat roof refurbishment page explains why the core sample comes first. You can send roof details through the form on that page.
Sources
- Liquid Roofing and Waterproofing Association, Generic types, Warm roof systems, Importance of reinforcement, FAQs, About, Technical downloads and Product register, checked 22 October 2026
- Triflex, Cold liquid applied resins (facts about Triflex products only), checked 22 October 2026
- Sika, Decothane Ultra (facts about that product only), checked 22 October 2026
- Kemper System, Kemperol 2K-PUR (facts about that product only), checked 22 October 2026
Common questions
Is PMMA better than polyurethane for a commercial roof?
Neither is better in general. PMMA cures extremely rapidly, which suits short weather windows, while polyurethane products offer published low-odour options for sensitive buildings. The certificate for your substrate, the detailing and the programme decide.
Can liquid roofing be applied in winter?
Within each product's limits. The LRWA says PMMA cure proceeds rapidly even at temperatures around 0°C, while Sika's Decothane Ultra states a minimum application temperature of +2°C. Check the data sheet limits for the product and the dew point, and ask what happens if conditions change before cure.
Will a liquid roof smell in an occupied building?
It depends on the product. Some polyurethane makers publish low-odour verification or describe their products as odourless, but the sources cited give no equivalent figure for PMMA or GRP. Ask for the safety data sheet and any odour testing, and agree air-intake protection in advance.
How do I check which system is certified for my existing roof?
Read the BBA, BDA or ETA certificate for the exact system, because the BBA says certificates should be read in full to confirm the parameters of use. The LRWA's online Liquid Roofing Finder uses third-party accreditation to show the substrates systems are approved for.