What the standing water was telling us
The giveaway on this roof was not the condition of the felt. It was the water sitting on it.
A flat roof is never truly flat. It is built with a slight fall so that water runs to an outlet rather than staying put. When water pools and is still there days after it rained, something has gone wrong with that fall, usually because the deck underneath has sagged, softened or taken on water itself.
Why ponding matters more than it looks
Standing water does not generally punch a hole through a sound covering. What it does is accelerate everything else.
It keeps the covering permanently wet, which ages it faster. In winter it freezes and expands, which works at every seam and blister. It adds weight to the exact part of the roof that is already sagging, which deepens the dip and collects more water next time. And it finds any weakness in a seam far more reliably than water that drains away in minutes.
By the time ponding is obvious, the covering is usually near the end anyway. On this roof the felt was worn and weathered across the whole surface, not just where water had collected.
What we did
The old felt came off and new OSB decking went down. That is the part that restores the fall, and it is the reason patching a ponding roof rarely works for long: you can repair the covering, but you cannot fix the shape of the roof without dealing with the deck.
The new covering is EPDM rubber, laid as a single sheet and dressed up the surrounding brickwork in one piece to form the upstands. Those junctions where a flat roof meets a wall are among the most common places for water to get in, and forming them from the same sheet rather than joining to them removes that risk entirely.
EPDM stays flexible rather than going brittle, and typically lasts around thirty years against ten to twenty for traditional felt.
If water is sitting on your flat roof
A puddle an hour after heavy rain is normal. A puddle three days later is not, and it is worth getting looked at before it turns into a leak rather than after.
The other signs worth watching are blisters, splits at seams or upstands, and any sponginess underfoot, which points at the deck rather than the covering. Our guide on why a roof leaks covers flat roofs alongside the usual pitched roof causes.
We carry out flat roof repairs and replacement, alongside all other roofing work, across Sunderland and the North East.
