Can You Put GRP Over an Old Felt Roof?

GRP stands for glass-reinforced plastic, a fibreglass mat set in resin that cures into a single rigid sheet. It depends on a clean, dry, firm surface to grip. Old felt is the opposite. It is a bitumen-based layer, often blistered, wrinkled and soaked with years of weather, and resin cannot key into it reliably. Even where the felt looks fine, it may be hiding trapped water that turns to vapour on a warm day and pushes up the new skin.

Movement is another problem. Felt flexes and softens in heat. A cured GRP sheet is far less forgiving, so the two layers pull against each other through the seasons, and cracks follow. Our answer on flat roof blistering describes what that looks like on the surface.

There is also a hidden cost. A new layer on top hides the deck. If the boards beneath have been getting wet, you will not know until a ceiling stains or the roof sags, and by then both layers need to come off. Seen from above, an overlay looks tidy for a season or two, which is why the trouble tends to show up late.

Some manufacturers publish a system for a specific old substrate, and a few allow an overlay in narrow conditions. That usually means a single layer, firmly stuck down, dry throughout, with no blisters or splits, over a sound deck, prepared in a way the maker states. Those conditions are hard to prove without opening the roof, which is why they are uncommon.

If a roofer proposes an overlay, ask for the manufacturer's written system for that exact situation, and ask how the dryness of the felt and deck will be checked. A plain, specific answer is a good sign. A vague one is not. Be careful if the main reason given is saving time or money, because a cover that fails after a few years costs more than doing the work properly once.

Part of this is about the future. A roof with two or three layers of old covering is heavy, hard to inspect and hard to repair later. Taking it back to the deck gives the next covering a clean start and makes any later repair simpler to carry out.

Stripping means removing the old felt layers, checking each board and cutting out anything soft or rotten. Then any dips are corrected, an insulation and vapour control layer is added if the design calls for it, and the new covering goes on. It is more work than an overlay, but the deck can be seen and judged, which is the point. Our answer on the best decking board for a flat roof explains what a sound deck looks like.

Approved Document L, the Part L energy rules, counts the fitting of a fresh roof membrane as renovation. Its 2021 edition gives roof values of 0.35 and 0.16 W/(m²·K) in Table 4.3 for existing elements, and the 2026 edition takes effect from 24 March 2027. Because the insulation question can change what a re-cover involves, raise it with your building control body at the planning stage.

The deck often decides how far a job grows, and that is why a written total follows the survey. Whichever route is chosen, the roof should arrive with a written total price after a survey. Photos of your own roof, including the exposed deck, arrive after the survey, showing where the money goes.

Whichever covering you choose, ask how the deck will be dried and tested before anything goes down, because a damp base shortens the life of any covering. Treat lifespans as guides rather than promises. Less, in an article last updated in March 2026, gives GRP 25 to 40 years and felt 10 to 20 years. A guarantee is a separate matter and not a life expectancy. Those figures show why replacing felt with GRP can pay off over time, but only if it is laid on a good base.

On cost, Checkatrade's September 2026 fibreglass roof guide gives GRP at £36 per m² at the low end for the covering only, and around £210 per m² fully insulated. The wide gap shows how much depends on what is included, such as deck repairs, insulation, edge trims and access. Take them as a rough guide, and ask each roofer whether their total covers VAT and scaffolding.

If you are comparing a felt replacement with GRP, our answer on how long a GRP roof lasts covers the fibreglass side in more depth. Our flat roofing page lists the coverings we fit.