What are flat roof edge trims and drip details?

Every flat roof has to stop somewhere, and the edge is where the covering turns over and is held. A trim, sometimes called an edge profile or drip bar, is fixed to the deck and the covering is dressed over it. It supports the edge, protects the end grain of the boards from rain, and gives the covering a clean line that shows from the street. A trim nailed on at wide spacing ripples along its length, so fixing at close centres into sound timber is part of doing it properly.

Trims come in aluminium, galvanised steel and plastic, and in a range of shapes to suit the system. Some are plain angles, some have a deeper face that hides the board edge, and some are made for a particular membrane. The right one depends on the covering, because felt, EPDM and GRP are each finished in a different way. Choosing the trim is part of laying the roof, not an extra, and a mismatched trim shows on the roof within a few years as a gap, a ripple or a stain on the fascia below.

A drip is the lip along the lower edge of the trim, projecting out and down. Water running off the roof follows the covering to the edge and then hangs on the underside by surface tension. Without a drip, it creeps back along the underside towards the wall, wetting the fascia, the brickwork and eventually the soffit. A drip gives it a point to fall from, and the gutter catches it. On a roof with no gutter, the drip has to overhang far enough that the water lands clear of the wall below.

The drip works only if it sits over the gutter. Where it stops short, water misses the gutter and runs down the wall. Where the gutter is fixed too high, the water splashes over the back. Our page on guttering, soffits and fascias covers how the roofline fits together, and the roof edge and the gutter need to be right as a pair or neither does its job.

Most edge failures are simple. The covering has lifted from the trim and let water in behind it. The trim has loosened or been bent by ladders. Sealant has dried and cracked. The corners, where two trims meet, were never properly formed, so water gets in at the join. Or there was never a trim and the felt was simply folded over the boards, where it splits with the weather.

Edge damage is easy to miss because it sits above the eye line. Look for a dark stain under the fascia, peeling paint on the board below, or a trim that has pulled away and waves along its length. A defect at the edge feeds water into the end of the deck, where timber rots quietly, so a small repair now beats a rebuild later. Green growth along the edge also shows water that is not leaving the roof cleanly.

The perimeter is more than straight runs. External corners, internal corners and the junction with a wall (the upstand, or short vertical return of the covering up a wall) are where the detail counts. A square finish at a corner means the trim has been mitred or fitted with a proper corner piece, the covering has been dressed round without wrinkles, and the sealed joints are tidy from every angle.

Where a flat roof abuts a wall, the covering turns up and a flashing, often lead, covers the top of the upstand. Our answer on finding a leak on a flat roof lists the edges and junctions we check first. A well-formed junction needs no sealant to keep it dry, and sealant on its own is a sign the detail underneath was weak.

A good edge is straight, firmly fixed at close intervals, with the drip running cleanly over the gutter and the covering stuck to the trim all the way along. There are no gaps, no waves, and no smears of sealant. It also looks right from the ground, which matters on a flat-roofed extension that faces the road, where the edge is the first thing a passer-by sees. We aim for a line that looks as if it was drawn with a rule, because an edge that is straight tends to be an edge that is secure.

Replacing trims is usually part of a re-cover, so the cost sits within the roof price. Less (last updated March 2026) puts small flat roof repairs at £150 to £500 across the UK, which is where a stretch of failed trim would land. Once we have surveyed, the quote arrives in writing with a single total. Our flat roofing page lists the full range of coverings and finishes.