Flat Roof Materials Compared: EPDM, GRP and Felt

EPDM rubber is the best all-round choice for most flat roofs: a single seamless sheet, no cracking under thermal movement, and a 20-year guarantee. GRP fibreglass is the pick for balconies and roofs with foot traffic because it cures hard and walkable. Traditional felt is the cheapest but the shortest-lived. For a standard extension or garage, EPDM usually wins on value over its lifetime.

If you have a flat roof on an extension, garage, or dormer, the material it is covered in is the single biggest factor in how long it lasts and how much trouble it gives you. The old image of a flat roof, cracked felt that blisters and leaks every few years, is well out of date. Modern systems handle the weather far better. Here is how the three main options compare so you can choose with your eyes open.

EPDM Rubber

EPDM is a synthetic rubber that comes as a single, seamless sheet. For most domestic flat roofs it is what we recommend, and for good reason. Because it goes down in one piece, there are very few joins to fail, and rubber flexes with the roof rather than cracking under the heat-and-cold cycle that destroys older coverings.

• Lifespan: a 20-year guarantee on the rubber, and often longer in practice.
• Best for: standard extensions, garages, dormers, and outbuildings.
• Strengths: seamless, flexible, low maintenance, good value over its life.

GRP Fibreglass

GRP, or glass-reinforced plastic, is the same fibreglass used on boat hulls. It is laid wet and cures to a hard, seamless, walkable surface, which is its big advantage. If the roof is going to take foot traffic, like a balcony or a roof terrace, GRP is the one to choose because it does not dent or scuff the way a softer covering would.

• Lifespan: comparable to EPDM, easily 20 years or more when laid properly.
• Best for: balconies, terraces, and any roof that will be walked on.
• Strengths: hard wearing, seamless, can be finished in different colours.

Traditional Felt

Felt is the original flat roof covering and the cheapest up front. Modern torch-on felt is a lot better than the old pour-and-roll stuff, and laid in layers by someone who knows what they are doing it can give a decent few years of service. But it is still the shortest-lived option, it has more joins than rubber or fibreglass, and it is more prone to blistering and cracking over time. We will fit it if budget is the priority, but we will always be straight with you about the trade-off.

Which Should You Choose?

For the typical extension or garage roof, EPDM rubber is the sensible all-rounder: seamless, long-lived, and good value once you spread the cost over its lifetime. If the roof will be walked on, GRP fibreglass is worth the extra. Felt makes sense only when the up-front price is the deciding factor and you accept it will not last as long.

Whatever the covering, the part that really decides how long a flat roof lasts is what sits underneath it and how the falls are set. A flat roof is not truly flat; it needs a slight slope so water runs off to the outlets instead of pooling. Get the deck and the falls right, fit a quality covering, and a flat roof will sit there doing its job for two decades without complaint.

Cost and Lifespan at a Glance

• EPDM rubber: mid-range cost, 20-year guarantee, best all-round choice for most homes.
• GRP fibreglass: similar cost to EPDM, very durable, the pick for foot traffic.
• Felt: lowest up-front cost, shortest lifespan, more joins to maintain.

Frequently Asked Questions