What Causes Leaks Around a Flat-Roof Edge?

The Four Edges

A flat roof has a perimeter on every side, and it is not the same detail all the way round. Working out which you have tells you what to look at.

The drip edge is where the roof discharges into a gutter. The covering has to be carried over the edge and down into the gutter so water leaves the roof rather than running back underneath.

The welted or bonded free edge is where the roof stops at open air with no gutter - over a canopy, or at a verge. The covering is folded and bonded to itself and fixed down.

The trim - a preformed metal or GRP section clipping or fixing over the edge, common on modern and recovered roofs.

The upstand, where the roof runs into a wall rather than stopping. A different problem and the commonest single cause of a flat roof leak.

Why Corners Go First

Wind loads a roof by suction, and the suction is strongest at edges and strongest of all at corners, where two edge zones meet.

On the West Kirby seafront a flat roof corner is in the most aggressive part of the wind field on the whole building.

A corner also has the most complicated geometry. The covering has to turn through ninety degrees while staying bonded, and a trim has to be mitred and joined there. Every system has a corner detail and every system's corner detail is the hardest part of the installation.

So the practical rule for inspecting a flat roof after a blow is: look at the corners first, then the edges, then everything else. A corner standing proud, or a ripple running back from one, means an air path has opened up and each subsequent gust will take it further.

How Each Edge Actually Fails

The drip edge: the covering has not been carried far enough over the edge, so water clings back underneath by surface tension and runs along the underside of the roof and into the fascia. A stained or rotting fascia below an apparently sound flat roof is nearly always this.

The welted free edge: the bond fails at the fold, usually starting at a corner. Once the fold opens, wind gets in under the whole covering.

The trim: two failure modes. It works loose - fixings back out, joints between lengths open up. Or, worse, it was fixed through the covering without the fixings being over-flashed, so there are screw holes in the waterproof layer at the exact point where water runs past. On a ponding roof those fixings sit under water permanently.

The upstand: carried up too low, so driving rain gets over the top of it; or the flashing capping it has lifted or was never chased into the wall properly; or it was sealed with mastic rather than formed.

The Coastal Multiplier

Two things make all of that worse on this side of the peninsula.

Uplift. Facing the mouth of the Dee with nothing in the way means more gusts, stronger, more often. A detail that is marginal inland is tested to destruction here.

Salt on the fixings. The trim itself is usually aluminium or GRP and copes fine. What corrodes is the screws and clips holding it, and salt accelerates that considerably. So the West Kirby version of a failed edge is usually a sound trim that has come loose, rather than a broken one.

That is worth knowing because it changes the repair. Refixing a trim with the same specification of fixing that just corroded is a repeat visit. Non-ferrous or stainless fixings are not a refinement here.

And sand. This stretch gets sand as well as salt, and sand in a gutter below a drip edge stops it draining, which backs water up to exactly the detail that is least able to cope with it.

What a Good Edge Repair Includes

Establishing which detail it is before pricing anything.

Removing rather than covering. A trim that has come loose gets taken off and refixed properly, not sealed over.

Fixings that are appropriate for the exposure, and over-flashed or sealed properly where they penetrate.

The corners done properly, because they are where it will start again.

And a look at the gutter below the drip edge while somebody is there, because on this coast it will have sand in it.