The short answer: a felt flat roof with one local fault, a puncture, a single failed lap, lifting flashing, can usually be repaired. A felt roof that leaks in more than one place, shows widespread cracking or blistering, or is past around 15 years old has usually aged across its whole surface, and patch repairs stop making economic sense. At that point the realistic choices are a full re-roof or a liquid-applied overlay that forms a new seamless membrane over the existing felt without stripping it.
How felt fails
Built-up bitumen felt is the most common flat roof covering in the UK, and it ages in a predictable way. Bitumen relies on oils that UV exposure slowly bakes out; the surface loses flexibility, then cracks under thermal movement. Laps between sheets, the joints the whole system depends on, open as the bitumen stiffens. Trapped moisture vaporises in summer heat and forms blisters that eventually split. The pattern matters because it is general: the sun does not age one corner of the roof. By the time the first leak shows inside, the rest of the covering is the same age and condition as the spot that failed.
Repairs that make sense
A repair is the right call when the damage is local and the surrounding felt is still serviceable:
- Punctures and splits from dropped tools, foot traffic or storm debris
- A single failed lap or blister, cut out, dried and patched properly
- Flashing and detail failures at walls, chimneys and edges, where the felt itself is fine but the junction has opened
Done competently, on felt that still has life, these repairs hold. Two warnings, though. First, the classic quick fixes, bitumen paint and self-adhesive flashing tape, are short-term measures; they buy a season, not years, and layers of old patch-on-patch make every future repair harder. Second, water travels: the leak inside rarely sits under the fault outside, so a repair without finding the actual entry point is a repair of the wrong spot.
The point where patching stops making sense
Use three tests:
- Count the leaks. One leak after a storm is an event. The second and third leak in different places within a year or two is a pattern, and the pattern is age.
- Look at the surface. Widespread crazing, bald patches where mineral finish has gone, multiple blisters: the covering is failing everywhere at once, just not equally fast.
- Do the arithmetic. Each call-out costs real money and buys less time than the one before. Two or three repairs often add up to a meaningful share of renewing the roof properly, with nothing lasting to show for it.
A 15 to 20 year old felt roof that has started leaking from age will not be patched back to health. The honest question is no longer “how do we fix this leak” but “how do we renew this roof”.
Renewing a felt roof: strip or overlay
Full replacement strips the old felt back to the deck and builds the covering again. It is the right answer when the deck is rotten or the insulation underneath is saturated, and the only answer when the structure needs work. It is also the most expensive and disruptive route: strip-off, skips, and days with the roof open to the weather.
A liquid-applied overlay renews the roof without removing it. The felt is surveyed, cleaned and primed, defects are made good, and the liquid system is applied with mesh reinforcement over the entire surface, including every outlet, upstand and edge, the approach that sealed an ageing felt roof in Shepherd’s Bush instead of chasing a single split. It cures into a seamless waterproof membrane formed in place, with no laps to open in future, which addresses precisely the way felt fails. Felt is, in practice, an ideal substrate for this: flat, stable and bonded, so the new membrane gets the foundation it needs.
HYDRONYLON® is a liquid-applied roof waterproofing system holding European Technical Assessment ETA-23/0735, with 10 years of confirmed durability, applied over existing felt by HYDRONYLON Approved Contractors. Completed installations are backed by a 10-year product guarantee issued by HYDRONYLON LTD to the Approved Contractor. Because the felt stays in place, there is no strip-off and the work typically takes days rather than weeks, at a cost typically well below a full re-roof.
The condition for an overlay is the same as ever: the substrate must be sound. Wet insulation or a soft deck means replacement first, and a proper survey settles that question before any quote.
Next step
If your felt roof is leaking, start with a survey rather than a patch. A HYDRONYLON Approved Contractor will tell you whether it needs a repair, an overlay, or honestly, a re-roof.