Torch on felt still covers thousands of Oxfordshire garages and extensions. This guide explains how it fails, which repairs genuinely hold, and the honest point at which another patch is wasted money.
Three layer torch on felt with a polyester reinforced cap sheet is a genuine 15 to 20 year roof. Single layer pour and roll felt from the 1970s and 1980s, still common on garages, was closer to 10 years when new and is now decades past it.
Working out which you have changes everything. Modern SBS modified felt takes a patch well. Old oxidised felt has lost its oils, cracks like a biscuit and will split again next to any repair you make.
The mineral surface weathers off first, leaving the bitumen exposed to ultraviolet light. The sheet then goes brittle, shrinks slightly and starts to split, typically over deck joints and along the laps where two sheets meet.
Blisters form where moisture trapped at installation expands in summer heat. Alligator style crazing across the surface is the clearest sign the roof is chemically finished rather than mechanically damaged.
A split in sound felt is cut back to good material, primed and covered with a torch applied patch lapping at least 150mm all round, with the edges sealed. A soft blister is cross cut, dried, dressed flat and patched over the same way.
Cold applied bitumen paint and self adhesive flashing tape have a place as a temporary measure, particularly in winter. They are not a permanent repair on a roof that is otherwise sound, and they make a proper repair harder later because the new felt will not bond to them.
The arithmetic of patching versus replacing
A felt patch repair typically costs £180 to £450. A new three layer felt covering on a garage sized roof is usually £1,400 to £2,400, and an EPDM re cover on the same roof is often within a few hundred pounds of that with a longer outlook.
Two or three patches in as many years pays for a decent slice of a replacement. Once the deck is soft underfoot, the felt crazes when you press it, or the splits are appearing away from previous repairs, replacement is the cheaper route over any sensible period.
What we replace felt with
For most small domestic roofs we recommend EPDM, because a single sheet removes the seams that felt fails at. Where the roof will be walked on we look at fibreglass instead. Felt still makes sense on larger, complex commercial roofs where sheet sizes cannot cover in one piece.
Whichever system goes on, the deck and falls get corrected first. Laying a new covering on a deck that ponds simply buys a newer version of the same problem.
How long should a felt roof last?
Modern three layer torch on felt gives 15 to 20 years. Older single layer coverings were nearer 10, which is why so many garage roofs from that era are failing at once.
Can felt be laid over the old covering?
Sometimes, if the existing felt is dry, well bonded and the deck is sound. It hides the deck condition though, so we prefer to strip and see what we are building on.
Why does my felt roof bubble in summer?
Trapped moisture under the sheet turns to vapour in heat and lifts the felt. Small firm blisters can be monitored, but soft or growing ones mean water is present and needs releasing.
Is torch on felt a fire risk during installation?
It is hot work, so it is done with care, proper equipment and a period of watching the area afterwards. Near timber and combustible detail we use self adhesive systems instead.
Do you still install felt roofs?
Yes, where it is the right specification for the roof. We will explain why we are recommending felt, rubber or fibreglass rather than defaulting to one system for every job.
Old felt roof on its last legs?
We will tell you honestly whether one more repair is sensible or whether your money is better spent on a new covering.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.