Water stains spreading across a loft room ceiling from the corner of a roof window are one of the most common calls a roofer gets in autumn. The window itself is often blamed, and sometimes rightly. More often the glass is fine and the fault lies in how the window is joined to the roof, or in something higher up the slope that sends water towards it. A roof window is a hole in a roof with a frame, a seal and a metal skirt around it, and any one of those layers can fail. This guide takes the causes in the order we usually find them, explains how to work out which one you have, and describes the repairs. Before you start, note the golden rule: do not seal the outside of the window with silicone and hope. A sealant smear covers a symptom, traps water and makes the proper repair harder.
How a roof window keeps water out
Knowing the layers helps you read the fault. The frame is fixed to the rafters, with a trimmer above and below. The roof covering stops short of the frame all round. A flashing, a skirt of lead, aluminium or a flexible material, fits over the frame and under or over the tiles, so that water running down the roof is guided around the window and not into the joint. Under the covering, the underlay is cut and dressed up to the frame, then sealed, as a second line of defence. Within the window itself, gaskets seal the sash against the frame, a drainage channel collects what gets past, and the glass unit is sealed at its edges.
Because of those layers, a leak at the ceiling does not tell you where the water got in. It can travel along the underlay, down a rafter or across the plasterboard before dropping. That is why tracing is part of the repair. Our page on roof window flashing kits shows how the pieces fit together.
Cause one: the flashing has failed
This is the commonest fault. The flashing kit is a set of pieces that wrap the window: a bottom apron, side pieces and a top piece that sit with the tiles or slates. If the kit was the wrong one for the roof covering, was cut to fit, or was installed without the correct laps, water will find the gap. Others fail through age. Lead flashing can crack where it has been bent repeatedly or has moved with temperature changes. Aluminium kits can be dented or split. Flexible materials degrade in sunlight. Sealant used instead of lapping breaks down within a few years.
Signs are water marks at the sides or top of the window lining after heavy rain, dampness that tracks down from a corner, and visible gaps or lifted metal when seen from outside. A fix means lifting the tiles around the window, removing the old flashing, checking the underlay, and fitting a correct new kit. That usually costs less than a new window and is routine for a roofer. See fitting a roof window in a slate roof if your covering is slate, because the lead dressing differs.
Cause two: the water is coming from higher up
A tile or slate slipped above the window sends water straight down onto it. So does a broken ridge or a gap in a flashing at a chimney higher on the slope. The window then gets a flood it was never designed to cope with, and its channels overflow. If the leak appears only in a driving south-westerly, or only after rain that lasts hours, a fault above is a strong suspect. We look at the whole slope above the window, and the repair there often clears the symptom. Our page on finding and fixing a roof leak sets out the full method.
Moss and debris cause a version of the same fault. A mat of moss or a drift of leaves above the window slows water, which then backs up under the tiles and pushes into the flashing joint. Cleaning the slope above and clearing the gutters is a cheap first step.
Cause three: seals and gaskets worn out
The rubber gasket between sash and frame does a lot of work, and it hardens, flattens and cracks with age. Open the window and run a finger around the seal: if it is brittle, torn or permanently squashed, rain driven at the sash can pass. Signs are drips from the bottom rail of the sash, and wet patches at the window sill after wind and rain together, though not after calm rain. Replacement gaskets are made for most current models and fit in minutes. For an out-of-production window the choice may be a new window. See replacing an old roof window for that decision.
Cause four: blocked drainage channels
Most roof windows have small drainage paths in the frame so that rain which gets past the seal runs out at the base. Spiders' webs, grit, moss and dead insects can clog them. When blocked, the water has nowhere to go and overflows into the lining. Clearing the channels with a thin brush or compressed air is simple, and a blocked weep hole is a good reason to check the window yearly. Look for any sign of water standing in the sash recess after rain.
