The short answer
A flat roof leaks at the edge trim when water gets behind the trim instead of off the roof. The usual causes are a drip trim that has lifted or cracked, gaps at the joints between trim lengths, a kerb trim that has pulled away, or a gutter that overflows and soaks the fascia. Most of these are local faults that can be repaired without replacing the whole roof.
How edge trims fail on a flat roof, what a leak at the edge looks like from inside and out, and how it is put right.
What the edge trim does
The edge of a flat roof is where water is meant to leave. A drip trim, sometimes called a drip edge, is a strip of metal or plastic fixed along the roof's lower edge. It carries the covering over the edge and throws rainwater clear into the gutter, so it does not run back underneath or down the face of the fascia, the board that closes the roof edge.
A kerb trim, found where the roof has a raised edge, does the same job along the sides. Between them, the trims hold the membrane in place, shield the timber behind, and give the covering a clean finish. They are simple parts, but a lot depends on them.
When one fails, the roof may look fine from a distance. The covering across the middle may be sound while rain gets in around the perimeter. That is why a leak near an outer wall or a ceiling edge often points to the trim and not to the roof sheet itself. The first step is a close look along every edge, with photos, before anyone talks about a new roof.
Drip and kerb trim failures
Trims fail for plain reasons. Fixings loosen as the roof moves with heat and cold. Sealant at the back of the trim dries, shrinks and splits. A trim fixed over a curled or lifted membrane edge leaves a gap behind it, and a trim set too flat sends water the wrong way. On older roofs, timber behind the trim may have softened, so even good fixings have nothing to hold.
You can often spot trouble from the ground. Look for a trim that sits proud of the fascia, a visible gap between trim and covering, rust streaks, or dark staining down the board below. Inside, a stain or bubbling paint at the top of an outside wall, near the corner of the room, is a common sign.
The fix depends on the cause. A loose trim can be refixed and resealed. A bent or corroded one is replaced. Where the covering has lifted behind it, the roofer lifts the trim, makes the membrane good, and fits it again with new fixings. If the wood behind has rotted, it is cut out first. Our answer on EPDM roof lifespan explains why edges often age before the middle of the sheet.
Gaps at joints and corners
Edge trims come in lengths, and wherever two lengths meet there is a joint. A good joint is covered by a cover strip or a sealed connector. A poor one is a small gap with sealant smeared across it, and sealant alone seldom outlasts a few winters of expansion and contraction. Water enters the gap, runs along the trim's back and drops onto the timber below.
Corners are the same story. An external corner has to be formed so that no water can slip round the end of the trim, and an internal corner needs the covering turned up without stress. Rubber and felt coverings both put strain on a corner, especially when they are pulled tight over a sharp edge.
If you see a thin dark line under a joint on the fascia, or a drip that appears in one spot only when rain is heavy, a joint is the likely cause. A roofer can check by running water on a short length at a time, starting low and working up, which narrows the source. Our guide on how to find a flat roof leak describes that process in more detail.
Gutters that overflow at the edge
Sometimes the trim is blameless and the gutter is the problem. A gutter blocked with leaves, or one that falls the wrong way, fills and spills over the front. The water then wets the fascia and the trim from the outside, soaks the timber behind, and then works under the covering at the edge. It looks like a roof leak, but the cause is below the roof.
Check the gutter during heavy rain. If water pours over the lip, or hangs from the trim, the gutter is overwhelmed or out of line. Clearing the run and checking the falls often cures it. Manufacturer guidance from Brett Martin gives 1mm of fall for every 600mm of gutter, which is a useful check. A sagging bracket or a split joint is a repair, and an old run may need replacing. See our gutters, soffits and fascias page for that work.
A blocked outlet can cause much the same trouble on the roof itself, which we cover in our answer on a blocked flat roof outlet. After a survey, you get one written price, with photos of the edge detail, and the flat roofing page sets out the roof work beyond the edges.
More on this in Flat Roofing in Crawley
Two more edge leak questions
Can I fill the gap with sealant myself?
A dab of sealant may slow a drip for a few weeks, but it rarely lasts. The gap moves with the weather, and sealant on a dirty or wet surface does not bond. It can also hide the true cause, such as a lifted membrane behind the trim. Photograph the leak and let a roofer judge whether it needs refixing.
Will a leaking edge damage the timber behind it?
It can. Water running behind a trim soaks the fascia and the roof edge timber, and damp timber softens over time. That is why an edge leak is worth sorting promptly. A survey checks how far the water has travelled, and if boards or fascia need renewing, that is included in the written price once we have seen them.