When a roof window is finished, what you see is glass, a frame and a neat border of roof covering. The border is where the flashing lives. It is a set of metal pieces that overlap the covering and the frame so that water running down the roof is carried round the window and back onto the slates or tiles below it.
This guide explains what a flashing does, how the kit differs from one roof covering to another and what to look for in the work. It is hidden once the job is finished, yet it keeps the window dry for as long as the roof lasts, so the fitters need to take real care over it.
What a flashing kit does
A roof window sits in a hole cut through the covering, battens and underlay. Rain that runs down the slope reaches the window and has to go somewhere. The flashing guides it round the sides, over the head and away from the sill, so none of it can find a way under the frame.
The kit is made for the window and for the type of covering. It has a lower apron, two side pieces, a top piece and a collar that sits against the frame. Each piece laps the next like a tile laps its neighbour, so water always passes over an edge and never into a joint. A kit that suits the roof covering also makes the finished window look as if it belongs in the roof.
Why the covering decides the kit
A slate is thin and flat. A plain tile is thicker and curved slightly. An interlocking concrete tile has a deep profile with ribs. A clay pantile has a pronounced wave. A flashing that fits one of these cannot be used on another, because the gap between the covering and the metal has to be closed along its whole length.
For that reason a flashing is chosen by the covering, not the window. Makers sell kits graded by the covering depth, and the installer reads the roof and picks the right one. A wrong kit leaves a gap at the sides, a bulge in the covering or a flashing that cannot lie flat.
| Covering | Profile | What the kit needs to do |
|---|---|---|
| Slate | Thin and flat | Lie low against the slate and let the slates lap over the side pieces |
| Plain tile | Slightly curved, laid in courses | Follow the course lines, with tiles cut cleanly to the flashing |
| Interlocking tile | Deep ribs | Fill the rib depth so water cannot slip under the side pieces |
| Pantile | Deep wave | Match the wave with a profiled apron and side pieces |
Slate roofs
Slate is the thinnest covering, and many York terraces and Victorian houses have it. The kit for slate is shallow, with side pieces that the slates lap over, so water runs from the slate onto the metal and back onto the next slate. The key is to lift the slates carefully, since old ones can be brittle.
We cut slates to fit round the window rather than forcing them, and we use new slates where an old one has broken. The result is a covering that closes tightly to the frame without visible gaps. Our roof window installation page describes the rest of the process.
Plain tile roofs
Plain tiles are laid in overlapping courses and give a roof its familiar scalloped look. A flashing kit for plain tile lies in the courses, and the tiles are cut round it so that each course lines up with the next. The side pieces are covered by the tiles, and the lower apron sits on top of the course below.
The challenge is matching the pattern. Cut tiles that look rough and uneven stand out, so we mark the cuts before we lift anything and keep spare tiles from the roof or from stock. The finished window sits in the roof as part of the pattern, not as a patch cut into it.
Interlocking tile roofs
Concrete interlocking tiles are common on post-war houses and newer extensions in the villages around York. They are deeper than slate and plain tile, and the ribs run down the slope. A kit for these has extra depth, so that the side pieces close the gaps under the ribs.
The tiles are heavy, and the window opening should be positioned to avoid cutting more tiles than necessary. A good position often lines up with the tile width, which leaves fewer awkward cuts and a tidier result. A careful dry run on the roof tells us where the cuts will fall before we begin.
Pantile roofs
Pantiles have a strong wave, and clay pantiles are a familiar sight on roofs in the eastern part of the region. The wave is deep, so the flashing needs a profiled apron and side pieces that match it. A kit made for a flatter tile will leave open channels beneath the metal.
Not every pantile roof takes every window, and it is a question that comes up often. We deal with it in can a roof window go in a pantile roof. As with other coverings, the fit depends on the profile, the lap of the tiles and the condition of the clay, which is checked at survey.
Lead, aluminium and other flashing materials
Most kits supplied with a roof window are made of aluminium that is coated to match the roof. They are light, they hold their shape and they are designed to fit the window's frame. Lead is the traditional alternative and is dressed by hand to the covering, which can suit a period house or an unusual profile.
Whichever material is used, the point is the lapping and the fixing, not the metal. A well-lapped aluminium kit performs better than a poorly dressed lead one, and the reverse is also true. We choose according to the roof and to what the window maker specifies, so the maker's warranty stays intact.
