A roof window sits in a hole cut through the very layer that keeps your house dry, so it is no surprise that it is a common place for water to appear. What surprises people is how often the window itself is fine. The glass, the frame and the opening mechanism are built to survive decades. The trouble is usually in what surrounds them: the flashing that ties the frame to the tiles, the felt or membrane behind it, and the way water is led around the unit.
This guide runs through the causes in the order we check them, and explains how to tell a true leak from condensation, which looks identical on a wet morning. Related reading: if the unit is beyond repair, our guide to roof window replacement sets out how a swap works, and the skylights hub lists every topic we cover.
Is it a leak, or is it condensation?
Start here, because the two problems need opposite fixes. A leak brings water in from outside, and it nearly always follows rain. Condensation is water from inside the room, settling on the coldest surface it can find. A roof window is often that surface, since it is a single pane in a warm room, and the bedroom or bathroom beneath it produces plenty of moisture.
Telling them apart
Look at the timing. Water that appears after a downpour, or only when wind drives rain from one side, points to a leak. Water that shows on cold, still mornings, or after a shower, points to condensation. Look at the position as well. Droplets on the inner face of the glass, or along the bottom of the frame, are condensation. A stain on the plasterboard splay, a drip from the top of the frame or a damp patch on the ceiling beside the window are more likely to be a leak.
Both can occur together, which confuses everyone. A window that has been dripping for a long time can also leave the lining damp enough to look like a leak after the leak has stopped. Wipe the sill dry, wait for the next rain, and note when it wets again.
Our guide to roof window condensation covers the indoor side, and this page concentrates on the outside.
The flashing kit around the frame
A flashing kit is the set of pre-formed metal pieces that fit around a roof window and tuck under the tiles or slates, so that rain running down the roof goes around the frame and not into it. It comes in versions to suit different coverings: one for flat profile tiles, another for deeper profiles such as pantiles or double Roman tiles, another for slate. Using the wrong one is a classic installation fault.
If the kit does not match the profile, gaps open up at the sides where the tile shape rises and falls. Fitters then close those gaps with mortar or sealant. Both give up within a few winters, because the metal and the tile expand and contract at different rates.
Soakers and the side flashings
Where a roof window sits in a plain-tiled or slated roof, the side flashings are often lead or a lead substitute, with a soaker laid in at each course so that water runs on top of the soaker and out over the tile. If a soaker is missing, bent or has slipped, the side of the window leaks at one particular height, which is a helpful clue. Lead flashings are normally Code 4 or Code 5, and soakers Code 3.
The top and bottom pieces
The top piece of the kit sheds water to either side, while the bottom piece lets it run out onto the tiles below. If the apron at the bottom is crushed, or tiles have been raised so that the apron sits proud, water can run back under it. Our page on roof window flashings explains the parts in more detail.
Seals, glazing and the opening sash
Most roof windows have a rubber seal between the opening sash and the frame, and another seal where the glazing unit meets the sash. After many years rubber hardens and shrinks, loses its spring and leaves a gap. You may see a faint line of daylight at the edge, hear whistling in wind or find water at one corner of the sill after driven rain.
A perished seal is a repair, not a replacement. Seals are made for each window model and can be swapped without taking the frame out of the roof. The catch is availability on older windows. When a manufacturer has stopped making a certain model, spares may be hard to find, and that is one of the factors in deciding between a repair and a new unit.
A failed sealed unit
The glass itself can fail. A double glazed unit that has lost its edge seal lets damp air in between the panes, and you will see misting or droplets inside the glass that cannot be wiped off. This is not a leak, but it often gets blamed for one. The remedy is a replacement glazing unit, or a new window if the frame is also tired.
Cracked or broken glass
A cracked outer pane lets rain in directly. Our guide to broken roof window glass covers what can be done and how to make the room safe in the meantime.
Installation faults we see most often
Many roof windows that leak are leaking from the day they were fitted, or from the first hard winter after. These are the faults that come up when we survey one.
