Flat Roof Edge Trims: What Do They Do?

The job the trim does

The edge of a flat roof is its most exposed part. Wind catches a loose membrane edge and lifts it, rain runs to the lowest point and the cut face of the deck is open to the weather. The trim solves three problems at once. It clamps or covers the covering at the perimeter so wind cannot get underneath. It hides and protects the edge of the deck and the timber below. And it has a drip, a lip that stands proud so that water falls free instead of running back under the roof or down the fascia.

That drip needs to line up with the gutter. If it does not, water either misses the gutter and runs down the wall, or it creeps back along the underside of the trim and soaks the fascia board behind it. Many cases of rotting fascias on extensions trace back to a trim fitted at the wrong angle or the wrong height.

Types of trim

The names vary, but the shapes fall into a few groups. A drip trim is the simplest: an angled strip that turns down at the front. A fascia trim extends further down and covers the top of the fascia board. A kerb trim finishes the sides of a raised roof edge, where the roof meets a parapet or a rooflight. Each is made to suit a particular covering, and the trim and membrane need to be compatible, because GRP, EPDM and felt are fixed to them in different ways.

GRP roofs often use a pre-formed trim over which the laminate is lapped, giving a bonded edge. EPDM is usually bonded to a trim or laid onto a drip edge with a clamping bar. Felt is dressed over a metal drip and sealed with a bitumen strip. Our guide to EPDM, GRP and felt compares the coverings.

Materials

Aluminium trims do not rust, are light and take paint, which makes them a common choice. Galvanised steel is stronger but rusts at cut edges and fixing holes in time. PVC and other plastics are cheap and easy to fit but can become brittle in cold weather and sag in heat, and they are less suited to a roof that is walked on or that receives heavy debris. Where the trim also carries a gutter bracket, strength matters. We specify the trim to suit the covering and the exposure.

Common faults

The first fault is the wrong height. A trim set too high holds water back on the roof, and our page on ponding on a flat roof explains how that makes a pool along the edge. Set too low, it leaves the membrane edge exposed. The second is a poor fixing. If fixings are too far apart, the trim lifts in wind, and screws that are not sealed let water into the deck.

The third is a lack of joints. Long runs of trim need properly sealed laps with a small gap for movement, because aluminium and PVC expand and contract with the temperature. A gap that is too tight buckles. A gap that is not sealed leaks. The last fault is a trim that does not discharge into the gutter. Watching the roof in rain shows this immediately, with a dark stain on the wall or fascia where the water falls. Our article on how flat roofs drain water shows how the edge and the gutter work together.

Wind and the edge

The edge is where wind does its damage. Air that gets under a loose trim or lifted membrane edge pushes upwards, and a flat roof with a poorly fixed perimeter can peel back from the corner like the lid of a tin. Corners and the windward side are the most exposed, which is why fixings are closer together there. A roof on an open site, or on the top of a hill, needs more care than one tucked between houses. If you can see a trim that rattles in the wind or has lifted away from the fascia, treat it as urgent, because the covering behind it is already being worked loose.

Replacing a trim

On a sound roof, the trim can sometimes be replaced without disturbing the whole covering, by lifting the membrane edge, fitting a new trim and re-bonding. On an old roof where the covering is also tired, it makes more sense to renew both together, since the edge is where the old and new have to join. Rotten fascia or deck timber behind the trim is cut out and replaced at the same time.

The free survey includes pictures of your trim, its fixings and the fascia behind, so you can see what is wrong. We then quote a fixed price in writing. If a trim alone will do, that is what we propose. The wider range of work is on the flat roofing in Basingstoke page.