Felt roof installation and renewal

Felt roofing uses bitumen-based membranes assembled in a specified build-up, often with a protective cap sheet. Application can involve torch-on, self-adhesive or other compatible methods. The suitable approach depends on the complete roof specification and fire risks around the work area; no method is automatically suitable everywhere.

The number of layers and the way they overlap at joints, edges and upstands matters more than the brand of felt chosen. A roof laid with careless overlaps or thin layers at the edges will struggle however good the material itself is.

The deck is checked and any soft or damaged boards are replaced before a single sheet goes down, since felt bonded to a wet or rotten base will fail early regardless of the layers above it. A vapour layer may be added, followed by insulation where the build-up allows for it, then the felt layers themselves, lapped to shed water downhill.

Edges, upstands and collars around rooflights or pipes deserve careful assessment because they interrupt the waterproof surface. Compare the material principles described on our EPDM rubber roofing and GRP fibreglass roofing pages rather than assuming that changing the covering alone will cure a poorly formed junction.

Felt remains a widely used covering, and a well-laid, well-maintained felt roof can perform perfectly well for ordinary garages, extensions and sheds. Problems tend to come from thin or single-layer coverings, poor detailing at edges and joints, or a deck that was never properly sound in the first place.

Material comparisons should consider roof geometry, exposure, maintenance and compatible detailing. EPDM rubber roofing and GRP fibreglass roofing work differently from layered felt; none removes the need for a sound base and drainage. Ask which specification is appropriate for the actual building instead of choosing from product advertising alone.

A small split, a lifted edge trim or a localised blister can often be patched without disturbing the rest of the roof, and our flat roof repairs page covers this kind of work in more detail. Where the felt has aged across large areas, where there are repeated leaks, or where the deck underneath has taken on water, renewal is usually the better long-term option.

Before committing to either route, it is worth getting a clear explanation of what has been found and why a particular repair or renewal is being suggested, rather than taking the recommendation on trust. Our flat roof replacement cost guide explains the main factors that affect pricing for this kind of project.