A roof window keeps the weather out in two places: the glass and the ring of metal around it. The metal is the flashing, and on most roof windows it arrives in a box with the window. A roof window flashing kit is that box, and picking the right one for your roof covering is as important as picking the window. This guide explains what is in a kit, why there are different ones, and how skylight flashing is fitted into slate, tile and stone.
What a flashing kit is
A roof window sits in a hole cut through the roof covering. Water running down the roof meets the window frame, so something has to carry it round the frame and back onto the roof below. That something is the flashing: a set of formed aluminium or lead-coated pieces, one for each side of the frame.
A kit holds those pieces cut and shaped to suit one window size and one type of roof covering. A typical kit has a bottom apron that sheds water over the covering below, two side pieces that run down each edge, and a top piece that sends water round the head of the frame. Some kit packs also include an insulating collar that fits between the frame and the roof structure, and an underfelt collar that ties into the roofing felt. A vapour barrier strip to seal the inside is a separate part.
Without a kit, a roofer has to form each of these pieces by hand in lead. That is sound practice on some roofs. A factory kit saves the time and gives the same shape every time.
Why there is more than one kit
Roof coverings are not all the same thickness, and the flashing has to match, because the metal rests on the covering and the covering rests on the battens beneath it. A kit for thin slate would not clear the ridges of a deep interlocking concrete tile. A kit for deep profiles would sit proud of flat slates and gather water.
| Roof covering | Type of kit | What to look for |
|---|---|---|
| Welsh and other flat slate | Flat-profile kit | Slates cut cleanly to the flashing, with a double lap at the sides |
| Plain clay or concrete tile | A plain-tile kit made for tiles up to 14 mm thick | Tile courses kept regular up to the frame |
| Interlocking concrete tile | Profiled kit with a deep side channel | Tile ridges that fit the side gutters with no gap |
| Stone slate | Lead flashing, formed on site | Heavy, uneven slates that a standard kit seldom fits |
Kits also come for single windows, for two windows side by side and for one stacked above another. A pair set in the same opening share their middle, so the kit has linking pieces instead of two separate side flashings. Using two single kits for a pair leaves a gap between them.
Reading the size code
Roof windows carry a code that names their size. In the VELUX range, for example, MK04 reads as a letter followed by two digits: the letter is the width of the frame and the digits are its height. The same code selects the flashing kit, because the flashing is cut to the outside of that frame.
That has a practical side. If you are replacing a window, the old one's code tells us what size the opening is, and so what kit and what new frame fit the existing rafter spacing. If the rafters were cut for an older size that has since been dropped from the range, the opening may need altering, and the quote will say whether that is the case.
The code does not tell you the roof covering. So the survey takes in the whole slope and not only the window.
Skylight flashing on a slate roof
Slate is thin and flat, so the flashing sits almost level with the surface. That makes a clean result possible, but it asks for care. Every slate that meets the frame is cut to fit the flashing's edge, and the side pieces are lapped so that each slate overlaps the flashing and the flashing overlaps the slate below it.
The old slates are lifted carefully and reused wherever they are sound. Welsh slates are often hard and long-lasting, but their nails rust through and corners crack as slates are moved. Any slate that will not survive the lift is replaced with a matching one. We discuss this in more detail in can a roof window go in a slate roof.
On a roof that has lost its original slate and been patched with whatever fitted, the flashing problem is that the course heights are no longer regular. We re-lay the courses around the window so that they run true to the flashing, which means the repair is confined to the area around the opening.
Skylight flashing on tile and stone
On concrete interlocking tile, the flashing has to cope with the profile. The tile has high and low parts, and the bottom apron and side channels are shaped to sit on those parts. If the kit is the wrong depth, a gap opens under the ridge of a tile and wind-driven rain can blow up it. We match the kit to the tile by measuring the profile at survey, not by guessing from the roof's age.
Plain tile, clay or concrete, is thicker than slate but still flat, and it takes a kit made for plain tile. The courses are small, so cutting round the frame is straightforward.
