Roof windows sit in the middle of a roof and interrupt the one thing a roof does well, which is shed water downhill in a continuous plane. Everything about a rooflight installation is therefore about getting water around the obstruction and back onto the covering below it. When one leaks, the window itself is very rarely the problem. It is nearly always the flashing kit, the underlay detail beneath it, or the way the tiles have been cut around it. Before you replace anything expensive, it is worth knowing that the frame is usually innocent.
Condensation or a leak? Settle that first
Condensation on a roof window is extremely common, particularly in bathrooms, kitchens and bedrooms in older houses over the winter. It appears on the glass itself, runs down to the bottom of the pane and collects on the internal sill or the timber below it. It shows up on cold nights whether or not it is raining, and often on the coldest, clearest night of the week rather than the wettest.
A genuine leak behaves differently. It appears during or shortly after rain, particularly wind driven rain from one direction, and it tends to arrive at the head or the sides of the window rather than as a film on the glass. Marks on the plaster at the top of the reveal are a leak. Water on the sill with dry plaster above it is usually condensation.
- Water only after rain, worse with wind from one direction: leak
- Water on cold mornings with no rain: condensation
- Stain at the top of the reveal: leak above the window
- Beads on the glass running to the sill: condensation
How a rooflight is meant to keep water out
A properly installed roof window relies on layers. Below the tiles, the underlay is cut and dressed so that any water reaching it is carried around the frame and back onto the underlay below, usually with a purpose made drainage gutter above the head of the window. Above that, the flashing kit provides a metal apron at the bottom, side sections and a head flashing, all designed to interleave with the covering.
If either layer is wrong, the roof leaks. A perfect flashing kit over a botched underlay detail leaks in driving rain. A perfect underlay detail under a poorly fitted flashing kit leaks in ordinary rain.
The faults we find most often
The wrong kit for the covering is common on older installations. Flashing kits are made for flat coverings such as slate and for profiled coverings such as pantiles, and using a flat kit on a deep pantile leaves a channel at each side of the frame that rain runs straight through in a squall.
Second is the head detail. The top of a rooflight is where the whole roof above it delivers its water, so any shortfall shows up there first. A head flashing dressed too shallow, or one that has been distorted by someone standing on it, funnels water sideways rather than over.
Third is mortar. Where the tiles around the frame have been bedded in mortar rather than cut and mechanically fixed, that mortar cracks within a few years and the joint opens.
Fourth, and easy to miss, is debris. Moss and silt building up in the channel above a rooflight dams the water and holds it against the flashing until it finds a way in. On a roof with overhanging trees, that alone accounts for a lot of leaks.
What you can check safely from inside
Go into the room during heavy rain and look at where the water first appears rather than where it pools. Photograph it. Then, if the window is in a loft space with an accessible void, look above the plasterboard with a torch on a wet day and see whether the underlay is wet, and where.
Check the window seal too. Roof windows have a rubber gasket around the opening sash that hardens over the years, and a hinged window that has been painted around, or one whose sash has dropped slightly, will let water past the seal itself. That is one of the few genuine window faults, and it usually shows as water along the bottom of the opening sash rather than at the head.
What we would not suggest is going onto the roof to look. Rooflights are usually mid slope, and the covering around them is the hardest place on a roof to stand safely.
What the repair involves
A proper repair means stripping the tiles or slates back around the window, exposing the underlay and the flashing kit, and rebuilding the detail in the right order. Where the kit is the wrong pattern for the covering it is swapped for the right one. Where the underlay has been cut short or has perished, a new drainage detail is formed so any future water is carried away rather than dropped into the ceiling.
This is not a job for sealant. Silicone smeared around a rooflight flashing traps water behind it and hides the fault for a season, and it makes the eventual repair messier. Any roofer offering to seal a rooflight leak from the outside with mastic is buying time rather than fixing it.
Roof windows and rooflights are ordinary pitched roofing work for us, and the inspection is free with a written quotation and no obligation.
Common questions
My roof window only leaks when the wind blows from one direction. Why?
Because driving rain is being pushed sideways into a gap that vertical rain runs past. It usually points to a side flashing or a covering profile mismatch rather than a fault at the head of the window.
Do I need a whole new roof window?
Usually not. Most rooflight leaks come from the flashing kit or the underlay detail around the frame, both of which can be rebuilt while keeping the existing window, provided the frame and glass units are sound.
Is the water on my window sill definitely a leak?
Not necessarily. Water on the glass and sill on cold mornings, with dry plaster above, is normally condensation from the room. Improving ventilation and reducing moisture in the room usually cures that, and no roof work is needed.
Can moss above the window cause a leak?
Yes. Moss and silt collecting in the channel above a rooflight dam the water and hold it against the flashing rather than letting it run past. Clearing it is often part of the repair.
Do you install rooflights as well as repair them?
Yes. New rooflights and roof windows are part of our pitched roofing work, and we can look at both repair and replacement during the same free inspection.