Roof Window Flashing Kit: Matching the Kit to Your Tiles and Window Size

The window turns up on a Tuesday and goes in by the weekend. The flashing doesn't, because nobody ordered it, or somebody ordered the wrong one. Now the roofer has a window sitting proud of the tiles with nothing weatherproofing the join, and a wait of several days for the right kit. The tiles are already laid, the scaffold is booked to come down, and one clean morning's work turns into a return visit. The flashing kit is the part of a roof window order most often forgotten, and most often ordered wrong when it isn't.

The window and the flashing kit are two separate purchases. That single fact catches more homeowners out than anything else about roof windows, so it is worth being clear about why, and about exactly what you are matching the kit to when you order it.

What it is and what it does

A roof window flashing kit is the set of pre-formed metal pieces that weatherproofs a roof window into the surrounding roof covering. It is the visible aluminium or grey-coated surround you see framing a roof window from outside, and it is what carries rainwater off the glass, around the frame, and back down onto the tiles or slates below the window.

The kit comes as a set of folded and pressed components: an apron at the bottom that lays over the tiles below the window, side flashings that channel water down each side, a head section or soakers at the top that tuck under the tiles above, and a top cover. On a tiled roof the side and head pieces include flexible soakers, lead-free pleated aluminium strips that the roofer dresses down over the contour of each tile so water cannot track sideways past the window. The whole kit is shaped so that water always runs over a joint and onto the next surface down, never into one.

This is different from the window's own weather seals, which keep water out of the glazing unit itself. The flashing handles the much bigger job of joining the window to the roof around it. Below the flashing sits the membrane-level waterproofing, the underfelt collar and breathable membrane that catch any wind-driven rain that gets behind the tiles. Flashing works at tile level, the collar works at membrane level, and a correct install needs both.

Why it is separate from the window, and the mistake that follows

A roof window fits the same way into a slate roof, a plain-tile roof, and a deep profiled-tile roof. The window does not change. What changes is how the surround has to be shaped to lap into that particular covering, because a flat slate, a small plain tile, and a chunky interlocking tile all sit at different heights and throw water in different ways. So the manufacturer makes one window and a family of flashing kits, one for each covering type, and you pick the kit that suits your roof.

That is why the two are sold apart, and it is also the root of the single most common ordering error: buying the window and either forgetting the flashing entirely or ordering a flashing that does not match the covering. A flashing made for slate will not seal correctly into deep interlocking tiles, because the soakers and apron are formed flat for a low covering and cannot dress over the high tile profile. A profiled-tile flashing dropped into a slate roof leaves the apron sitting proud with gaps under it. Either way the join leaks, or the roofer refuses to fit it and you wait for the right one.

Flashing types by roof covering

The covering your roof window sits in decides the flashing code you need. Velux, the dominant brand in the UK, uses a two-letter code for each covering type, and the other major brands map onto the same idea even where their letters differ. The covering type is the first thing to get right.

EDW is for profiled and interlocking tiles, the standard concrete or clay tile with a raised roll or rib, up to roughly 120mm in total thickness. This is the most common kit on UK extensions because interlocking concrete tiles are the most common covering. The apron is deeply profiled so it dresses down over the tile rolls.

EDZ is for low-profile coverings: flat interlocking tiles, slate-effect tiles, and thin profiles up to around 30mm. The apron and soakers are formed shallower than the EDW because the covering they lap onto is flatter.

EDL is for natural and fibre-cement slate up to about 8mm thick, the thinnest common covering. The flashing sits almost flat to the roof and the soakers are dressed tight to the slate.

EDN and EDJ are recessed flashings. Instead of the window sitting on top of the roof plane, a recessed flashing drops it slightly into the roof so the glass sits closer to the tile line for a flatter, more discreet look. EDN suits tiles, EDJ suits slate and low-profile coverings. These cost more and need the rafters set out to suit, so they are a design decision made early, not a swap you make on site.

Fakro and Keylite, the two main alternatives to Velux, sell the same covering-matched families under their own codes. Fakro uses EHV-A and EZV-A type codes for tiles and ESV-A for flat coverings; Keylite splits its kits into tile flashing and slate flashing ranges. The principle is identical: match the kit to the covering. If you are buying a non-Velux window, buy that brand's flashing, because the kits are shaped to that brand's frame and are not cross-compatible.

Matching the kit to the window size code

Covering type is only half the order. The flashing also has to match the window's size code exactly, because the kit is pressed to fit one frame footprint and no other.

Velux size codes pair two letters with two digits. The letters give the width band (CK is 55cm wide, MK is 78cm, PK is 94cm, SK is 114cm, UK is 134cm) and the digits give the height (02 is 78cm, 04 is 98cm, 06 is 118cm, 08 is 140cm). So an MK04 window is 78cm by 98cm, and the flashing for it is also coded MK04. A CK02 flashing will not fit an MK04 window: it is 23cm narrower. Order the flashing in the same letter pair and the same number as the window.

The number to trust is the one stamped on the data plate inside the window frame, not the size of the hole in your roof. The rough opening is always a little larger than the window. People who order a flashing off a tape measure of the hole sometimes land one size out. Read the code off the window itself, or off the window's order confirmation, and match it.

Single vs combination flashings

Everything above covers a single window in its own patch of roof. The moment you put two or more roof windows close together, the flashing changes.

A single flashing weatherproofs all four sides of one window. Where two or more windows sit side by side or stacked one above another with a small gap between them, the gap between them is a trap for water, and a pair of single flashings butted together does not close it cleanly. The answer is a combination flashing, designed to weatherproof a group of windows as one unit and to seal the strip of roof between them.