Stone slate and heavy stone tiles, found on old cottages along the Peak edge and in the stone terraces of the western suburbs, are a separate case. The slates vary in thickness, so a factory kit seldom lies flat on them. Lead flashing, formed on the roof to the shape of each stone, is the sound answer. It is slower and it asks for a skilled hand, and it is the reason a quote on a stone roof reads differently from one on a slate roof.
How the kit goes in, step by step
The order of work is the same on every roof, and it starts at the bottom.
First the covering is stripped back to the line of the opening and the rafters are trimmed and framed where a new hole is needed. Then the window frame is fixed to the structure, and the insulating collar is set round it. The bottom apron goes on, lapped over the covering below. The side pieces follow, working up. The covering is relaid against them. The head flashing goes in last, tucked under the course above so water cannot get behind it.
Inside, the vapour barrier strip is taped to the ceiling lining and to the frame, so moist air from the room does not reach the cold timber round the opening. The reveal is then boarded and finished.
How long this takes depends on the roof, so we plan the time at survey and not before we have seen the covering. Access and the condition of the battens decide more than the window does.
Weather is the other fixed point. The roof is open from the moment the covering is lifted until the head flashing is tucked in, so we work from the forecast, sheet the opening whenever we stop and do not leave a half-flashed window overnight if rain is due.
Inside, a dust sheet and a clear floor are the only things we ask of you. Everything above the ceiling line is ours to manage.
Signs that flashing has done its time
Flashing is a wearing part, and it is easy to forget because it is on the roof. Some signs that it needs attention:
A brown stain on the ceiling beside the window frame, not under the glass. Moss or debris collected in the side gutter. Metal that has lifted at a corner. A slate that has slipped beside the frame. Sealant that has cracked or pulled away from the flashing.
If you see these, a replacement kit is usually the answer, and fitting one means lifting the window frame's surround, but not necessarily the window. Where the window is in good order, the kit can be changed with the frame left in place in some cases, though the covering round it must be lifted. The survey tells you whether the kit, the window or both are the right thing to replace.
Our guide to where a roof window can go and the page on centre-pivot or top-hung roof windows cover the rest of the choice. If the window serves a bedroom in the roof and needs to double as a way out, see can a roof window be a fire escape, since the height of the frame above the floor matters.
On the quote, the guarantee and the paperwork
A quote names the kit by type as well as the window. It says what covering the kit was chosen for, whether any slates or tiles will be replaced, and how the flashing is joined to the covering. The life plan records the same, with the kit as one of the parts that will need a look later and a note of how it would be changed.
On a roof window, our 10-year guarantee leans hardest on one place: the flashing joint, where metal meets slate or tile. That is where roof windows most often show problems, so it is what we put our name to. The glass and the frame carry their makers' own terms.
Part L (2021) asks a replacement roof window on an existing home to reach 1.4 W/m²K or WER Band B. The council or a private approver has to be told about the fitting, and we make that notification ourselves. When the work is signed off, a certificate comes to you, and it is worth filing with the house papers. Planning rules for roof windows on houses are covered in our planning guide, and the overview of roof windows sets out the whole service.
About flashing before you choose
Is the flashing kit included with a new roof window?
The kit is a separate item chosen for your roof covering. In our quotes it is listed beside the window, so you can see which kit goes with which covering.
Can the old flashing be reused with a new window?
Not in general. Flashing is shaped to one frame size and the metal has been dressed to the old covering. A new frame gets a new kit, so the joint is made fresh.
Does lead last better than aluminium?
Each has a place. Lead is the choice on stone slate and uneven roofs because it is worked by hand to fit. Factory aluminium kits suit regular slate and tile and are made to a consistent shape. We state the checkpoints for the one we use in the life plan.
Will the flashing show from the street?
It sits close to the roof surface, in a colour chosen to suit the covering, so on slate it reads as a thin dark line round the frame.
Does a conservation area change the flashing?
It can. Conservation areas and listed buildings have their own rules for roof windows, and a stone or slate roof may need lead rather than a factory kit. We check the council's position at survey.