DIY roof window fitting goes wrong in five predictable places: a missing insulation collar, a missing underfelt collar, the wrong flashing kit for the covering, a frame set out of square, and disturbed or badly lapped underfelt. None of them leak on the day. They surface months later as a black mould line at the head, a driven-rain leak at the bottom corners, a sash that will not seal, and cold draughts.
That is the pattern. The job looks perfect when the last tile goes back, which is exactly the trap. Every one of these faults is invisible from the finished roof and shows up in the first hard winter instead. Below is each fault, why it happens, and the symptom it produces.
The two collars that get left out
The insulation collar and the underfelt collar are supplied as separate items, they are not glamorous, and they are the two components a self-fitter most often skips because the window looks complete without them. Both do real work.
Missing insulation collar
The insulation collar is a pre-formed jacket that wraps the outside of the frame and closes the gap between the frame and the trimmed timber opening. Leave it out and you have a continuous cold bridge running right round the window, straight through the thickness of the roof.
The symptom is a cold reveal, and in January that reveal runs cold enough for room moisture to condense on it. You get a black mould line on the plasterboard, first at the head, and it comes back however much anti-mould paint goes over it. Most people report it to us as a leak. It is not a leak. No flashing work fixes it because nothing outside is wrong. The collar costs a small fraction of the window and it is the single most common omission on self-fitted units.
Missing underfelt collar
The roofing underlay has to be connected to the window frame so that any water getting past the tiles drains over the head of the frame and back out onto the covering below. The underfelt collar is the pre-formed part that makes that connection cleanly.
The DIY version is to cut the felt roughly, lap it over the frame and staple it. That holds until the first serious easterly, when driven rain gets under the covering, hits an unsealed lap and runs into the timber rather than over it. The symptom is a damp patch at the head or a stain tracking down one side, and on a shallow-pitch slate roof it is a matter of when, not if. We cover the whole sequence a professional install follows on our how VELUX windows are installed page.
The wrong flashing kit
A roof window does not keep water out on its own. The flashing kit does, and it has to match the roof covering. Flashing kits come in families: slate and low-profile for anything up to about 8mm thick, profiled tile for concrete interlocking tiles and pantiles with a deep contour, and combination or recessed kits for windows set deeper into the roof plane.
Fit a flat slate kit onto pantiles and the skirt cannot dress down into the tile valleys, so it bridges over them and leaves a channel for wind-driven water. Fit a deep profiled kit onto flat slate and there is a visible gap under the skirt. Either way the failure shows up as a leak at the bottom corners in driven rain, because that is where the water concentrates and where the mismatch leaves the biggest gap. This is the most common specification error we find on other people’s work, and it is worth understanding properly on our flashing kit explained page.
A frame set out of square
The frame is set into the opening on brackets, then packed, levelled and squared before anything is fixed. Squareness matters more than it sounds. A frame out of square by a few millimetres will not let the sash pull evenly onto the gasket. One corner touches, the opposite corner stands off.
The symptom is a sash that never quite seals. You feel it as a draught, you hear it as a whistle in wind, and on an exposed slope you eventually see it as water getting past the gasket where it does not close. People chase this with new gaskets and adjusted hinges for years without success, because the fault is not the gasket. It is the frame geometry, and that was set on the day of fitting.
Disturbed or badly lapped underfelt
Cutting the opening for a new window means working around the existing underlay, and it is easy to tear it, or to re-lap it the wrong way so an upper sheet tucks behind a lower one. Water then runs to the tear or the reversed lap instead of down and out.
This is the hardest fault to diagnose from inside, because the leak often appears well away from the window as the water tracks down a rafter before it drips. By the time it shows, the timber it has been running over has been wet for months. It is also the fault most likely to be blamed on the window itself when the window is fine.
Why these faults all replace rather than repair. By the time a built-in fault shows, the roof has been open to water at a hidden point for a season or more. We do not patch roof windows. On a self-fitted unit that is already leaking or mould-lined, the honest fix is to strip back, correct the collars and flashing, and reset the window properly, with a 10-year workmanship guarantee behind it. A misted pane on top of any of this is a separate signal, covered on our roof window misting page.
What it means locally
Ipswich and the villages around it have a lot of shallow-pitch slate terraces and a lot of loft-conversion windows fitted in a hurry between 2000 and 2010. Shallow pitch and driven easterly rain are the exact conditions that expose a missing underfelt collar or a mismatched flashing kit fastest. On the coast at Felixstowe and Harwich, add salt to the same faults and the timeline shortens again. If you have a self-fitted window and you are seeing any of the four symptoms above, it is worth a survey before the damp reaches the trimmers.
We survey, tell you which fault is actually causing the problem, and give you one fixed written price. Roof window installation is the whole of what we do, and our installers have 25 years fitting them.
Request a fixed quote and we will tell you honestly what went wrong and what it takes to put right.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.