Cause five: condensation mistaken for a leak
Water on the glass or frame in cold weather might not be rain at all. Warm indoor air meets cold glass and drops water, which runs down and may even stain the lining. The clues are timing and pattern. Condensation appears on cold, dry mornings, is on the room side, and covers the glass or frame evenly. A leak appears after rain and drips from a point. Because the fix is completely different, ventilation and insulation instead of flashings, we recommend ruling it out first, and our guide on condensation on roof windows explains how.
Cause six: a failed glass unit or cracked pane
A cracked outer pane lets rain straight in, which is easy to see. A failed sealed unit has a worn edge seal that lets moisture and sometimes water into the cavity between panes, and it shows as misting or droplets inside the glass. That is not a roof leak but it does mean the unit has to be replaced, and our answer on replacing roof window glass covers the options. Hail or a branch can crack glass, and the damage should be made safe quickly.
Cause seven: a low pitch or the wrong window for the roof
Every roof window has a minimum pitch below which rain cannot be relied on to run clear, and a flashing kit designed for one covering cannot always be used on another. A window fitted in a shallower slope than its limit will leak in heavy rain no matter how well it is flashed. If the roof was altered, or the window moved, this can be the hidden reason. Our answer on the minimum pitch for a roof window explains the limits. Where the pitch is too shallow, the answer may be a different product, such as a low-pitch window, or a flat rooflight on a kerb, covered in flat roof rooflights.
How to trace the leak yourself
Start inside. Look at the lining and ceiling in daylight and note where the stain is, and whether it is wet now or dry. Check the loft side if you have access, looking at the timbers around the window for dark tracks. Then look outside from a safe place such as a ground-level view with binoculars or from a window. Missing or slipped tiles above, bright metal at the flashing and moss on the slope are all clues.
If you have help, a hose test works: one person soaks one section at a time, working from the bottom up, while another watches inside. Start at the lowest point and move up in stages, so that you can tell which section is at fault. Do not use a pressure washer, which forces water uphill under the tiles. Stay off the roof. A roof window and the slope around it are slippery, and a fall from a ladder onto a scaffold-free house is a serious risk. We carry out this tracing as part of a survey and photograph what we find.
What to do while you wait for a repair
If water is coming in now, deal with the room first. Move furniture and bedding out from under the drip, put a bucket or a tray under it, and lay towels or plastic sheet on the floor. If the ceiling is bulging with trapped water, make a small hole at the lowest point of the bulge with a screwdriver, with a bucket underneath, so the water drains in a controlled way instead of bringing the whole section of plasterboard down. Switch off lights and sockets near the wet area and keep an eye on any ceiling light fittings, which can fill with water.
Take photographs of the staining and the window, with the date, because they help the roofer and any insurance claim. If you can, note the wind direction and when the water appears. We prioritise leak calls and will make a leaking roof safe first, using a temporary cover or a patch to stop more water getting in, and then explain the lasting repair. Do not climb onto the roof to attempt it yourself: wet tiles and glass are slippery, and a fall is far costlier than a leak.
Repair or replace?
Many leaks mean a flashing, a gasket or a few tiles, and a repair is the right answer. We say so, because a window with a sound frame and glass may have many years left. Replacement makes sense when the frame has rotted, the glass unit has failed and the model is out of production, or when the window is so old that the flashing cannot be matched. Before deciding, we look at the lining, the timber trimmers and the insulation. If the water has been running for a while, rot may extend beyond the window itself, and a repair that does not deal with it will not last.
On cost, trade guides give typical UK ranges for a fitted roof window of £680 to £1,300, and a manufacturer's figures of £1,623 to replace and £1,906 to add, including VAT. A flashing replacement or a reseal costs a fraction of that, though we only give you a number after the survey: your fixed quote follows the survey, with photographs. If the leak is active, we make the roof safe first, covering the area to stop further damage, and then discuss the lasting fix. Our roof windows page lists the whole service, and you can phone 01256 213956 to arrange a visit.
