Insights — 14 October 2026
How a fabric-reinforced liquid roof membrane works
A liquid-applied roof membrane goes on by roller, squeegee or spray rather than in rolls. It follows every drain, upstand and penetration and cures into one continuous membrane. A fabric-reinforced membrane adds a layer of polyester fabric inside it, with the fabric laps bedded in resin.
This article explains what each layer of such a membrane does, using the ResinTech RMS system we install as the example. Every figure in it comes from the RMS system data sheet.
The layers
Four layers, each with one job
| Layer | Material | Wet film | Its job |
|---|---|---|---|
| Base coat | ResinTech FPU, a flexible polyurethane | 25–28 mils | The first half of the waterproofing; the fabric is laid into it |
| Reinforcement | Polyester fabric, one layer | — | Reinforces the membrane from inside |
| Second base coat | ResinTech FPU | 25–28 mils | Encapsulates the fabric and completes the waterproofing |
| Top membrane | ResinTech PPM polyurea, or HPP polyaspartic | 18–20 mils (PPM) or 10–15 mils (HPP) | Covers the base coats and protects them from sunlight |
A primer is optional. On sound, dense, dry concrete it is not needed; it is used on porous, marginal or moisture-risk substrates, wherever the test patch shows it is needed, or where the project specification calls for one. A colour finish over PPM is optional too, for a set colour or gloss.
The base coats
The waterproofing is two coats of FPU
FPU is a single-component, moisture-curing polyurethane: there is no mixing of two parts on the roof. It is flexible and crack-bridging, with 800% elongation and flexibility down to −35 °C in its data sheet tests. That flexibility is the point of it: the base coats are crack-bridging and stay flexible in the cold. Expansion and active movement joints still get their own detail.
It goes on in two coats of 25–28 mils wet each, never more than 28 mils in one coat. The drains, upstands, wall junctions, penetrations, corners and seams are done first, with FPU and fabric, before the field coats go down.
The fabric
Why the fabric sits inside the membrane
The fabric is laid into the first coat while it is still fresh and embedded until it is fully wetted out, with no wrinkles, folds or trapped air. The second coat then goes over it, either straight away, wet-on-wet as the fabric is laid, or once the first coat can be recoated, typically 8–12 hours later. Either way the fabric ends up fully encapsulated, inside the membrane rather than on top of it.
On a concrete deck the fabric covers the whole surface. On an existing roof it is recommended over the whole surface, and it is never left out at seams, laps, flashings, drains and transitions: the places where the roof changes direction or material. Fabric joints overlap at least 3 inches (75 mm) at the sides and 6 inches (150 mm) at the ends, with resin between the laps.
The top membrane
Why the base must always be covered
FPU stays tacky for days, and it is not designed to face sunlight or traffic. So it always gets a top membrane, applied once the FPU can be recoated, typically 8–12 hours after its last coat, and within 72 hours. A base left longer, or one that has picked up dirt, is cleaned and test-patched before it is top-coated.
| PPM — standard | HPP — fast | |
|---|---|---|
| Chemistry | UV-stable polyurea, single component | Polyaspartic, two-component kit |
| Wet film | One coat, 18–20 mils, never more than 20 | 10–15 mils |
| Colour | Medium grey | Tinted on site to any colour |
| Elongation | 648% | 20%: a hard topcoat, not a crack-bridging membrane |
| Drying and cure | Tack-free 2–3 hours, through-dry about 9 hours, full cure 24–48 hours (20 °C, 50% RH) | Tack-free about 1 hour, light foot traffic 3–4 hours (a cure stage, not a traffic rating), full cure 5–7 days (23 °C, 50% RH) |
Thickness
The wet film is the specification
A liquid membrane is only as good as the thickness it is applied at, and that is measured, not estimated. Every coat is checked with a wet-film gauge as it goes on. The data sheet sets maximums as well as minimums: no more than 28 mils of FPU in one coat, and never more than 20 mils of PPM over the FPU.
Before any optional colour finish, the whole membrane comes to about 68–76 mils wet with PPM on top, or 60–71 mils with HPP. After the second base coat the membrane is inspected for pinholes, voids, exposed fabric and thin spots, which are repaired before the top goes on.
Conditions
What it needs to go on
Dry, stable weather, with the roof surface at least 3 °C above the dew point and never damp, frozen or contaminated. FPU needs a surface between 5 and 35 °C during application and cure; HPP needs a surface of 10–30 °C, air of 10–32 °C and humidity no higher than 85%. A new concrete deck needs a minimum 28-day cure. An existing roof has to be sound, dry, clean and firmly bonded, and it gets an adhesion test patch before the full application.
The membrane is then kept closed to traffic and protected from rain and standing water until it cures, and the batch numbers, film builds and weather are recorded for the handover.
Limits
What it is not
It is not a traffic surface: roof terraces and walkway routes with regular foot traffic take the ResinTech PTS+ pedestrian system, and rooftop parking takes VTS-D+. It needs positive drainage, and service under ponding or standing water is not covered by the RMS data sheet. It does not bridge expansion or active movement joints on its own; they get their own detail. PPM is not a vapour barrier, and the system is not for pools, tanks, potable water or submerged service unless ResinTech approves it for that use. New wood or plywood roof decks are not covered by the RMS data sheet and are assessed first.
We install this system as our flat roof coating and restoration service. Whether your roof is a candidate is the subject of our article on restoring or replacing a commercial flat roof.
Questions we get asked
How a fabric-reinforced liquid roof membrane works: FAQs
What makes a liquid roof membrane fabric-reinforced?
A layer of polyester fabric is laid into the first base coat while it is fresh, embedded until it is fully wetted out, and encapsulated by the second base coat. The fabric ends up inside the membrane rather than on top of it.
Why does the membrane need a top coat?
The FPU base coats stay tacky for days and are not designed to face sunlight or traffic. A UV-stable top membrane, PPM or HPP, is applied once the FPU can be recoated, typically 8 to 12 hours after its last coat, and within 72 hours.
How thick is a fabric-reinforced liquid roof membrane?
In the ResinTech RMS build, two base coats of 25 to 28 mils wet each, plus 18 to 20 mils of PPM or 10 to 15 mils of HPP on top: about 68 to 76 mils wet in all with PPM, or 60 to 71 mils with HPP. Every coat is checked with a wet-film gauge.
Can a liquid membrane go over an existing roof?
Over existing flat roofing that is sound, dry, clean, firmly bonded and drains, after the defects are repaired. The roof type is confirmed with ResinTech and an adhesion test patch is applied and checked first, because whether the membrane bonds to a particular roof is tested, not assumed.
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