Comparing flat roof coverings for extensions and outbuildings

EPDM rubber membrane

EPDM is a single-sheet rubber membrane, often supplied as one piece for small to medium roofs, which removes the seam joints that are the most common failure point on other systems. It's highly flexible, tolerates thermal movement well, and has a long service life when properly installed, commonly outlasting felt and matching or exceeding GRP.

It needs a skilled installer for detailing around upstands and penetrations, since the adhesive and trim work at these junctions is where most EPDM failures occur rather than in the membrane itself. On larger roofs needing multiple sheets, the seam quality becomes critical.

GRP fibreglass

GRP is applied wet and cures into a fully bonded, seamless surface, which makes it strong at complex detailing around multiple upstands, rooflights and changes of level, since the resin can be worked into corners in a way sheet materials cannot. It also gives a walkable, durable surface that suits roofs used as a terrace or regularly accessed for maintenance.

It's less tolerant of significant structural movement than EPDM, since the rigid cured surface can crack if the deck beneath flexes more than expected, so the roof deck needs to be stable and well-fixed before GRP goes down. It also requires dry conditions during application, which can affect scheduling.

Modern felt systems

High-performance torch-on or self-adhesive felt systems, laid in multiple layers with a mineral or reflective cap sheet, perform considerably better than the older two-layer felt systems that gave flat roofing its poor reputation. They remain a cost-effective option on straightforward roofs without excessive detailing, particularly on garages and simpler extension roofs.

Felt is more dependent on installation quality than EPDM or GRP, since correctly heat-welding or bonding laps is critical to the roof's performance, and poor lap work is the most common cause of felt roof leaks.

Falls, insulation and detailing that apply to all three

Every flat roof needs a minimum fall to shed water rather than let it pond, typically achieved with firrings or tapered insulation beneath the covering. Ponding water shortens the life of any covering and is one of the most common design faults on flat roofs that were built without enough fall in the first place.

Upstands at abutments, correct trims at edges, and properly detailed penetrations for soil pipes or rooflights matter more to the roof's success than the choice between the three coverings above. A cheap covering perfectly detailed will often outlast an expensive covering poorly detailed.

• Check the roof has adequate fall before choosing a covering
• Match the covering to the number of upstands and penetrations on the roof
• Confirm the insulation and vapour control build-up beneath the covering
• Ask about installer experience with the specific system being quoted

How to specify a flat roof covering

Ask for the covering, insulation thickness, fall specification and detailing at every upstand and penetration to be set out in writing rather than a single line for 'flat roof recover'. Where the roof has multiple complex junctions, ask specifically how GRP or EPDM would handle those points compared with felt.

Get clarity on the warranty offered, since manufacturer warranties on these systems usually depend on installation by an approved installer, which is worth confirming before work starts.

Common questions