Flat roofs have an unfair reputation — a well-built one is genuinely reliable. But Bristol has a large stock of ageing felt roofs over extensions, garages and bay windows, and those do leak, in predictable ways. Here are the causes we find most often, and what fixing each one properly involves.
Ponding: water that never leaves
Flat roofs aren’t flat — they should have enough fall to shed water within a day or two of rain. When the deck sags or was built too level, water pools in the same spots, and standing water works on every seam and blister beneath it, especially through freeze-thaw. Small ponding over a sound membrane can be tolerable; ponding over ageing felt is a countdown. The real fix addresses the fall, not just the surface — sometimes with tapered insulation when the covering is renewed.
Failed seams and flashings
Water rarely comes through the middle of a membrane; it comes through the joints — laps in the felt, junctions where the flat roof meets the house wall, upstands around rooflights. Felt laps open as the bitumen ages; wall junctions fail when the flashing was mastic-and-hope rather than proper lead. Local repairs work when the surrounding material is sound: heat-welded patches, renewed lead flashings at the wall.
Blistering and cracking: the felt endgame
UV embrittles felt; trapped moisture blisters it. A few blisters can be repaired, but widespread blistering with surface cracking means the material is done — patching becomes whack-a-mole, each repair outliving its neighbour by months. Felt typically gives 10–20 years; if yours is there and leaking in multiple places, replacement is the honest recommendation.
Punctures and traffic
Flat roofs collect foot traffic — window cleaning, aerial work, retrieved footballs — and old felt punctures easily. A single puncture is a quick patch. If the roof needs regular access, that’s worth designing for at replacement time with walkway protection.
Why we replace with EPDM
When a felt roof reaches the end, we fit EPDM rubber membrane: one seamless sheet on most domestic roofs (eliminating the lap problem entirely), UV-stable, flexible through frost, with a 20-year guarantee on our installations and a realistic life of 40–50 years. It costs more than re-felting; per year of service it’s the cheaper roof.
The diagnostic difference
Water shows up indoors away from where it enters — it travels along the deck before dropping. Finding the true entry point beats patching where the drip appears. That’s the first job of a proper inspection, and it’s free: get in touch and we’ll trace it properly.