A pitched roof rooflight flashing leak almost always comes from one of four things: the frame was set too high in the roof so the head flashing bridges the tiles instead of sitting into them, the side soakers were left short or omitted, the underlay was cut and never formed into a collar, or somebody used mortar and mastic where interleaved metal should have been. None of those are product faults. All four are geometry, and geometry cannot be corrected with sealant. That is why a diagnosed flashing failure ends in replacement rather than a patch.
This page is about diagnosis. If you already know your flashing is wrong and you want to understand which kit should have gone on and in what order, that is covered on choosing and fitting the right flashing kit. If your roof is slated, the detailing differs enough that we have given it its own page at fitting rooflights into slate roofs.
How flashing is supposed to work
A pitched roof does not waterproof itself with a continuous membrane. It sheds water by overlap. Each course of tile or slate discharges onto the course below, and the whole surface works because every joint is covered by the piece above it. Put a rectangular obstruction in the middle of that and you have interrupted every course it crosses. The flashing kit is what rebuilds the overlap around the interruption.
Four components do it, and each has a distinct job:
- The apron sits below the window and is dressed down over the covering in the course beneath, throwing water clear onto the roof surface where the ordinary lapping takes over again.
- Side sections and soakers run up the two vertical edges. Each soaker is interleaved with one course of tile or slate, tucked under the course above and lapping over the course below, so water travelling down the side of the frame is handed from soaker to soaker and never touches the timber.
- The head flashing goes under the covering above the window and is dressed down into its profile. Everything arriving from up the slope runs over the top of it and is deflected around the frame.
- The drainage gutter behind the head flashing on most modern systems catches anything that does get behind and discharges it sideways.
Read that list again and notice what is missing. Nothing in it is glued, sealed or bedded. The assembly is watertight because of how the pieces sit relative to each other, not because of any material bridging a gap. A correctly flashed rooflight would still shed water with every scrap of mastic scraped off it.
The single test that tells you most. Look at the head of the window from the ground with binoculars, or from a neighbouring upstairs window. If you can see daylight or shadow under the top flashing where it meets the course above, it is bridging rather than sitting down into the tile. That is the commonest cause of a leaking pitched rooflight and it cannot be fixed from outside. Do not go up to check. A photograph on a phone from a bedroom window tells us what we need.
The four failure modes, and how each one behaves
1. Head flashing bridging the course above
Every manufacturer publishes a fitting depth for the frame relative to the tile batten, and it differs by covering. Set the frame 20mm too high and the head flashing no longer reaches down into the trough or the surface of the tile above it. It spans across instead, leaving a channel running the full width of the window.
The signature is a leak that only appears in heavy or wind-driven rain, arrives at the top of the reveal, and often shows first as a brown tide mark on the plasterboard rather than as dripping water. In light rain the volume is small enough to run away down the underlay, so the owner concludes it only leaks in storms and assumes that is normal. It is not.
2. Side soakers omitted or short
The sides are the fiddly part of the job. Each soaker has to be cut, positioned and interleaved as the covering is relaid course by course, and on a 1400mm tall window that can be a dozen separate pieces. Someone in a hurry fits half of them, or fits them all but with too little lap, or lays the tiles back over the top without tucking the soaker under.
Short soakers work for roughly two years. What fails first is not the sheer volume of water but capillary action drawing it sideways along a lap that is 30mm instead of 100mm. The signature is water appearing at one side of the reveal only, usually the side the prevailing weather comes from, and worsening progressively year on year rather than starting suddenly.
3. No underlay collar
Beneath the battens is a breather membrane or, on older Suffolk roofs, bituminous felt. It is the second line of defence and it is not optional, because some water always gets past a tiled roof in driven rain. When the opening is cut, that membrane is cut with it. Done properly it is formed into a collar, taken up the outside of the frame and dressed out over the underlay of the course below so anything it catches is returned to the drainage plane.
Done badly it is slashed with a knife and left. Everything that gets past the tiles then lands on the trimmers and the top of the ceiling. This one is cruel because it hides. Water is arriving on timber for three or four years before a stain reaches the plaster, and by the time it does the trimmer and the rafter ends are wet and sometimes soft. The signature is a stain that appears some distance from the window rather than at its edge, and timber that smells damp in the loft when the ceiling below still looks perfect.
