The appeal of overlaying is obvious: less mess and less labour. The risk is sealing a wet or rotten deck under a new membrane, where it keeps decaying out of sight until the roof starts to sag.
When an overlay can work
An overlay is only worth considering when the existing felt is well stuck down with no blisters, the surface is flat with no loose chippings, the deck is firm underfoot everywhere, and there is no sign of damp on the ceiling below. The felt usually needs covering with a suitable overlay board so the rubber has a smooth, compatible surface to bond to.
Why strip and replace is usually better
A leaking felt roof has normally let water into the deck. Chipboard and oriented strand boards soften and swell when wet, and timber joists can start to rot at the edges. Stripping back reveals that damage so it can be cut out and replaced.
It also gives the chance to correct the falls so water runs off rather than ponding, and to rebuild the roof as a warm roof with insulation above the deck.
- Wet or delaminated deck boards found and replaced
- Falls corrected with firrings or tapered insulation
- Insulation brought up to current standards
- New edge trims, upstands and outlets rather than patched ones
Building regulations and insulation
Replacing most of a flat roof covering is normally treated as renovation of a thermal element, which can bring in a requirement to upgrade the insulation where it is practical to do so. That is another reason stripping back often makes more sense than overlaying, since the work can be done in one go.
EPDM, felt or GRP?
EPDM is a synthetic rubber sheet, laid in one piece where possible so there are few joins. Modern torch on or cold applied felt systems are laid in layers. GRP fibreglass cures to a hard surface. Each can perform well; edge details and falls matter more than the choice. Our handbook comparison of EPDM and felt flat roofs looks at the two in more depth.
How to check your roof before deciding
Walk the roof carefully, or look from a ladder, for soft spots, blisters, splits at upstands and standing water. Inside, look for stains on the ceiling, especially near walls and outlets. Any of these points towards stripping back rather than covering over.
Repair, overlay or replace: making the call
A single split or a failed upstand on an otherwise sound roof can often be repaired locally. An overlay only makes sense on a dry, well bonded roof with a firm deck. Widespread blistering, soft areas or repeated leaks point to replacement.
Age matters as well. A felt roof near the end of its life will keep failing in new places, so money spent patching or overlaying can end up wasted when the deck finally has to be exposed anyway. Getting the deck checked before choosing avoids paying twice.
How we approach flat roof replacement
We carry out roofing with our own team and have more than 40 years of building and carpentry experience. We start with a free site visit to check the covering, the deck and the falls, then give a fixed-price written quote setting out whether an overlay is realistic or whether the roof should be stripped and rebuilt.
Common questions
Can EPDM be installed over felt?
Occasionally, when the felt is firmly bonded, flat and dry and the deck is sound, usually with an overlay board between. On a leaking roof, stripping back is normally the better option.
Why not just cover a leaking felt roof with rubber?
The deck beneath is often already wet. Sealing it in lets the boards and joists keep decaying out of sight.
Is EPDM better than felt?
Both can last well when laid properly. The falls, upstands, outlets and edge trims matter more than the membrane.
Do I need to add insulation when replacing a flat roof?
Replacing most of a flat roof covering can require the insulation to be upgraded where practical, which is easiest when the roof is stripped back.
Do you give a fixed price?
Yes. After a free site visit we provide a fixed-price written quote.