Felt vs EPDM vs GRP: Which Flat Roof Is Best for Your Home?

Flat roofs are everywhere in Cumbria — on kitchen extensions, garages, dormers and bay windows. When one starts to leak or reaches the end of its life, the first question most homeowners face is which material to replace it with. Felt, EPDM rubber and GRP fibreglass each have strengths, weaknesses and a very different way of coping with our weather.

This guide compares the three main flat roof materials honestly, so you know what you're being offered, why, and what to ask before you commit.

What Are the Three Main Types of Flat Roof?

Most domestic flat roofs in the UK use one of three systems:

• Felt — layers of bitumen-based membrane bonded to the deck, today usually applied as a torch-on system
• EPDM — a single sheet of synthetic rubber membrane, bonded to the deck
• GRP — glass reinforced polyester, better known as fibreglass, laid as a liquid resin with glass matting and finished with a hard topcoat

You'll also come across liquid-applied coatings, single-ply PVC and TPO membranes, and lead or zinc on period and high-end properties. For a typical house extension or garage, though, the choice almost always comes down to felt, EPDM or GRP.

Felt Flat Roofs

Felt is the traditional choice, and the one many older flat roofs in Carlisle still have. The old three-layer built-up felt roofs laid with hot bitumen had a reputation for blistering and cracking after 10–15 years. Modern torch-on systems use polyester-reinforced, polymer-modified membranes that are far tougher and more flexible.

Pros

• Usually the most economical option, particularly when repairing or overlaying an existing felt roof
• A mineral-finished cap sheet gives good protection from UV and foot traffic
• Familiar to almost every roofer, so repairs are straightforward

Cons

• Shorter lifespan than EPDM or GRP — traditional felt typically lasts 10–20 years
• Torch-on installation uses a naked flame, which needs care near timber fascias, soffits and roof voids
• Multiple laps and joins mean more potential weak points than a single-sheet system

EPDM Rubber Flat Roofs

EPDM (ethylene propylene diene monomer) is a synthetic rubber membrane. On most domestic roofs it's supplied as one continuous sheet cut to size, so there are no seams across the main roof area.

Does EPDM have to be glued down?

On houses and extensions, EPDM is almost always fully bonded to the deck with adhesive, with a stronger contact adhesive around the edges and upstands. On larger commercial roofs it can also be mechanically fixed or held down with ballast. What matters most is that the deck underneath is sound, dry and smooth.

What goes under an EPDM roof?

EPDM is laid over a timber deck, typically OSB3 or plywood. On a warm roof build-up, there's a vapour control layer and rigid insulation boards between the structure and the deck. Getting that build-up right is what prevents condensation problems underneath the membrane.

Can water sit on an EPDM roof?

EPDM tolerates standing water better than most materials, but no flat roof should be designed to hold water. A properly built flat roof has a fall so rain drains to a gutter or outlet. Persistent ponding adds weight, collects debris and puts extra strain on joins and details.

Pros

• Long lifespan — a well-installed EPDM roof typically lasts 25 years or more
• Flexible, so it copes well with temperature swings and minor building movement
• Cold-applied, with no naked flame
• Quick to install, which helps in an unsettled Cumbrian forecast

Cons

• Can be punctured by sharp objects, falling slates or careless foot traffic
• Not designed as a walking surface without walkway protection
• Complex roofs with lots of upstands, rooflights and pipes need careful detailing
• The black rubber finish doesn't suit every property

GRP Fibreglass Flat Roofs

GRP stands for glass reinforced polyester. Resin and glass fibre matting are laid over a timber deck and cure into a hard, seamless shell. A coloured topcoat finishes the surface, and pre-formed GRP trims are used at the edges.

Pros

• Seamless, rigid surface with no joins across the roof
• Walkable, and suitable for balconies and terraces with an anti-slip topcoat
• Long lifespan — typically 25 years or more when laid correctly
• Handles complex shapes and detailing well
• Available in a range of colours

Cons

• Needs dry weather and suitable temperatures to cure properly — a real consideration in Cumbria, so installation has to be planned around the forecast
• Rigid, so it can crack if the deck moves or wasn't prepared properly
• Quality depends heavily on the installer's skill
• Repairs are possible but can be visible against the original finish

EPDM vs GRP vs Felt: Side-by-Side

Lifespans are typical ranges, not guarantees. A badly installed EPDM or GRP roof will fail sooner than a well-installed felt one.

