Do rooflights leak?

On a pitched roof

Nearly always the flashing kit. The most common single defect in this trade is a flat profile kit fitted to a profiled tile, leaving a gap under every roll.

After that: the underlay collar omitted or badly dressed, so water on top of the membrane goes into the roof rather than around the opening. And battens left unsupported over the opening, dropping and taking the tiles with them.

On a flat roof

Nearly always the kerb. Too low for the covering to terminate, no roof margin to detail into, or water ponding against the upstand.

The one that is not a leak

A large share of reported leaks are condensation: an uninsulated kerb or an unsealed vapour control layer producing water on the inside. It looks identical and it is fixed completely differently.

The test: does it happen in cold dry weather?

Getting it right

The correct kit for the covering, the underlay collar dressed properly, the insulation collar not omitted, the kerb at the right height and square, the covering terminated under the flange, and the unit positioned where water reaches it moving.

None of that is difficult. All of it is easy to skip.

Our guarantee

Covers the workmanship for ten years. If water gets in because of how we fitted it, we come back and fix it, and the labour is included. Detail on the guarantee page.

Properly fitted ones do not

A rooflight fitted correctly into a sound roof should not leak for twenty five years. The units themselves are not the problem: they are manufactured to a tight tolerance and tested.

Almost every leaking rooflight we are called to is an installation fault, and it is nearly always one of four things. All four are invisible once the job is finished, which is exactly why they get skipped.

1. The wrong flashing kit

Flashing kits come in profiles matched to the covering: flat for slate, deep for profiled interlocking tiles, and specific patterns for pantiles and other shapes.

A flat kit fitted to a profiled tile leaves open voids under the tile channels where it crosses them. Water gets under within a season, and because it enters below the flashing it appears inside as a leak that seems to come from nowhere.

This is the most common fault we find on other people’s work, and it is exactly where a cheap quotation saves money, because the kit is a separate order line and nobody can see which one was used.

2. The underlay collar

Every tiled or slated roof has an underlay beneath the covering, and water reaching it must be carried around the opening rather than into it.

The collar has to be dressed and lapped so the membrane above the window drains onto the collar and the collar drains onto the membrane below. Get the laps the wrong way round and the roof funnels water into the opening rather than past it.

3. Sequence

The covering goes back over the flashing, never under it. Reversing that is a guaranteed leak, and it happens more often than it should on rushed work.

4. Standing water, on a flat roof

A different failure mode. Where the deck does not fall away from the kerb, or the kerb is too low, or the outlet is blocked, the unit sits in a puddle after every shower. A seal designed to shed water is not designed to be submerged.

What is not a leak

Water on the inside face of the glass on cold mornings, unrelated to rain, is condensation. Water on the plaster reveal rather than the glass is usually a missing insulation collar. Moisture between the panes that cannot be wiped off either face is a failed sealed unit at end of life.

Three of the four things reported to us as leaks are one of those.

What to ask before you buy

Which flashing kit, by name and part number, for your covering. Whether the insulation collar is included. On a flat roof, the kerb height and whether its outer face is insulated. And what the workmanship guarantee covers and for how long.

Properly installed ones do not

A rooflight is not a hole covered over. It is a designed junction, weathered by four layers working in sequence, and when that sequence is right it is as reliable as the rest of the roof.

The units that leak leak because of how they were fitted, and the faults are the same small set every time.

The installation faults

The wrong flashing profile. A flat kit on a deep profile tile bridges the rolls and leaves open channels in the troughs, straight under the flashing.

No underlay collar, or one not dressed and lapped. Water that gets past the covering in driven rain reaches the membrane, runs down, arrives at the opening and has nowhere to go.

The covering laid under the flashing rather than over it. The lap runs the wrong way and water is asked to climb.

Reliance on sealant. A bead of silicone is a detail finish, not a weathering strategy.

Too close to the ridge, with fewer courses above the head than the flashing kit requires.

The maintenance faults

Debris in the sill drainage channel, holding water against the gasket. This is what causes a sound window to start letting water in after fifteen quiet years, and a wipe once a year prevents it.

The thing that is usually not a leak

Water appearing in cold weather without rain. That is condensation, from an uninsulated kerb or reveal, or from a vapour control layer never sealed to the frame.

The diagnostic is timing, and it costs nothing. After rain means water from outside. In cold weather regardless of rain means moisture from inside.

Getting that distinction right before anybody goes on the roof saves a great deal of money.