4. Mortar or mastic used as the weathering
This is the one we see most on older installations across the Suffolk villages, and it is almost always a second-hand fault: the original fitter used the wrong kit for the covering, left voids, and filled them. Mortar around a rooflight cracks. It is a rigid material bonded to two things that move at different rates and different temperatures, sitting in a position that freezes and thaws. Three winters is a fair average. Silicone lasts longer and fails the same way, except that it fails invisibly because it stays stuck at the edges while splitting through the middle.
The signature here is a leak that was fixed, stayed fixed for a season or two, then came back. Then was fixed again. Each round of sealant lasts a little less than the one before, because the surfaces are progressively more contaminated and the underlying gap is progressively wider.
Is it actually a leak?
A significant share of the calls we take about water at a pitched rooflight are not flashing failures at all. They are condensation from a missing insulation collar, and no amount of work on the outside will ever find anything. The two have different fingerprints and you can usually tell them apart from inside the room.
| Observation | Flashing leak | Condensation |
|---|---|---|
| Timing | During or shortly after rain | Cold mornings, whether or not it has rained |
| Season | Any time of year, worst in driven rain | October to March, worst in a still cold snap |
| Position | One edge or one corner, usually the head or one side | Evenly around all four reveals |
| Colour of the mark | Brown or grey staining, sometimes tannin from timber | Clear water, no staining until mould develops |
| Response to heating the room | None | Improves markedly with ventilation and steady heat |
| Loft above | Damp underlay, wet trimmers, sometimes visible drip path | Loft dry, the problem is entirely at the reveal |
If both fingerprints are present you may have both faults, which is common when a unit was fitted quickly with neither collar.
Condensation traced to a cold perimeter is a build-up problem, not a weathering one, and it is dealt with by fitting a proper insulation collar and taping the vapour control layer to the frame. The pitched roof skylight installation hub covers both collars in detail.
What we do at survey, and what we do not do
We survey from the ground, from inside the loft, and from the roof once access is properly set up. What we ask you to do is stay inside. Nobody should be on a pitched roof to investigate a leak, and a wet or frosted slope is genuinely dangerous. Photographs from an upstairs window, or from the garden with a phone zoomed in, are enough for us to form a view before we come out.
Inside the loft we look for the things that tell the story:
- Whether the underlay was formed into a collar or cut and left. This alone decides a lot of surveys.
- Water tracks on the trimmers and the rafter ends, and whether the timber is sound or has started to soften.
- Staining on the underside of the underlay above the window, which points at the head flashing rather than the sides.
- Whether an insulation collar exists at all, and whether the vapour layer is sealed to the frame.
- Salt staining and rust bleed at the fixings, which on coastal roofs at Felixstowe or Harwich tells us the original job used A2 grade 304 rather than 316L stainless.
From outside we check the frame depth against the manufacturer’s dimension for that covering, the profile depth of the tile or slate, whether the soakers are present and how far they lap, and whether the tiles around the opening were cut to the metalwork or the metalwork forced to the tiles.
Why the answer is replacement
Apex does not patch, reseal or make good around a failed flashing. That is a commercial position and it is also an engineering one, and it is worth explaining honestly rather than asserting.
Once a flashing has failed for any of the four reasons above, the fault is in where the frame sits relative to the covering, or in laps that were never formed. You cannot change frame depth without lifting the covering and taking the frame out. You cannot retrofit an interleaved soaker without stripping the courses it needs to interleave with. And you cannot form an underlay collar with the window still in the hole. By the time you have opened up enough to do the work correctly, the unit is out and you are fitting a rooflight. The only question left is whether you put the old one back.
Usually you should not. A unit that has been leaking for a few years has generally taken water into the frame, and on older stock the sealed unit is at or past the end of its service life anyway. A 1990s or early 2000s rooflight will be running a U-value nowhere near the 2.2 W/m2K that Part L asks of a replacement rooflight in an existing dwelling, where a modern thermally broken unit sits at 1.1 to 1.4 W/m2K. Refitting old glass into a correctly formed opening spends the labour and keeps the worst part.
The other side of the honest picture is what patching costs. A resealing visit is cheap. Two or three of them across five years, plus the damage while the leak continues, comes to a meaningful fraction of the replacement, and at the end of it the geometry is still wrong. This is the arithmetic behind our whole position, and it is set out on the hub page along with published prices for a like for like replacement of an existing unit.
