Skylight leak repair that finds the real gap first

A skylight is a hole in the roof with a frame around it, and the roof has to shed water around that frame on all four sides. That job is done by a flashing kit: a back tray above the skylight that catches water running down the roof and diverts it to either side, side flashings that run down each edge, and an apron at the bottom that laps out over the tiles or the sheet. On a tile roof the tiles either side are cut to fit and sit on the side flashings. On a metal roof the flashings are dressed into the ribs of the sheet.

When a skylight leaks it is usually one of these parts, not the glazing. The back tray was made too narrow and water runs over the end of it in a downpour. Leaf litter and moss build up behind the back tray until water dams and climbs over the top. A cut tile beside the skylight cracks and lets water under the side flashing. The sealant along the apron dries out and lifts. On older units the flashing was galvanised steel and has rusted at the folds. On a tubular skylight the seal between the dome and its base perishes in the sun.

All of that is flashing and seal work, which is why a leaking skylight is a job for a licensed roof plumber even when a tiler or a skylight company put the unit in.

Water runs downhill, so we start above the skylight and work down. The back tray first: how wide it is, whether litter has dammed behind it, whether the tiles above lap onto it properly. Then the side flashings and the cut tiles or sheet edges that sit on them, because a cracked cut tile is one of the most common things we find. Then the apron at the bottom and the sealant along it. Then the frame itself: the seal between the glazing and the frame, the dome seal on a tubular unit, and the sash seal on a roof window that opens.

Inside, we look at the shaft. Where the stain sits tells us which side the water is coming from, and whether it is a leak at all. A skylight over a bathroom, laundry or kitchen often has condensation running down the inside of the shaft on cold mornings, which looks like a leak and is not one. We say which it is, because the fix is ventilation, not flashing.

If the entry point is not obvious from the roof we run water over the skylight from a hose, one section at a time, and watch the shaft. Everything is photographed from the roof and from below before any repair, so you see exactly what failed.

Many skylight leaks are fixed on the roof without touching the unit. A narrow back tray is replaced with a wider one, with a diverter so water is pushed clear of the skylight instead of over it. Litter behind the tray is cleared and the tiles above re-seated. Cracked cut tiles either side are replaced and bedded so they sit flat on the side flashings. Rusted galvanised flashings are replaced with Colorbond steel or lead, dressed to the tile profile or the sheet rib. A dried apron seal is cut out and re-sealed with a flexible sealant rated for roofs, not a tube of bathroom silicone.

Tubular skylights get their own treatment: the dome is lifted, the base seal and the dome gasket are replaced, and the base flashing is checked and re-dressed. Roof windows that open are checked at the sash seal and the drainage channels around the frame, which block with litter and back water up into the room.

Skylights on metal roofs are quicker, because the flashing kit is a single piece; skylights on tile roofs take longer because the cut tiles come out and go back. Either way the work is photographed finished, and we check the rest of the roof around the skylight while we are up there, because a skylight near a valley or a wall often shares its leak with a flashing next door.

Sometimes the flashing is fine and the unit has failed. An acrylic dome that has gone brittle and crazed in the sun will crack, and once it cracks it will not seal. A double-glazed roof window whose edge seal has failed fogs permanently between the panes; it is not leaking yet, but the seal that failed is also the one keeping water out. A timber-framed roof window that has been wet for years can rot at the sill. A skylight that was fitted with a flashing kit meant for a different roof pitch or a different tile profile will never sit right no matter how much it is re-sealed.

When that is what we find, we say so before any flashing work, so you are not paying to re-flash a unit that needs to go. The office will tell you whether a like-for-like replacement is a job we can quote or one for the skylight supplier; either way, the flashing around a new unit is roof plumbing work, and our roof plumber can do that part rather than leaving it to whoever fits the unit. If the honest answer is that a re-seal will hold for now and the replacement can wait, we say that too.

Every skylight job comes with photos from the roof and from inside the shaft, before and after, so the decision is made on what is actually there.