What Type of Roofing Is Best for a Flat Roof?

There's no single best material — there's a best material for your roof. The factors that decide it are:

• How the roof is used. If you'll walk on it or use it as a balcony, GRP is the natural choice.
• Size and shape. Large, simple roofs suit EPDM. Small roofs with lots of edges and details often suit GRP.
• The condition of the deck. If the existing boards are rotten or uneven, they need replacing whatever goes on top.
• Access and timing. EPDM can be laid in a wider range of weather, which can matter in a Cumbrian winter.
• Budget and how long you'll stay. Felt can make sense for a garage or an outbuilding; EPDM or GRP usually works out better value over the life of the roof on a home.

What Are the Downsides of a Flat Roof?

Flat roofs have a reputation for problems, and some of it is deserved. Compared with a pitched roof, they:

• Drain more slowly, so poor falls or blocked outlets lead to ponding
• Generally have a shorter lifespan than a slate or tile roof
• Need more regular checks for debris, blocked outlets and damaged edges
• Are more exposed to condensation if the insulation and vapour control aren't built correctly

Most flat roof failures we see in Carlisle come down to poor drainage, poor detailing around edges and upstands, or an old roof that has simply reached the end of its life — not the idea of a flat roof itself.

How Long Will a Flat Roof Last?

Material sets the ceiling, but installation and maintenance decide how close you get to it. Keep outlets and gutters clear, deal with blisters, splits or lifted edges early, and have the roof checked after severe weather. If your flat roof is showing its age, our guide to signs your roof needs replacing covers what to look for.

Should You Avoid Buying a House With a Flat Roof?

No — but go in with your eyes open. Before you buy, find out:

• How old the flat roof is and what it's made of
• Whether there's a transferable guarantee and who installed it
• Whether there's any sign of ponding, cracking, blistering or damp staining on the ceilings below

Some insurers also ask what proportion of a property's roof is flat, so check that with your insurer early. Our guide on whether home insurance covers a leaking roof explains why.

How Much Does a Flat Roof Cost?

Every flat roof is priced on its own details. The main factors are the size of the roof, the material, the condition of the deck, whether insulation needs upgrading, access, and how much detailing there is around edges, rooflights and upstands.

For an up-to-date guide price, use our roof cost calculator, or ask us for a free quote and we'll give you a written price based on your actual roof.

If you're replacing most of a flat roof, Building Regulations may require the insulation to be brought up to current standards. We'll tell you whether that applies when we quote, and your local building control team can confirm the position for your property.

Flat Roofing in Carlisle and Across Cumbria

EA Roofing Services repair and replace felt, EPDM and GRP flat roofs across Carlisle, Longtown, Brampton, Penrith and the rest of Cumbria. We'll look at how your roof is built and used before recommending a system, and we're fully insured for every job we take on.

Call us on 07341569754 or request a free quote.

Frequently Asked Questions

What are the three main types of flat roof?

The three most common flat roof systems in UK homes are bitumen felt (usually torch-on), EPDM rubber membrane and GRP fibreglass. Other options include liquid-applied coatings, single-ply PVC or TPO membranes, and lead or zinc on higher-end work.

Which is better for a roof, EPDM or GRP?

Neither is better in every case. EPDM is flexible, quick to install and copes well with movement, which suits larger or simple roofs. GRP is rigid, seamless and walkable, which suits smaller roofs, balconies and roofs with lots of detailing. The right choice depends on the roof's size, shape, use and deck condition.

How long does a flat roof last?

As a rough guide, traditional felt lasts 10–20 years, modern high-performance torch-on felt can last longer, and a well-installed EPDM or GRP roof typically lasts 25 years or more. Installation quality, drainage and maintenance matter as much as the material.

Can you walk on an EPDM or GRP flat roof?

GRP can be walked on and, with an anti-slip topcoat, used as a balcony or terrace. EPDM can take occasional foot traffic for maintenance, but it isn't designed as a walking surface unless walkway protection is added.

Is EPDM better than felt?

For most domestic roofs, EPDM outlasts traditional felt, has fewer joins and is installed cold, with no naked flame. Modern torch-on felt systems have narrowed the gap and can be a good value option, particularly for repairs to existing felt roofs.

Should I avoid buying a house with a flat roof?

Not necessarily. A flat roof in good condition is not a reason to walk away. Ask how old it is, what it's made of and whether there's a guarantee, and have it checked for ponding, cracking or blistering before you buy.