Flat roofs on Kent extensions and garages fail for predictable reasons. Here's what's usually going on and what actually fixes it.
A flat roof should last 20 to 25 years. If yours is leaking every winter, the problem is usually one of four things — and none of them are fixed by another tin of sealant.
1. Ponding water
Water sitting on a flat roof after rain is the single biggest killer. It finds every weak point and works at it constantly. Ponding usually means the roof has insufficient fall, or the outlets aren't sitting at the lowest point. Re-sealing without fixing the drainage just moves the problem.
2. Failed seams and joints
Older felt and GRP roofs fail at the seams first — that's where the material flexes as the temperature swings. Kent gets a wide range: hot summers, frosty winters, and that daily expansion and contraction cracks aged felting along its joins.
3. Perished flashings
Where a flat roof meets a wall, the lead or trim flashing takes the weather. When it lifts or cracks, water tracks behind it and appears inside metres from where the roof actually failed — which is why flat roof leaks are famously hard to trace by eye.
4. Blocked outlets
Moss and grit wash down and block the outlet. In heavy rain the roof fills faster than it drains. Simple to fix, but if left it damages the deck underneath.
What we'd actually recommend
If the deck and insulation are sound, a single seamless EPDM membrane laid over the whole roof removes the seam problem entirely — no joins to fail, and it copes well with temperature movement. If the deck is spongy or the insulation is wet, no covering will hold; the deck has to be replaced first. Anyone who quotes you a membrane without checking the deck from underneath is skipping the part that decides whether the job lasts.
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Costs and timings in this article are guides for Medway and Kent properties. Every roof is different — a survey is the only way to get an accurate figure.