A roof window flashing kit is the set of weatherproofing sections that keeps water out where the window meets the roof, sitting between the frame and the surrounding covering. It is not part of the window and it is not one-size-fits-all. The kit has to match the roof covering profile and the depth the window is set at, because both change how water has to be led around the frame.
Get this right and the roof is watertight for the life of the window. Get it wrong and you have built in a leak on day one that only shows up in driven rain. Here is how the kits divide up and why the match matters twice over.
What the flashing kit actually does
Water arriving on a roof runs downhill over the covering. A window interrupts that path. The flashing kit takes the water that would have run where the window now sits and leads it around the frame and back onto the covering below, without letting any of it reach the timber. It does that with a head section above the window, side sections down each edge, and an apron skirt below, all lapped so that every joint sheds water downhill onto the piece beneath it.
The skirt is the part that has to sit against your particular covering. That is why the same window takes different kits on different roofs, and why you cannot treat flashing as a generic accessory.
The four kit families by covering profile
Flashing kits are grouped by the profile and thickness of the covering they seal against. Match the covering to the family and the skirt sits tight.
| Kit family | Suits | Approx covering thickness or profile | Where we fit it locally |
|---|---|---|---|
| Slate / low-profile | Natural slate, fibre-cement slate, plain clay tiles | Up to about 8mm, near-flat | Victorian terraces off Norwich Road and Spring Road |
| Profiled tile | Concrete interlocking tiles, pantiles | Up to about 120mm profile depth | Post-war estates and Suffolk pantile roofs |
| Combination / recessed | Any covering, window set deeper for a flatter look | Recessed into the roof plane | Conservation streets, listed and Article 4 areas |
| Twin / multiple | Two or more windows side by side or stacked | Any, with a designed gutter between units | Larger loft conversions and new-build plots |
Slate and low-profile
For natural slate, fibre-cement slate and plain clay tiles, anything up to about 8mm thick. The skirt is shallow and sits tight to a near-flat covering. On an Ipswich Victorian terrace with Welsh slate this is the correct kit, and fitting a deep profiled kit instead leaves a gap under the skirt you can see daylight through.
Profiled tile
For concrete interlocking tiles, pantiles and anything with a deep contour, up to around 120mm of profile depth. The skirt is deeper and pleated so it can be dressed down into the tile valleys and up over the rolls. Suffolk pantiles need this. The wrong kit here is what produces the classic wind-driven leak at the bottom corners, because a flat skirt bridges the valleys and leaves a channel straight to the frame.
Combination and recessed
Used where the window is set deeper into the roof plane so it sits flatter against the covering, which conservation officers often ask for on sensitive properties. More sections, more labour, and a genuinely better look on a period roof. This is frequently what gets approved in central Ipswich, Lavenham and Dedham where anything standing proud of the roof is restricted.
Twin and multiple
Where two or more windows sit side by side or one above another, the kit includes the vertical or horizontal jointing sections and a designed gutter that carries water between the units and out. You cannot make a twin installation by butting two single kits together, though we have found plenty of roofs where somebody tried, and the joint between them is always where it leaks.
Why the kit must match both the covering and the depth
Covering is the obvious half. The less obvious half is installation depth. The same window can be set flush with the roof plane or recessed into it, and the flashing has to suit whichever you have chosen. A standard kit on a recessed installation, or a recessed kit on a flush one, leaves the skirt at the wrong height relative to the covering and the lapping geometry stops working.
So there are really two questions the flashing answers, not one. What is the covering, and how deep is the window set. Both have to be right for the water to be led cleanly around the frame. This is also why you cannot reuse an old kit on a new window even at the same size code: the frame depth changes between generations, and the flashing that suited the old depth may not suit the new one.
The flashing kit is a specification decision, not a leftover part. A cheaper quote that does not name the covering it is priced for is a quote that has not decided the flashing yet. Ours states the covering, the profile depth and the kit family before you commit. It is one of the built-in faults we most often find on self-fitted and cut-price work, set out on our DIY installation problems page.
What it means for your roof
At survey we check the covering type, measure the profile depth, and confirm the depth the window will be set at, then specify the kit against all three. On slate we also allow for the shallow pitches common on Suffolk terraces, where a low pitch lets driven rain further up under the covering and the flashing lap has to be generous. The detail specific to slate is covered on our slate roof installation page, and the full fitting sequence on our installation process page.
If you are replacing failing windows on a slate roof, the flashing choice runs alongside the slate reinstatement decision, which we cover on replacing a VELUX on a slate roof. And roof window installation as a whole is the only thing we do, so the flashing gets specified properly on every job rather than assumed.
Request a fixed quote and we will confirm the exact flashing kit for your covering at survey, priced in and stated on the quote.
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.