Where the price lands. A like for like replacement of an existing pitched unit up to 780 x 1180mm, with the correct flashing kit for your covering, an underlay collar, an insulation collar and a taped vapour seal, runs £1,250 to £1,850 supply and fit. Where wet timber has to come out and be replaced, that is quoted after the survey and before we start. You get one fixed price, and every installation carries a 10-year workmanship guarantee.
Local patterns we see
The failures cluster by housing stock, and after enough surveys the patterns are predictable.
On the Victorian and Edwardian slate terraces across central Ipswich and the Norwich Road area, the problem is nearly always at the sides. Slate is thin and the temptation is to cut it tight to the frame, which leaves no clearance for the soaker to sit in and no room for thermal movement. The slate then cracks at the cut edge and water goes straight through. That detail has its own page at fitting rooflights into slate roofs.
On the clay pantile roofs across the Suffolk villages, it is nearly always the deep profile. A flat kit was fitted to a covering with a 50mm S-curve, every roll left a void, and someone filled them with mortar. Those are the roofs where we find fillets cracked along their whole length.
On the interlocking concrete tile of the 1960s to 1980s estates at Chantry, Rushmere St Andrew, Whitton and Kesgrave, it is the raised interlock rib. A standard kit bridges a deep rib and the gap sits directly over the side of the frame.
On the coastal belt at Felixstowe and Harwich, the flashing geometry is often perfectly good and the fixings have gone instead. A 304 stainless fixing within a mile of open sea shows rust staining in two or three winters, and once the fixing loses section the flashing lifts in a gale and the geometry follows. Coastal work from us gets 316L throughout, because the molybdenum content resists chloride pitting.
Common questions about flashing leaks
Can a leaking rooflight flashing be resealed?
Sealant can slow a leak for a season. It cannot fix one, because a flashing leak is caused by the frame sitting at the wrong depth or by laps that were never formed, and neither is a gap that sealant can close permanently. Silicone bonded across two components that move at different rates, in a position that freezes and thaws, splits through the middle while staying stuck at the edges, so it looks intact while leaking. We do not offer resealing. Once a flashing has failed, correcting it means lifting the covering and taking the unit out, at which point you are fitting a rooflight.
How do I know whether it is the flashing or the glass unit?
Water at the perimeter of the reveal, arriving during rain and staining brown, is weathering. Misting or streaking between the panes of glass that you cannot wipe off is a failed sealed unit, which is a completely different fault and does not wet the ceiling. Clear water on all four reveals on cold mornings with no rain is condensation from a missing insulation collar. If water appears at the top of the reveal only in heavy rain, look at the head flashing first.
Should I get up and look at the flashing myself?
No. A pitched roof is dangerous when it is dry and considerably worse when it is wet, frosted or mossy, and slate in particular gives no warning before it slides. Everything we need can be seen from a bedroom window, from the garden with a phone camera zoomed in, or from inside the loft. Photographs of the head of the window and of the underlay above it are more useful to us than anything you could gather at close range.
Why did the leak only start after ten years?
Because most of these faults are marginal rather than absolute. A short soaker lap or a slightly high frame will shed ordinary rain perfectly well and only fail under wind-driven rain from a particular direction. What changes over ten years is that fixings corrode slightly, laps open a fraction with thermal movement, mortar cracks, and moss builds up and holds water where it used to run away. The geometry was wrong from day one. The weather simply took a decade to find it.
Is wet timber around the opening a big job?
It depends entirely on how long the water has been arriving. Surface staining on a trimmer that is still hard is cosmetic and we leave it. Timber that a screwdriver enters easily has to come out, and that means new trimmers and sometimes new rafter ends, which is structural work rather than glazing. We check with a moisture reading and by probing at survey, and if replacement timber is needed it is priced into the fixed quote before anything starts rather than discovered on the day.
Get a fixed price
Send us what you can see. Where the water appears, when it appears, whether it follows rain, roughly how old the unit is and what the roof is covered with. We will come out, get into the loft, check the frame depth against the covering and tell you plainly which of the four failure modes you have. Then you get one written fixed price for the replacement, with the unit, the correct flashing kit for your covering and both collars itemised on it, and a 10-year workmanship guarantee behind the work.
Request a fixed quote for a leaking pitched rooflight anywhere in Ipswich, Suffolk or north Essex, or read how the flashing should have gone on in the first place on our flashing kit installation page.
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.