A roof window is a hole in a roof with a window in it, and water has to be led around that hole by something. The something is the flashing kit. It is a set of purpose-made parts, usually metal, that wraps the window frame and overlaps the roof covering at the sides, the top and the bottom, so rain runs over the joins and away.
Kits come in different forms because roof coverings differ. A kit shaped for thin slates is wrong for thick profiled tiles, and one for plain tiles sits differently again. The frame depth matters too, since a window may sit proud of the roof or be recessed into it. Ordering the right kit means knowing the covering and the fitting height before the window is bought.
This guide explains what a kit contains, the main types by covering, why flashing faults are a common cause of leaks, the order in which the parts go on, what is renewed when a window is swapped and what to ask before the work starts, for roof windows near Farnborough and in the surrounding area.
What a flashing kit is made of
A kit is a set of pieces designed to work together, and each has a job on one side of the window.
The parts
- Bottom apron: the piece below the window, which carries water from the glass and frame onto the covering beneath.
- Side flashings: pieces up each side, which stop water getting between the frame and the covering at the sides.
- Top flashing: the piece above the window, which tucks under the covering higher up and overlaps the head of the frame.
- Underlay collar: a membrane sleeve that connects the roof's underlay to the window frame so that any water reaching the underlay is led around the window.
- Insulation collar: a frame of insulation around the window, which limits cold bridging at the edge of the opening.
The materials
The flashing pieces are normally aluminium, finished in a colour to suit the covering, or lead in some types. The collars are foil or membrane. The exact materials depend on the maker and the series.
Why it is a kit
The parts are cut to fit a given window size and a given depth of covering, so they fit without bodging. Mixing parts from different kits, or cutting down a kit made for another window, risks gaps at the corners.
What the kit does not do
It does not make a poor roof sound. A kit relies on the covering around it being in good order, with tiles properly fixed and the battens firm. If the surrounding roof is worn, our guide to replacing a Velux window explains how the window and the covering are planned together.
Kits by covering type
The thickness and shape of the roof covering decides which kit fits. Makers publish the options in their data sheets, and a roofer matches the kit to the covering and the window.
Slate
Slate is thin and flat. A slate kit has shallow flashings that lie close to the surface, with side pieces that step under and over the slates as the courses rise. The thinness means less height to bridge, and the flashings can be slim.
Profiled tiles
Profiled tiles, such as pantiles and interlocking concrete tiles, have raised and lowered sections. The flashing has to follow the profile or be formed with a soft edge that conforms to it. A kit for profiled tile is deeper and more flexible than one for slate, and the bottom apron has to clear the raised sections. The kit's data sheet states the range of profiles it suits.
Plain tiles
Each plain tile is small and every course overlaps the one below, which leaves the flashing a lot of small steps to follow. A plain-tile kit usually has side pieces designed to be tucked between the courses, and extra care is needed at the lap lines.
Recessed fitting
A recessed kit sits the window lower in the roof, so it is flush with or just below the covering. It gives a smoother look and a lower profile, and it is used where the owner wants a neat finish. A recessed kit has its own components and drainage detail, and cannot be swapped with a standard one.
Choosing
The roofer names the covering, measures its depth and picks the kit from the maker's list. If the roof has two types of covering, say an extension in a different tile, each window gets its own kit.
Why flashing faults cause leaks
A roof window can leak because of the glass, the seals or condensation, but a failure at the flashing is a common cause. The reason is geometry.
A join in a sloping surface
The flashing is where two different systems meet: the window and the covering. Rain runs down the slope, hits the window frame and has to be directed around it. Every joint is a path water can follow if it is poorly overlapped, and the ones that matter most are at the corners and at the top, where water arrives with the most force.
Common faults
- An old kit reused with a new window, or the wrong kit for the covering.
- Pieces overlapped the wrong way, so water runs under instead of over them.
- Sealant used as the main defence, which dries, cracks and lets go.
- A top flashing that does not tuck far enough under the covering above.
- Corners cut or folded badly, leaving a gap.
