A roof window arrives in a box with a frame, a sash and glass. Beside it, and sold separately, is a second box that matters just as much: the flashing kit. It holds the metal or flexible pieces that join the window to the roof covering and make the joint waterproof. When a roof window leaks a year or two after fitting, the glass is almost never the cause. The flashing is. Either the wrong kit was used, or the right one was fitted badly, or the kit was left out in favour of sealant. This guide explains what a flashing kit does, what is in it, why there are so many versions, and how we choose and fit one for each roof.
What flashing is for
Water runs down a roof slope and must be led around the window, not into it. A flashing is a strip of waterproof material that overlaps the joint between two surfaces so that the water runs over the top of the joint and not through it. On a roof window the flashing does this on four sides at once. At the head, above the window, it sheds the water that arrives from higher up the slope to either side. Down both sides it channels water past the frame and out onto the covering below. At the bottom, it carries everything over the lower edge and onto the tiles below.
The idea is lapping, not sealing. Each piece overlaps the next like the tiles themselves, so gravity does the work. Sealant is a back-up for details that cannot be lapped, not the first line of defence. A kit that depends on sealant to keep out water will fail when the sealant ages, which it does. Our guide to leaking roof windows describes what the failures look like.
What is in a typical kit
Kits vary by maker, but most contain the same elements:
- A bottom apron, which sits over the tiles below the window and turns up against the lower edge of the frame.
- Side flashings, two long pieces that run from the bottom to the top of the frame. On profiled tiles they are often shaped to follow the tile profile, and in two parts, a lower gutter piece and an upper cover.
- A top apron, which sits over the head of the frame and under the tiles above, directing water to the sides.
- A cover or trim, the outer metal skin of the frame, often aluminium in grey, black or copper tones.
- Fixing plates and screws, which hold the pieces to the frame and rafters.
- An underlay collar or sealing sleeve, a flexible membrane that connects the roof underlay to the frame so water under the tiles is led around it.
- An insulation collar, sometimes supplied separately, which wraps the frame in insulation to prevent cold bridging.
The underlay collar is easy to overlook, because it is hidden. It is the second line of defence, and it saves a roof when wind-driven rain gets under the tiles. Without it, any water that gets past the main flashing reaches the frame and the room. The insulation collar affects comfort and condensation more than leaks, and our page on condensation on roof windows explains how.
Why the kit has to match the covering
Roof coverings are not alike, and the flashing has to follow the shape of whatever it sits against. A flat plain tile lies close to the battens and overlaps by a few centimetres, so a kit for plain tiles has a thin, flat profile. An interlocking concrete tile has deep troughs and ridges, and the kit has a gutter piece shaped to catch and carry the water that runs down them. A natural slate roof takes a flat flashing with soakers or lead-dressed details. Corrugated sheets, metal profiles and thatch each have their own kits or need custom lead work. Using a flat kit on a profiled tile leaves gaps along the sides. Using a profiled kit on a flat tile raises the tiles around it and makes them rock.
Makers publish a code for each combination of window size and covering, and the code must be ordered exactly. Some kits can be used on a range of coverings up to a given tile thickness, and others cannot. The thickness of the covering matters too, because the flashing is designed to sit at a particular height above the battens. This is why we check your covering at the survey and why the quote lists the kit. A kit bought online for the wrong roof is a false saving. Our page on fitting a roof window in a slate roof shows how lead-dressed details are used for slate.
Lead, aluminium and flexible flashings
The material of the flashing matters. Aluminium kits are standard on most roof windows. They are light, easy to fit and available in colours that match the frame cladding. They suit most profiled and flat coverings, though the metal can dent. Lead is the traditional material, soft enough to be dressed to any shape, long-lasting, and the right choice for natural slate and period roofs where a bespoke finish is wanted. It can crack if bent repeatedly or set in a way that prevents movement, which is why the way it is cut and fixed matters. Our section on lead roofing and leadwork explains how lead is installed. Flexible membranes and butyl-backed sheets are used for awkward details, but they can degrade in sunlight and are usually concealed beneath other layers.
