The short answer
Most flat garage and porch roofs leak at the edges and junctions, not in the middle of the covering. Splits at the upstand, a blocked outlet, a gutter that has dropped, or ponding in a dip are the usual causes, and a survey finds which one it is before anyone talks about replacement.
A flat or shallow roof is a sheet of waterproofing laid over a deck, with a slight fall to move water to one edge. When it leaks, people tend to assume the whole covering has failed. Often it hasn't, and treating it as if it had can mean paying for a new roof when a repair would have held.
Q1.1Why do edges and upstands fail first?
The upstand is the strip where the waterproofing turns up a wall. If the wall is brick, the top edge needs a flashing or a chased-in lead strip that sheds water over it. Sealant alone dries and cracks. A failed upstand lets rain run down behind the covering, and the stain appears inside several feet away from the real fault.
Q1.2Does ponding on a garage roof matter?
Water that sits on a flat roof for days is a warning, and a pool that is still there two days after rain deserves a proper look rather than a hope that it will dry. Standing water speeds up the ageing of every covering, from felt to rubber. Dips form over weak decking or where insulation has compressed, and the waterproofing wears quickly under a permanent pool. Outlets and gutters need to be clear and set low enough for water to leave the surface. Where a dip is shallow, a repair may be possible; where the deck has softened, the deck needs replacing first.
Q1.3Which covering is on my Rowde flat roof?
Garage roofs in the village could be torch-on felt, EPDM rubber or GRP fibreglass, or something older and bituminous. Each has its own repair method, and mixing them rarely works. Our flat roofing page explains how they compare, and the survey photographs show you which one your roof has.
A flat roof does not always need to be taken off to be fixed, but it should never be left leaking over winter. First we make a leaking roof safe, and the longer-term fix is discussed once the water is stopped.