A drip on the sill or a brown ring on the reveal sends most people straight to the glass. In practice the glass is rarely the culprit. A pitched roof window is a hole cut in a sloping roof and then sealed with a frame, a sheet-metal flashing, a layer of underlay and a ring of insulation, and the leak is nearly always at one of those joins. Sometimes the water is not coming through at all, but forming on a cold surface inside the room.
Here we cover the pitched case: the flashing kit, the seals, the water channels around the frame, condensation and the repair itself. Water on a flat roof is a different problem with different causes, so for that see our answer on a flat roof skylight that is leaking. For the whole range of work on windows in the roof, start at our roof windows and skylights page.
Rain, condensation or a real leak: telling them apart
Before anyone goes up a ladder, it helps to decide what kind of water you are looking at. Condensation is moisture from the air in the room settling on a cold surface, and a roof window frame or glass pane is often the coldest surface in a bedroom. It shows as misting on the glass, beads of water along the bottom of the frame, or black mould on the lining. It tends to come with cold mornings and wet clothes drying indoors, and it often eases when the room is ventilated.
A true leak behaves differently. It follows rain rather than temperature, so the stain grows after a downpour and stops when the roof dries. The water may arrive some distance from the window, running along a rafter before it drops. A dry spell followed by a wet one that brings new staining is the strongest clue.
Quick checks you can make
Look at when it happens. Water at the top corners of the frame after heavy rain points to the head flashing or the underlay. Water at the bottom corners points to the apron or the cill. A ring of damp around the whole lining, worse on cold still nights, points to condensation or a missing insulation collar. We cover the second group in our answer on roof window condensation.
Why the distinction matters
Replacing a flashing kit to cure condensation wastes money, and treating a flashing leak with a dehumidifier delays a repair while the timber gets wetter. We would rather spend an hour working out which one it is than guess. Our survey photographs the opening from outside and in, so you see the same evidence we do.
Flashing kits that have failed or never fitted
A flashing is the metal strip that bridges the gap between the window frame and the roof covering. For roof windows it is usually supplied as a kit, matched to the model and to the covering it sits in. The kit includes an apron at the bottom that sheds water onto the tiles below, side pieces that channel water past the frame, and a head piece that covers the top.
Wrong kit, wrong covering
The most common installation fault is a kit made for one covering used on another. A kit for flat interlocking tiles sits at a different height from one for plain tiles, and a kit for slate behaves differently again. Where the covering is thicker or uneven, as it is on Cotswold stone slates, the standard pieces may leave gaps that only show in heavy rain. These roofs often need lead dressed by hand instead of the stock pieces.
Age and movement
Metal flashings move as they warm and cool. Over many years the edges lift, the fixings loosen and the laps open. In older homes the original flashing may have been bodged by a later repair, with sealant smeared across a gap that the metal was supposed to cover. Sealant is a stopgap: it hardens, splits and lets water in behind itself, so a window with a lot of mastic around it is often a window that has leaked before.
Lead as a repair material
Where a flashing needs reforming rather than replacing, lead is the traditional material. Leadworx calls Code 4 the standard lead for general flashing. Checkatrade's repair guide, updated October 2026, puts lead flashing repairs at £50 to £70 per metre and stepped flashing at £80 to £120 per metre in Code 5, excluding VAT. Those figures are for flashing in general, not for roof windows alone. See our guide to roof window flashings for how the pieces fit together.
Seals, gaskets and the frame itself
The next place to check is the window. A roof window has a rubber or silicone seal between the opening sash and the frame, and sometimes a second seal inside it. These perish with sun and age, and a seal that has flattened or cracked lets rain in at the sill when wind pushes against the glass. You can often see the problem: a gap along one edge, a corner where the gasket has pulled away, or a patch that has gone hard and shiny.
Replacing a seal
Seals are made for the model, so a replacement has to match. If the manufacturer still makes the part, fitting it is a short job that does not involve touching the roof covering. If the model is old and the part no longer exists, the repair choice shifts towards replacing the unit, covered in our guide to replacing an old roof window.
Timber and glazing
Painted timber frames need their finish kept up. Where the coating has worn at the bottom rail, the wood takes in water and swells, and the gap that results is mistaken for a leak at the seal. Failed glazing is a different matter: when the sealed unit misting between its panes has lost its edge seal, the problem is the glass, not the roof, and the answer is to replace the glass.
