A VELUX flashing kit is the set of metal pieces that seals the gap between a roof window and the roof covering around it. The kit you need depends on what the roof is covered with. In VELUX's naming, EDW is for profiled roofing such as interlocking tiles, EDZ is for flat roofing such as plain tiles or thicker slate, and EDL is for thin slate. Choosing the wrong one is a common cause of a window that sits badly in the roof, so the kit is worth settling before the window is ordered.
This guide explains what a kit contains, how the letters map to the roofs of Exeter and Devon, how kits are laid in, and what to look for on a quote. The kit is a roof-side matter, but it also affects what you see from the room, because the depth of the roof covering and the flashing decide how the frame sits and how deep the lining must be.
What a flashing kit does
A roof window sits in a hole cut through the roof, and the roof covering has to be brought up to the frame and sealed against it without trapping water. The flashing is the part that does this. It is a set of shaped pieces, made of coated aluminium in VELUX kits, that overlap the frame and slot under or over the tiles or slates around it.
The rule behind every flashing is the same one that governs the rest of the roof: water runs downhill, and each layer must lap over the one below. At the sides, the kit channels rain past the window into the gutter line. At the top, it sends water around the frame instead of into it. At the bottom, it lays over the tiles below so water that runs down the glass is carried onto the roof covering.
Because every roof covering is a different thickness and shape, one kit cannot fit all of them. That is why there are several, and why the code on the box is not a detail you can ignore.
EDW, EDZ and EDL compared
The three kits you will meet most often are set out below. If you are unsure of the code for your own roof, describe the covering in words to the surveyor and let them confirm it.
| Kit | Roof covering | Typical Devon examples |
|---|---|---|
| EDW | Profiled roofing | Interlocking concrete or clay tiles on newer housing and extensions |
| EDZ | Flat roofing, including plain tiles and thicker slate | Plain clay tiles on Victorian and Edwardian houses, thicker slate on older terraces and cottages |
| EDL | Thin slate | Thin slate used on later nineteenth-century roofs and some modern natural slate |
The difference between EDZ and EDL comes down to how thick the covering is. Thicker materials stand higher off the roof, so the flashing has to step up further to clear them. Thin slate lies tighter to the roof, and a flashing built for a thicker material would sit proud and look wrong.
- EDW for profiled tiles
- Interlocking tiles have a raised profile, with rolls and troughs running down the slope. The flashing for these is shaped to follow the profile of the tile instead of lying flat.
- EDZ for flat coverings
- The tiles lie flat and overlap each other in courses, so the flashing sits against a straight line of covering. This is the kit for plain tile roofs and for slate above a certain thickness.
- EDL for thin slate
- Same principle as EDZ, scaled down for a thinner covering.
These codes apply to VELUX roof windows. Other makes use their own systems, and a rooflight on a kerb is flashed in a different way. We would not guess at a code: the manufacturer's instructions for the exact window and roof covering decide it.
Slate, plain tile and concrete roofs around Exeter
The roofs of the area are mixed, and that is why the kit must be chosen house by house. Georgian and Victorian terraces in the city and in St Leonards commonly have natural slate. Villas in Torbay and Exmouth have slate or clay tiles. Cob cottages in the Culm and Exe valleys have a mix of reclaimed slate, clay tiles and, in places, thatch edges. The 1930s semis in Heavitree and the postwar estates tend to have concrete tiles, and modern new builds such as those in Cranbrook are covered in interlocking tiles.
A single street can contain three of those. A house that has been re-roofed may have a covering that does not match the one next door. And some roofs mix materials: a slate main roof with a tiled extension, or a section re-slated in a different stone. The surveyor looks at the covering where the window will go, not at the neighbours.
Old slate roofs deserve a careful look. The slates may be thick and irregular, laid on riven laths, with a wide variation in size. The kit sits on the surface, so it depends on the roof being sound beneath it. If the slates around the opening are slipping or the battens are soft, those need dealing with as part of the work, because a good flashing on tired roofing does not hold. Our guide to roof window flashing for slate and tile covers that side of the job in more depth.
Single windows, side-by-side windows and windows one above the other
A kit is chosen for a single window by default. When two windows go side by side, or one above the other, there are connecting flashings that join them into a group, so the gap between the frames is sealed as one unit instead of with two separate surrounds. That is common in kitchen extensions, where two or three windows share a slope to light one room.
Grouped windows change a few things. The roof opening is bigger, so the rafter trimming is bigger. The distance between frames is fixed by the connecting pieces, so it cannot be adjusted on site to line up with something inside. And the inside of the room needs thought: two windows with a gap between them can share one splayed lining, which reads as a single opening in the ceiling, or each can have its own. Which looks better depends on the width of the ceiling and the position of the rafters. We discuss this at survey, with the ceiling in view.