- The flashing kit does not match the tile profile, or the supplied kit was left out and the gaps filled with sealant.
- The underlay or membrane was not dressed into the window collar, so water that gets past the tiles has nowhere to drain and runs into the opening.
- Battens were cut back too far around the frame, so tiles near the window sit loose and tilt.
- The window was fitted too low on the slope, which leaves too few courses of tile below it to shed water cleanly, or too close to a valley or hip.
- The window was fixed to the rafters without packing, so the frame is twisted and the sash does not close evenly.
The underlay point is the one most often missed, and it is the hardest for a homeowner to spot. Water that gets under the tiles in any roof has to run down the underlay to the eaves. A roof window interrupts that path, so the underlay has to be formed into a collar or gutter that carries the flow around the frame. If it was not, a pitched roof with tiles in perfect condition will still leak at the window.
Where the original fitter has left, our roof window for any roof page explains how kits and methods differ across tile and slate types.
The inside: lining, insulation and cold bridges
Some leaks start with the finish inside the room. Between the frame and the plasterboard sits a packed insulation layer, and a vapour barrier or sealing strip ought to tie the frame to the ceiling lining. If that insulation was packed in carelessly, or the layer was left out, warm air reaches the cold frame and condenses inside the reveal. The staining that results is dark, mottled and appears along the splay. It can look like a leak that has soaked the plaster, but the water is coming from the room.
Rain noise and drips
Sometimes the complaint is sound, not water. Heavy rain sounds loud on a small pane, and some people mistake a drumming noise for a drip. Our guide to rain noise on roof windows goes through what helps.
Debris and standing water
Leaves, moss and bird nesting material can collect on the flashing above the frame, particularly where a window sits near a tree. The pile holds water against the metal and stops it shedding. Clearing it by hand, from a safe position, is a routine task, and our roof window maintenance page lists what to look at each year.
If you spot bats or nesting birds near a roof window, stop and get advice before disturbing anything, because both are protected.
Repair, re-flash or replace?
Once the cause is known, the choice is usually among three jobs. They differ a lot in size and cost.
Re-sealing and seal replacement
If the fault is a perished seal or a small gap in the flashing, the fix is small. A replacement seal, a tidied flashing or a single new soaker costs far less than a window, and the window can stay in place.
Re-flashing the window
If the kit is wrong, damaged or built from sealant, the tiles around the window need lifting, the old flashing comes out, and a correct kit is fitted with the underlay collar. This is more work and needs a dry day, but it deals with the cause. MyBuilder's roof window guide, updated 21 August 2026, puts installation at £350 to £600 for a standard job, labour included, with scaffolding at about £500 where it is needed. Those are typical UK figures, and a repair costs well below them.
Replacing the unit
If the frame is rotten, the glazing has failed, or the window was badly sized or placed to begin with, a new unit is the sounder route. Because the opening and flashing are already there, a like-for-like swap is simpler than a first-time fit. Whichever way it goes, you will know which of these jobs you are paying for, because our price is set down on paper after we have looked at the roof.
Putting sealant on a leaking flashing slows the water for a few weeks, then hides the evidence a roofer needs. Photograph the stain, the date and the weather first.
What to do while it is still dripping
Put a bowl under the drip, lay a towel on the sill and shift anything precious clear of the spot. If water is running down the wall, switch off lights in that area and check whether any electrics are wet. Stay off the roof to look; a damp slope with an open window beside it is a bad place to be.
If the room is usable, leave the window shut and the blind up so you can see what is happening. Wait for dry weather, then take a photograph of the outside of the window from the ground, using zoom, and of the inside showing the stain. Both help a roofer judge the job before arriving.
A leaking roof gets priority from us, and we make it safe before anything else. After the survey you get a written quote and photos taken on your own roof. Workmanship is guaranteed for 10 years on paper, and nothing is left behind when we go. Our guide to leaking flat skylights may be more useful if your unit sits on a flat roof, and the enquiry form or WhatsApp is the fastest way to send us pictures.