What goes round the flashing
The flashing is only part of the wet-side detail. Beneath it, a membrane collar and a lining of underlay carry any moisture that gets past the covering down and out. Round the frame, insulation closes the gap between the window and the rafters so that warm air does not meet cold metal and condense.
These pieces are hidden within minutes of being fitted, and that is why we photograph them first. The photographs show the membrane laps, the insulation and the trimmers before the covering goes back, and you receive them at handover. They are a record of work you cannot see again.
Fitting several windows in a row
When two or more windows sit side by side, they can share a single flashing run if the kit allows it, with a narrow section of covering between them. The windows must be set at the right spacing for the kit, and the surveyor will check that on your roof.
Setting windows too close together can leave too little covering between them to lap properly. A little more room gives a neater and longer-lasting result. Our roof window cost guide explains how fitting several at once affects the price.
The order the pieces go on
Flashing works like a run of overlapping steps, so each piece has to go on in the right order. The covering is opened up first, the window frame is fixed between the rafters, and the membrane collar is lapped over the frame and down onto the underlay. The metal then goes on from the bottom of the window upwards: the lower apron first, then the side pieces, and the top piece last.
That order means every upper piece sheds water over the one below it. Reverse it and the water can run behind an edge. It is also why a flashing cannot be tidied up after the roof has been closed over: the laps are only visible while the work is open, which is another reason the hidden stages are photographed as they are completed.
Old coverings, spare slates and matching
An older roof rarely has spare covering waiting in the loft, and the slates or tiles round a new window are the ones most likely to be disturbed. Before any work starts, we look at the condition of the covering at the survey and note whether the pieces that must be lifted are sound enough to go back. Brittle slates, crumbling clay or tiles with worn nibs may need replacing, and that is a line on the written quote rather than a surprise on the day.
Where new pieces are needed, we try to match the colour and texture of what is there. A new slate beside a century-old one will weather in time, but a close match keeps the window from looking like a patch. Where a roof has been re-covered in patches with mixed tiles, a good match can often be found on a less visible slope and swapped round.
Chimneys, hips, valleys and neighbouring windows
The kit needs room to work. A window set too close to a chimney stack, a hip, a valley or the edge of the roof leaves no space for the flashing to lap properly on that side, and a custom lead detail is needed instead. The maker publishes minimum gaps for each kit, and we check them against your roof at survey so the position of the window is chosen with the flashing in mind.
Moving a window a little up or across the slope often solves the problem. A shift of a few courses can put the side pieces on clear covering, and in a pitched roof it costs nothing in daylight. If the ideal position really does collide with a stack or valley, we tell you plainly and show you the alternatives.
Why the maker's kit matters
A roof window and its flashing are designed and tested together. Using the kit the maker specifies for your covering keeps the product warranty intact, whereas a home-made or mismatched flashing can invalidate it. A kit also arrives with the right fixings and a set of fitting instructions, so the work is carried out to the maker's own standard.
There are cases where a standard kit does not suit, for example a very deep profile or a stone slate. A specialist kit or a hand-dressed lead flashing is then used, and we record that on the quote with the reason, so you can see why the line is different from the standard one.
What a good flashing leaves behind
- A flat apron with no lifted edges at the bottom of the window.
- Side pieces tucked under the covering, with an even margin on both sides.
- A top piece fully covered by the course above it.
- Cut tiles or slates that follow the course lines.
- No daylight visible under any edge of the metal.
Looking after the roof around the window
A well-fitted flashing needs very little attention. The things that do cause trouble are moss and leaves that pile up above the window and hold water against the head, and a gutter that overflows onto the covering below. A look from the ground after a storm, and a clear-out of debris when you clean the gutters, is usually all it takes.
If you find a slipped slate or a cracked tile close to the window, mention it before it gets worse. A small gap in the covering above the flashing can let water get under the metal, even though the flashing itself is sound.
What to look for after the work
Stand outside and look at the window. The flashing should lie flat against the covering, with an even gap and no daylight under it. The slates or tiles should sit in neat courses up to the sides, and the upper edge should be covered.
Inside, look at the lining. Stains round the frame after rain are a sign that something is wrong, and so is condensation that forms in a regular line around the edge. If you see either, tell us so we can look. At the walk-through we show you the finished flashing, hand over the maker's product details and warranties, and add our 10-year workmanship guarantee. Where the job was notifiable, the Building Control certificate goes in the same folder.