If you are planning a group, tell us early. The size of the combined opening is easier to plan around before the windows are ordered. The codes behind the window sizes are explained in VELUX window sizes and codes explained.
The insulation collar and the vapour barrier
The flashing keeps rain out. Two other parts of the kit manage what is going on inside the roof build, and they are easy to overlook because they are never seen.
- Insulation collar
- This is a pre-formed insulating surround that sits around the frame in the roof build, between the frame and the rafters. Without it, there is a gap in the insulation around the window, and that gap becomes the coldest part of the ceiling, where condensation and mould appear.
- Vapour control layer
- Warm air from a bedroom or kitchen carries water vapour. If it reaches the cold side of the roof build, it condenses in the timber and insulation. A membrane lapped onto the window frame and onto the room-side vapour layer stops that. It must be continuous, with no unsealed tears.
On a replacement job, we check the existing build for both. Older windows often went in without them. If the old frame sat directly against the rafters with a gap of filler, the new one should not be fitted the same way, because that is the junction that the lining finish will show first, as a ring of damp staining.
Both parts hold the finish together, too. A lining that is warm to the touch takes paint evenly and stays free of stains, and a cold one does not. That is why we described the insulation behind the lining in our guide on how a rooflight is finished inside.
How a kit is laid in
The order matters more than any single piece, because each part must lap over the one below it. In outline, it goes like this:
- The roof window frame is fixed to the rafters or to a trimmed opening, at the height set by the roof covering, so the glass stands at the correct level in the roof.
- The underlay and any membrane are dressed up to the frame, so water that gets under the tiles is carried to the surface.
- The bottom apron goes on first, lapping over the covering below.
- The side flashings follow, each overlapping the apron and tucked under or over the covering at the side according to its profile.
- The top piece goes on last, lapping over the side pieces, and the covering is cut and laid to meet the flashing neatly.
On a slate roof, cutting the slates at the edge of the flashing is a skilled job. A badly cut slate shows as a ragged edge on the roof, and it is also where wind can lift. On a profiled-tile roof, the tiles around the opening are cut to the profile of the flashing, and the lines of the courses should run on past it.
We do not hurry this stage. It is the part of the job you will never see again once the scaffold is gone, and the flashing is covered by our 10-year workmanship guarantee because it is our workmanship.
Replacing a window and its flashing
When an old roof window is replaced, the flashing must be replaced with it. It is tempting to reuse the old surround to save work, but the old flashing was shaped to the old frame, and a new frame of a slightly different size will not seal against it. Mixing a new frame with an old flashing is a common reason for a replacement that looks fine on the day and starts to go wrong in the first winter.
It is also an opportunity to correct what was wrong before. If the old kit was the wrong type for the roof, the new window will be fitted with the right one. If the roof covering has been renewed since the old window went in, the new kit is chosen for the covering it has now. Our answer on whether a VELUX window can be replaced like for like sets out what has to match and what can change.
A whole-window replacement, frame and glass, is notifiable to Building Control, and we handle that notification. The same applies to fitting a new window in an existing roof, which is building work because the rafters are cut. Replacing only the sealed glass unit in an existing frame is not notifiable, and does not involve the flashing at all.
How the kit changes the room below
It is easy to treat the flashing as a roof matter and the lining as a ceiling matter. They meet at the frame. The covering sets how high the window stands off the rafters, and that height is part of the depth the lining has to span. A thick slate on an old cottage roof lifts the frame a little more than a thin tile does, and the extra thickness adds to the reveal.
The effect is small on a sloping loft ceiling and larger where the lining runs through a cold loft to a flat ceiling below. In both cases the depth is measured on site, once the covering is known, not taken from a standard drawing. That is why we survey the roof from outside and the ceiling from inside before writing the finish schedule, and why the kit and the lining shape appear on the same page of it.
Reading a quote for the flashing
A good quote names the kit, not just the window. It will say, for example, "VELUX window MK04 with EDZ flashing for plain clay tiles", or it will describe the same thing in words. If the quote says only "flashing included", ask which kit and why.
Questions worth asking:
- What is the roof covering, and which kit suits it?
- Is the insulation collar included, and the vapour control layer?
- If windows are grouped, which connecting pieces are being used?
- Will the existing roof covering be reused, or does any of it need replacing around the opening?
- Is scaffold or access equipment included?
We write the answers into the finish schedule, so the roof build above the room and the lining below it are recorded together. You can see what a schedule covers on our finish schedule page.
Glass is the other half of a window specification. Our guide to VELUX glazing options compared covers the panes. The whole subject is brought together on the VELUX window installation page. When you want a window specified for your own roof, ask for a quote and we will survey the roof covering before we name a kit.