Where several windows sit side by side, special combination kits join them with shared flashings, so that the gaps between the frames are covered. Using separate kits for each window in a bank leaves the join between them as the weak point. Windows stacked above one another need a connecting kit as well, so water from the upper window passes over the lower.
How the kit is fitted
The work goes in a set order, from the bottom up. First the frame is fixed to the rafters and the underlay is cut, dressed up to the frame and sealed with the collar. The battens around the opening are fixed to give the tiles a proper support. Then the bottom apron goes on, with tiles laid over its sides, and the side flashings are fitted from the bottom, each piece overlapping the next. The top apron is fitted last, tucked under the tiles above. Finally the outer cover is clipped on. At each stage the laps run downhill, and no piece depends on sealant to work.
Tiles around the frame are cut to fit, not forced. They need to sit flat on the flashing, with a gap that lets water run past, and be fixed with the correct clips or nails. Cutting a tile too close to the frame or leaving it hanging off the batten is a common fault and a cause of broken tiles in the first frost. A good fitter checks each tile around the kit, and we photograph the finished edge before the scaffold comes down.
Common mistakes
A few come up often enough to list:
- Using the wrong kit for the covering, or an off-the-shelf kit that was cut to fit.
- Leaving out the underlay collar, so there is no second line of defence.
- Fitting side flashings the wrong way round, so the laps point uphill.
- Filling gaps with silicone or expanding foam instead of correcting the lap.
- Reusing an old flashing on a new window, so the pieces no longer fit the frame.
- Skipping the insulation collar, which causes cold bridging and mould on the lining.
- Failing to renew battens or tile fixings round the frame.
Each of these may pass an initial look and leak months later. That is why we use the maker's complete kit with the window, and why a replacement window gets a new kit as a matter of course. Our guide to replacing an old roof window describes how the whole job runs.
Special situations
Some roofs ask more of a kit than a plain slope does. A window set close to a hip or a valley has little room for the side flashings to fall, and the water from the valley can pour straight onto the frame. In that case the window is better moved, or the valley is relined with lead or GRP so it carries the flow cleanly past. A window near the ridge has little roof above it to shed water onto the head flashing, which makes it easy to flash but exposed to wind, so we pay extra attention to the head apron and tile fixings. A window beside a chimney or a dormer cheek often needs lead work to join the kit to the existing flashings, and the two have to be dressed so that neither stops the other draining.
Thatch, stone slate and old handmade clay tiles are rarely a match for off-the-shelf kits. The tiles vary in thickness and the courses are uneven, so a bespoke flashing in lead is the sounder way to go. On a flat or near-flat roof, a window is replaced by a kerbed rooflight, and the waterproofing becomes the membrane turned up the kerb, as described in flat roof rooflights.
Checking and maintaining flashings
Flashings need little care, but they benefit from a yearly look. From the ground, check that the metal is still firmly in place with no lifted edges, and that no tiles around the frame have slipped. Clear moss and leaves from the slope above and from the gutter piece at the sides, since debris holds water against the joint. After a storm, look for movement. A flashing that rattles or has bent corners should be checked by a roofer. If you can see daylight in the loft around the frame, tell us.
What it costs
A flashing kit is a small part of the price of a window. A trade guide quotes anything from £680 up to £1,300 for a standard window supplied and fitted, and a manufacturer's indicative figures run higher once VAT is counted, so the kit is a small share of the total. Those totals include the kit, but not every extra, such as bespoke lead work, scaffold or a combination kit for several windows. The kit we specify for your covering is named in the written quote, which follows the survey. For the cost breakdown in more detail, see how much a roof window costs to fit.
If your window is leaking and you suspect the flashing, we can inspect it and photograph the problem. Our roof windows page sets out the full service, and the number is 01256 213956.
