Flat Roof Conversions in Liverpool: What It Costs and What to Expect

By Chris, owner of Everlast Roofing North West (20+ years on Merseyside roofs) · About Everlast Roofing · Last updated 2026-06-29

I had a call in March from a woman in Maghull. She had a 1970s back kitchen extension with a flat roof, and she had been patching the same felt leak for four years. The felt was original. It had been cold-applied over the top twice. She could see the deck was starting to go at the edges, and she was getting a damp patch on the back bedroom ceiling every time it rained heavily. She had three quotes. One was for felt again. One was for GRP. One she did not understand. She rang to ask me which was right.

The answer in her case was GRP, and I can tell you exactly why in this article. But more importantly, I want to walk you through what a flat roof conversion actually involves in Liverpool, what the real costs are, and how to tell whether you need a material conversion or something bigger. If you have a flat roof on a back extension, a garage, a dormer, or a porch, and it is giving you trouble, read on.

What is a flat roof conversion and which type do you need?

The term covers two different jobs, and they are not the same price or the same process. A material conversion means stripping the old flat roof covering and replacing it with a better system. The deck and joists usually stay. You are just changing what sits on top. This is the right job for the majority of Liverpool flat roofs that have reached end of life. A pitch conversion means adding a new timber frame and turning the flat roof into a sloped one with tiles or slates. This is a bigger, more expensive structural job and is only right in specific circumstances.

Material conversions are the most common call we get in Liverpool. The back-extension felt roof that is 15 or 20 years old, has been patched once or twice, and is now letting water in despite the patches. The garage flat roof that looks fine from the ground but is actually blistered and split across the whole surface. The kitchen extension where the deck has started to move and the felt is cracking at the joints. These are all material conversions, and the choice is usually between GRP fibreglass, EPDM rubber, or felt again.

Pitch conversions come up when a homeowner wants to stop dealing with a flat roof permanently, when the flat roof is on a building that has planning permission for a raised pitch, or when the deck and joists have failed and the roof needs a full structural rebuild anyway. If you are rebuilding from the joists up, it sometimes costs only a little more to go pitched at the same time. We will tell you honestly whether a pitch conversion makes sense for your property, or whether a quality flat roof system is the better call.

How much does a flat roof conversion cost in Liverpool?

The cost depends on the system you choose, the size of the roof, the condition of the existing deck, and whether any joists or timbers need replacing. These are the realistic prices we quote for standard Liverpool residential work.

For a material conversion on a typical back-extension or garage flat roof in Liverpool, expect to pay: felt replacement, £350 to £700 for a 15 to 25m² roof; EPDM rubber, £700 to £1,500 for the same size; GRP fibreglass, £1,200 to £2,400. If the deck is sound and the job is straightforward, you are looking at a day on site for any of these. Where the price goes up is when the deck needs replacing (add £200 to £600 depending on size and timber specification), or when there is significant joist rot underneath (add more, priced once we are up there and can see what is there).

For a flat-to-pitched conversion on a back extension in Liverpool, costs start at around £4,500 for a simple mono-pitch addition to a small extension and go up to £10,000 to £15,000 for a larger hip or duo-pitch structure with slates or tiles. The scaffold is the same for both, the structural timber is additional, and you will likely need building regulations sign-off rather than just a like-for-like replacement. We will handle the written quote and advise on whether you need to notify your local authority. All costs include VAT, which is at the standard rate for flat roof conversions on existing residential extensions.

GRP fibreglass, EPDM rubber or felt: which system is right for your Liverpool roof?

This is the question most Liverpool homeowners get wrong, usually because they are looking only at the upfront price. Here is the honest comparison.

Felt is the cheapest option upfront, but it has the shortest life of the three. A properly installed, high-quality mineral felt system will last 10 to 15 years on a residential extension in Liverpool. The key word is properly installed. A cold-applied felt overlay on top of old felt will give you maybe five years before the lap joints start to fail. Felt is the right call when you have a simple, accessible roof, a limited budget, and you understand you are buying 10 to 15 years, not 25. It is also fine for a building you are planning to sell or redevelop soon.

EPDM rubber is a single-ply membrane that is glued to the deck in large sections. It is flexible, handles Merseyside weather and movement well, and will last 25 to 30 years with minimal maintenance. It is better value than GRP over a 20-year horizon even though it costs more upfront. The main limitation is that it uses bonded laps, which need to be dressed correctly. Reputable installers use proper lap sealant and two-part adhesive. A sloppy EPDM install is almost always a lap problem, so this system rewards choosing a roofer who knows what they are doing. EPDM is our recommendation for most garage and extension roofs in Liverpool where GRP is not practical for access reasons.

GRP fibreglass is a two-part liquid system (chopped strand mat and polyester resin) that cures to a seamless, rigid, waterproof shell. There are no laps, no joins, no future weak points. It is the most durable of the three and the one we recommend most often for flat roofing in Liverpool on extensions, porches and dormers. It takes longer to apply than EPDM and costs more upfront, but a GRP roof installed correctly will last 25 to 30 years with no maintenance beyond an annual check of the upstands. The one limitation is temperature: GRP needs to be applied above 5 degrees Celsius to cure properly, which means winter applications require careful scheduling. We do not apply GRP in cold or wet conditions.

What happens on the day of a flat roof conversion in Liverpool

For a standard material conversion on a Liverpool back extension or garage, here is the typical sequence. We arrive in the morning with the materials and set up access (usually a ladder, occasionally a small scaffold tower for higher roofs). The old covering comes off first. Once the deck is exposed, we check every board and joist for rot, movement, or damp damage. If anything needs replacing, we agree it with you before we continue, show you the photos, and update the written quote accordingly. No surprises on the final day.

Once the deck is confirmed sound (or repaired), the new system goes on. For GRP this means applying the chopped strand mat, saturating it with resin, then applying a second coat and the coloured topcoat. For EPDM, the adhesive is applied to the deck, the membrane is rolled on, the laps are sealed, and the upstands are dressed to the walls. For felt, the layers are applied in sequence according to the specification. On a typical 20 to 25m² roof, a GRP conversion is done in a day. EPDM is usually done in a day. Felt is also a day or less.

We finish by fitting drip edge trim, new guttering upstands where needed, and a final watertight check. We photograph the finished job from every angle and send them to you along with the guarantee. The site is cleaned up before we leave. If you are in Maghull, Crosby, or anywhere across Liverpool and Merseyside, we aim to leave the site as though we were never there, apart from the new roof.

• Check the deck, not just the covering. A blistered felt surface might mean the deck beneath is damp or rotting. A good roofer will expose and check the deck before quoting a fixed price.
• Get the system lifespan in writing, not just the price. A felt quote at £400 and a GRP quote at £1,800 are not comparable unless you know one lasts 12 years and the other lasts 30.
• Ask what is included in the quote. Specifically: deck replacement if needed, new drip edge, guttering connection, waste removal, and VAT.
• For GRP and EPDM, check that the roofer has installed at least 20 systems of that type. Both materials are unforgiving of technique errors.
• Booking deposit covers materials and secures the slot. Balance is due on completion once you have inspected the finished job.
• Request photos of the deck once the old covering is stripped, before the new system goes on. This is your evidence if any dispute arises later.
• If the quote mentions a flat-to-pitched conversion but your extension is under 3m in height, check whether building regulations apply in your local authority area before work starts.

Frequently asked questions

For more local roofing advice, browse the Everlast Roofing blog or call Chris on 07933 828 045.