A drain channel that has blocked
Many frames have small channels or weep holes designed to let any water that gets past the first seal drain away. Debris, paint and spiders can block them, and the water then has nowhere to go but inside. Clearing the channel with a soft brush costs nothing and is one of the first checks we make on any window that leaks only in wind-driven rain.
Underlay, battens and the water you never see
Behind the covering, the roof window opening is lined with underlay, and that layer is the second line of defence. Underlay is meant to be cut and lapped around the frame so water is carried to the edges and out at the bottom. If it is cut short, folded the wrong way or left with holes from old nails, water runs behind the flashing and drips down the lining.
Signs of a hidden fault
A leak that gets worse over several winters, or that appears only in prolonged rain, often means the underlay has failed rather than the visible flashing. The timber around the opening is the evidence. A soft batten, a dark patch on a rafter or a stained trimmer shows that water has been getting in for some time. We probe these with a bradawl during a survey, because rotten timber that is left in place will undermine whatever is put on top of it.
Insulation collar and cold bridge
The ring of insulation around the frame does two things: it keeps the cold out and it stops warm air meeting a cold surface. When it is missing or squashed, the lining around the window runs cold, and condensation forms there. People often take the resulting damp patch for a roof leak. If the collar is the issue, the cure is cheap compared with a flashing replacement.
If a stain keeps coming back after the flashing has been resealed, the cause is more likely to be under the covering than on top of it.
Gutters and overflow
A window set in a slope below a blocked gutter or a valley can be swamped by water overflowing from above. Check that the roof above the window is shedding water as it should before blaming the window itself.
How the repair is carried out
What the repair involves is set by the findings, and no two roofs are alike. A local fix suits a lifted flashing edge, a dry seal or a blocked channel. A rework suits a flashing that has failed throughout or an underlay that needs to be redressed. A replacement suits a frame that is rotten, a unit whose parts are no longer made, or glass that has lost its seal.
The sequence on a flashing repair
The roofer lifts the covering around the window for a few courses either side and above, and sets the slates or tiles aside in order. The old flashing and any sealant come away, the underlay is inspected and renewed where damaged, and the battens and trimmer are checked. A new or reformed flashing is fitted, the underlay is dressed over it so water runs outward, and the covering is returned. Every slate goes back in its own course.
What it costs
MyBuilder's roof repair guide, updated 21 August 2026, puts minor repairs at £100 to £300 and moderate repairs at £500 to £1,000. Scaffold, which the same publisher's roof window guide puts at about £500, is the item that moves a small job into a larger one, and on a steep Stroud lane the access can matter more than the repair. Your own written quote, issued after the survey, covers flashing, materials, labour, access, waste and making good.
Stained plaster and lining wait until the roof has dried out. A fix is only proven by the next heavy rain, so let us know if any staining comes back.
Choosing between a repair and a new window
Repairing is usually the first thing to try, and it is often the right answer. A window with a sound frame, good glass and a flashing that has simply aged can be given a new kit and carry on for years. There are limits, though. If the frame has rotted, the glass has failed, the mechanism no longer works, or repeated patching has made the roof around it a mess of sealant and offcuts, a new window is the better investment.
What tips the balance
Three things push towards replacement. Age comes first, because an old unit may not meet the current energy standard and may not have spares. Condition comes second, since rot in the frame or trimmer is expensive to chase repeatedly. Cost is third: if the repair needs scaffold anyway, the extra to fit a new unit is often smaller than the first quote suggests, because the access is already paid for.
Different roofs, different problems
Not every window sits in a plain tiled roof. On a conservation-area house with a low-profile steel-look unit, repair parts and flashing details differ, and our guide to conservation rooflights explains that product. For a flat roof with a rooflight on a kerb, the logic is closer to a kerb and membrane problem, covered in our guide to rooflights for flat roofs.
After the repair
The work carries our 10-year workmanship guarantee in writing, covering our installation and not the window's own parts. Site clearance and waste removal are part of the job. If a leak is active and the room below is at risk, tell us when you call, since a leaking roof gets made safe first and leaks and storm damage are our priority.