EPDM and the Flat Roof Problem
Flat roofs got a bad name from felt. Rubber membranes changed the maths, but only if the deck underneath is right.
Why Felt Failed
Three-layer built-up felt has a service life measured in low double figures, and it fails by blistering and splitting as the bitumen dries out and the roof moves. Every flat roof horror story your parents tell is a felt roof.
EPDM is a single sheet of synthetic rubber, usually laid in one piece on a domestic roof, so there are no seams across the field of the roof to fail. Manufacturers quote fifty years and the material has been on commercial roofs long enough for that not to be fantasy.
The Deck Is the Job
A rubber membrane is only as flat as what it is laid on. If the existing deck is soft, uneven or has ponding, the membrane will follow it exactly and you will have a beautiful new roof with standing water on it. Water sitting on EPDM will not leak, but it collects dirt and it tells you the falls are wrong.
A proper job strips back to joists, checks the falls, and lays new deck. A quote that involves laying rubber over the existing felt is not a fifty-year roof.
Edges, Upstands and Outlets
Membranes almost never fail in the middle. They fail where they turn up a wall, where they meet a rooflight, and at the outlet. Those details are trims and adhesives rather than sheet, and they are where the skill is.
If you are getting quotes, ask what happens at the abutment with the house wall. The answer separates the roofers from the people who bought a roll of rubber.
