Chapter 1: Why does the flat roof over the garage and porch of our mid-century Crowthorne house keep leaking?
What we find
Houses designed in the mid-century modern manner use flat roofs generously: over the garage, the entrance porch, the link between house and outbuilding. However well such a section was built, a flat roof has no slope to throw water off, so every flaw is felt quickly. Bitumen felt cracks along its upstand (the vertical turn-up where the roof meets a wall), and edge trims lift where the old timber behind them has softened.
Inside, the clue is a stain that spreads slowly across a ceiling lined with timber boards or plasterboard, often worse a day or two after rain because the water travels along the deck before it drops. Ponding, with shallow pools lingering after a shower, usually means the deck has sagged between joists.
What we fix
We open the roof at a few points on the survey to find out whether the deck is dry. If it is, a new membrane over the existing structure may be all that is needed. If boards are soft, they come out and new ones go in, with a proper fall built in so water runs to the gutter. For the covering we usually talk through EPDM (one sheet of synthetic rubber), GRP (fibreglass resin that sets into one hard shell) and felt. Typical UK ranges per square metre are £80 to £120 for EPDM, £100 to £160 for GRP and £60 to £100 for felt, and the written price only arrives once we have seen the roof.
Matching a modern membrane to an original architect's detail takes care at the edges, so we photograph those joints before and after. Our flat roofing page sets out the full range.