Three jobs on one strip
An edge trim is a metal or plastic profile fixed along the perimeter of a flat roof, where the covering would otherwise end in a raw edge. It does three things at once.
First, it finishes the edge. A felt, single-ply or GRP covering needs a clean termination, and the trim gives it one. Second, it carries water. The lower lip projects slightly, so rain leaves the roof as a drip and falls into the gutter or clear of the wall, instead of running back under the edge. Third, it holds. The trim is fixed through the covering or clamped over it, which anchors the edge against uplift.
A roof with no edge trim, or one with a poor one, behaves differently. Water clings to the underside of the fascia and tracks back toward the building. The edge of the membrane lifts, curls and splits. Inside, this shows up as a stain along the outside wall, a long way from where anyone would think to look.
It is worth knowing the trim is part of the waterproofing, not a cosmetic extra.
Drip trims and kerb trims
Two kinds are common, and the choice follows the shape of the edge.
| Trim | Where it sits | What it does |
|---|---|---|
| Drip trim | Roof edge over a gutter | Throws water into the gutter |
| Kerb trim | Edge with a raised lip | Holds the covering and shapes the finish |
| Wall flashing | Where roof meets wall | Seals the junction |
A drip trim is the usual finish on the lower edge of a roof that drains to a gutter. A kerb trim, or raised edge, is used where the roof is edged with a small upstand that stops water running off at that side. Which edge needs which is worked out on site.
The wall junction is a separate detail with its own leak risk; how a flat roof is sealed where it meets a wall goes through it.
The trim must also suit the covering. A trim made for one system is not always right for another, so it is chosen together with the membrane, not afterwards from a catalogue. A mismatch shows up as a gap that no amount of sealant will hold shut.
Wind at the edge on a coastal house
The roof edge takes the brunt. Wind passing over a flat roof creates suction at the perimeter, and it pulls hardest at corners and eaves. A loose trim gives that suction something to grab, and a lifted edge can start a tear that runs across the roof.
Near the sea, the edge of a roof is often the first thing the wind meets. The Met Office station at Manston records a mean wind of 9.86 knots over 1991 to 2020, with January's mean at 11.21 knots. Those are long-term averages, and single gusts run higher. Fixings are worked out for the exposure of each site, so ask how far apart the screws and clips on your trim are spaced instead of assuming.
Check an edge by eye after a windy spell. Look for gaps along the trim, lifted ends at the joins, bent sections and bare screw heads. A trim that moves when pressed from a ladder is already loose. Salt in coastal air can also wear fixings, so rusted screws are worth noting down for the roofer.
Why a loose trim lets water behind the membrane
Once a trim loosens, a gap opens between it and the covering. Rain blown into that gap runs behind the edge of the membrane, where nothing seals it. From there it wets the deck, the fascia board and the timber beneath, and the first sign is often rot at the roof edge.
The fault grows slowly. A trim that is slightly lifted lets in a little water on each wet day, and the deck stays damp for weeks. By the time a stain shows indoors, the edge boards may be soft.
Repair depends on what is found. A trim that is sound but loose can be refixed and the edge resealed. One that is bent or corroded is replaced, and where the deck edge is damaged the boards are cut out first. If there are cracks in the surface along the edge, cracks in a GRP roof explains why edges are often where they begin.
Send photos of the edge from the ground and we say what we see. A single total in writing follows the survey. The full range of this work is under flat roofing.
