What covering is on my flat roof, and what are the options?
Felt, EPDM rubber, GRP fibreglass, single-ply and mastic asphalt, how to tell which you have, and the warm deck and cold deck distinction that explains most flat roof failures.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 5 min
- Safety
- warning
Short answer
Look at the surface: overlapping sheets with visible joints and often mineral chippings is built-up felt; a single seamless black rubber sheet is EPDM; a hard, shiny, seamless resin surface with a moulded edge trim is GRP; a smooth grey or black plastic sheet with welded seams is single-ply. Underneath, whether the insulation is above the deck or between the joists matters more than the covering does.
Flat roofs have a poor reputation earned almost entirely by one detail done badly: where the insulation sits, and whether the space above it can breathe. Get that right and modern coverings last a long time. Get it wrong and the best covering in the world is sitting on a deck that is quietly rotting.
Safety
Step by step
- Identify built-up felt.Layers of bitumen sheet bonded together, usually with a mineral-finished top layer, often with loose chippings. Look for overlapping joints running across the roof and a mineral or grey surface. It is the traditional covering and by far the most common on older garages and extensions.
- Identify EPDM.A single sheet of synthetic rubber, matt black, laid in one piece across most domestic roofs so there are no joints in the field. Flexible enough to push in with a thumb. Widely used for domestic re-covering because it goes down without heat.
- Identify GRP.Glass reinforced polyester laid wet and cured in place, giving a hard, slightly glossy, completely seamless surface, usually with a moulded plastic trim at the edges and a slight texture. Rigid — it does not give under a thumb.
- Identify single-ply and asphalt.Single-ply is a thin plastic membrane in grey, white or black with heat-welded seams, more common on larger roofs. Mastic asphalt is a thick, hard, seamless black layer, often with a sand or chipping surface, and it is heavy and long-lived — mostly found on older, better-built roofs.
- Then look at where the insulation is.This matters more than the covering. A warm deck has the insulation above the structural deck with a vapour control layer beneath it, so the deck and joists stay warm. A cold deck has the insulation between the joists, leaving the deck above it cold.
- Understand why a cold deck needs ventilation.Moist air reaching a cold deck condenses on its underside. A cold deck therefore requires a continuous, cross-ventilated air gap above the insulation, with vents at opposite edges. Without it, the deck rots from the inside while the covering looks perfect.
- Recognise the classic well-intentioned failure.Topping up the insulation in a cold deck by filling the ventilation gap. It improves the U-value on paper and converts a working roof into a rotting one. If you are improving insulation on a flat roof, the design of the whole build-up has to be reconsidered.
- Know what triggers building control.Re-covering more than a set proportion of a flat roof is controlled work, and it generally brings a requirement to improve the thermal performance at the same time. That is why a full re-cover is often a warm deck rebuild rather than a new sheet on the old build-up.
Tips
- Standing water after rain is not automatically a defect on a very shallow roof, but a puddle that is still there two days later points at a fall problem or a sagging deck. GuideHQ has a guide on flat roof ponding.
- The upstands, the edge trims and the abutment flashings are where flat roofs actually leak, far more often than the middle of the field. Look at the edges first.
- A soft, springy patch underfoot at the edge of a roof is a rotted deck. That is a reason to stay off it entirely, not to press harder to check.
Common mistakes
- Filling the ventilation gap of a cold deck with insulation — It removes the only thing keeping the deck dry. The roof then rots invisibly from above the ceiling, and the first symptom is often a ceiling stain years later.
- Painting a leaking felt roof with a liquid coating — It bridges the symptom, traps moisture in the existing build-up, and makes a proper re-cover more expensive because the coating has to come off. Liquid systems have legitimate uses; blanket-coating a failed roof is not one.
- Laying a new covering straight over a wet or rotted deck — The new covering is only as good as what it is fixed to. Any re-cover starts with establishing the deck is sound and dry, and a quote that does not mention the deck is not a complete quote.
If it doesn't work
A stain on the ceiling below that appears in cold weather rather than after rain
Cause: Condensation in the roof build-up rather than a leak — typically a cold deck with blocked or absent ventilation — Fix: Note the weather it correlates with, because that is the diagnostic. Fixing the covering will not help; the build-up and its ventilation are the problem.
Blisters and bubbles in a felt roof
Cause: Moisture trapped between layers expanding when the sun heats it — Fix: Blisters that are not split are not leaking, and cutting them open creates one. They are an indicator of trapped moisture, which is information about the roof's remaining life.
Cracking along the edge trim of a GRP roof
Cause: Thermal movement, often where the trim was bonded rather than allowed to move — Fix: GRP is rigid and the detail at the edges is where it is stressed. It is a repairable material but the repair has to address why it cracked, not just fill it.
Questions people ask
How long does a flat roof last?
It depends far more on the build-up, the falls and the detailing than on the covering. Manufacturers publish guarantees for their systems, which is the honest place to look. A well-detailed modern roof outlasts a badly detailed one of the same material by a wide margin.
Can I lay EPDM myself?
It is the most DIY-accessible covering because it goes down with adhesive and no flame, and the details at edges, upstands and outlets are still where it succeeds or fails. The material is the easy part.
Is a warm deck always better?
It is the arrangement used for new work and it removes the ventilation problem entirely, so as a rebuild it is generally the right answer. It also raises the roof level, which can be a problem at a door threshold or under a window, and that is the constraint that sometimes decides against it.
What is an inverted roof?
A warm deck variant with the insulation above the waterproofing rather than below it, held down by ballast or paving. It protects the membrane from temperature swings and is used on terraces and green roofs.