- The underlay collar left out, so water in the underlay reaches the frame.
How leaks show up
Stains on the lining around the window, drips on the sill, damp patches on the ceiling below and mould at the corners all point toward the flashing. Condensation can look similar, so a diagnosis comes first. Our guide to why a Velux window leaks sets out how to tell the causes apart.
Why repair is not always a sealant job
A line of mastic can mask a fault for a season. A proper repair resets the flashing so that its overlaps do the work, and the sealant is only a finish.
The order a kit is fitted
Water runs downhill, so each piece of a flashing has to sit in the right relationship to the pieces above and below it. The order of fitting follows that logic, and it is the part most often done wrongly.
Preparing the opening
The frame is fixed to the rafters and the header at the heights the maker specifies, the underlay is cut and dressed to the frame and the underlay collar is fitted so that the underlay is sealed to it. The insulation collar goes in next. The battens around the frame are positioned so the covering will finish at the right level.
From the bottom up
The fitting is done from the bottom of the window upward. The bottom apron goes on first, lapping over the covering below. The side flashings follow, set so their lower ends overlap the apron and their edges are tucked between or under the covering. The top flashing goes on last, tucked beneath the covering above and lapping the sides.
The covering around it
Slates or tiles are then laid up to the flashings, cut where needed and fixed as the roof requires. Edge pieces are cut and fixed individually, so none can lift in the wind. Our guide to slipped roof tiles explains why cut tiles at an opening are the first to move if they are poorly fixed.
The finish
The cover trim is clipped on, the lining inside is completed and a final check is made against the maker's instructions. Photographs are taken of the flashing before the final pieces cover it.
The instructions that come with a kit set the order and the overlaps. A roofer who follows them, piece by piece, is far less likely to leave a leak.
When a window is replaced or added
The flashing job differs depending on whether the window is new, swapped or already in place.
Swapping a window
A new window normally comes with a new kit, and the old flashing is removed. Reusing the old flashing risks leaks, because its dimensions and fixings do not match the new frame. The covering around the opening is lifted and relaid, and any slipped or cracked tiles are replaced at the same time.
Adding a window
A new opening needs structure, then the kit. The rafters are trimmed, the frame set and the kit fitted as above. Our roof windows and skylights service covers both routes.
When the roof is being re-covered
If the roof is being re-roofed, the flashing kit goes in as part of the work, and it is easier to do properly because the covering is off. It is the moment to renew an old window's flashing even if the window itself is staying, and to check the lining and frame. The window and the roof then finish together.
Repairing without replacing
A leaking flashing can sometimes be reset without replacing the window. The covering is lifted, the flashing is checked and reset, and tiles are relaid. If the frame is sound, that costs less than a new window.
Planning and approvals
Planning and Building Regulations apply to the window, not the flashing. For Class C permitted development and Part L figures, see our guides to the other roof window topics.
Whatever the case, a photograph of the flashing before it is covered is a good record to keep.
What to ask before work starts
A few questions tell you whether a roofer has thought about the flashing, which is the part that decides whether the window stays dry.
About the kit
- Which kit is being used, and is it made for my roof covering and window series?
- Is it a standard or recessed fitting?
- Does the kit include an underlay collar and an insulation collar?
About the fitting
- Will the covering around the window be lifted and relaid?
- Will tiles cut at the edges be fixed individually?
- Is sealant used as a finish only, and not as the main seal?
About proof
- Will there be photographs of the flashing before it is covered?
- What does the maker's instruction sheet say, and will it be followed?
Cost context
MyJobQuote (Sep 2026) gives a typical UK range of £1,100 for a standard mid-range Velux window, and around £100 less for a straightforward swap. The kit is part of that job, and its type depends on the covering. The written price after a survey states what is included, with VAT shown.
After the job
Keep the data sheet, the kit type and the photographs. If a leak appears later, they tell a second roofer exactly what is behind the finish.
When to call
If you see a stain or a drip around a roof window, do not wait. Water in the lining and insulation damages the structure, and a small leak is far easier to fix than a rotted rafter.