Velux calls these EKW for tiles and EKL for slate, supplied to suit a given number of windows in a given layout. You specify how many windows, the size code, and whether they are arranged side by side, stacked, or in a block, and you order one combination kit for the whole group rather than separate single kits. The gap between the windows is set by the kit, so the rafters and the windows have to be positioned to suit it. This is a layout decision made on the drawings, with the structural opening built to match, not something improvised once the windows arrive.

If your design has two rooflights next to each other to bring more light into a kitchen extension, this is the kit that joins them. Do not let two single windows go in with two single flashings and a hand-formed lead gutter between them; a proper combination flashing is the detail that does not leak in five years.

The collars that go with it

A flashing kit weatherproofs the window at tile level. Two more components weatherproof it below the tiles, and they are easy to overlook because they are not part of the flashing itself.

The insulation collar (Velux BDX) is a frame of foam insulation that fits in the structural gap between the window frame and the rafters, stopping a cold bridge and the condensation that forms on cold metal flashing in winter. The underfelt collar (Velux BFX) is a shaped piece of breathable membrane with a drainage channel that joins the window to the main roof membrane, catching any wind-driven rain that gets behind the tiles. On Velux Pro+ flashing kits (the 2000-series), both collars are already in the box; on the basic 1000-series kits, neither is, and you buy them separately. The detail of which kit includes what, and why the underfelt collar matters even when building control does not strictly require it, is covered in the underfelt collar guide. The short version for ordering: buy the Pro+ kit and the collars come with it.

Pitch range and the right covering, two things to confirm

Two checks save a wasted order beyond covering and size.

First, the window has to suit your roof pitch. Roof windows have a minimum and maximum pitch they are rated for, commonly around 15 to 90 degrees for a standard centre-pivot window, with some products dropping to lower pitches and others needing a special low-pitch flashing below about 15 degrees. A near-flat extension roof at 10 degrees needs a flat-roof rooflight or a specific low-pitch kit, not a standard tile flashing forced onto too shallow a slope. Confirm the pitch of the roof the window is going into against the window's rated range before ordering.

Second, confirm the covering with the merchant supplying the tiles or slates, not from memory. "Concrete tiles" is not specific enough; the flashing cares whether they are deep interlocking (EDW), flat or slate-effect (EDZ), or true slate (EDL). The tile supplier can tell you the profile and thickness, which is what decides the code.

How it goes in, and who fits it

You are specifying and checking this, not fitting it, unless you are confident working on a roof off a tower or scaffold. Knowing the sequence lets you check your roofer's work before the access comes down and the opening is closed for good.

A competent roofer fits a standard flashing in well under an hour once the tiles are up. The work that goes wrong is rarely the flashing itself; it is the wrong kit arriving, the collars skipped, or the soakers left undressed so they bridge the tile rolls instead of following them.

Cost and where to buy

Single flashing kits for the common coverings sit close in price, with the recessed EDN and EDJ kits and the combination EKW and EKL kits costing more because they are larger and more complex. Price rises with window size: an SK06 or UK08 flashing costs more than the MK04 shown in the table above. The table below gives current UK retail for the common single sizes and a two-window combination kit.

Specialist roofing suppliers (Roofing Superstore, JJ Roofing, Burton Roofing) carry the full range of codes and sizes and are usually the cheapest on the premium Velux kits, with delivery in a few days. Screwfix and Toolstation stock the common Velux sizes for same-day collection, which is the route to take if a kit was forgotten and the roofer is waiting. Travis Perkins and other builders' merchants stock them too, useful for consolidating with a wider tile order. Amazon UK lists Fakro and some Velux kits, but check the seller is supplying the exact covering code and size before buying.

Buy the window and its matching flashing from the same supplier in the same order where you can, so the codes are checked together and nothing arrives orphaned. The flashing is a small fraction of the window's cost, and ordering it as an afterthought is exactly how it ends up wrong or missing.

Alternatives

The honest alternative to a manufactured flashing kit is a roofer hand-forming a lead surround around the window. On a one-off, an awkward junction, or a heritage roof, skilled leadwork can weatherproof a roof window without a kit. For a standard extension it is the wrong call: it relies entirely on the roofer's skill, takes far longer, and a manufactured kit shaped for the exact window and covering seals more reliably and keeps the window's warranty intact. Manufacturers' guarantees expect their own flashing to be used. A reused flashing off an old, smaller window is not an alternative either; it will be the wrong size code and will not seal.

The other choice sits further upstream: whether to fit a roof window at all, or a flat-roof rooflight (a glazed unit set on a kerb on a flat or near-flat roof) instead. Those are a different product with their own upstand-and-kerb weatherproofing, not a flashing kit, and the decision turns on the roof pitch and the look you want.

Common mistakes

Wrong covering type. Ordering an EDW for a slate roof, or an EDL for deep interlocking tiles, gives a flashing whose apron and soakers cannot dress to the covering. It leaks or will not fit. Confirm the covering profile with the tile supplier before ordering.

Wrong size code. A flashing one size off the window leaves a gap or overlaps wrong. Read the code off the window's data plate, not the rough opening, and match the letters and digits exactly.

No collars. Skipping the insulation and underfelt collars leaves a cold bridge and a gap in the membrane-level waterproofing, the cause of most roof window leaks and condensation. Buy the Pro+ kit so both are included.

Pitch out of range. Forcing a standard flashing onto a roof shallower than the window's rated minimum pitch. A near-flat roof needs a low-pitch kit or a flat-roof rooflight, not a standard tile flashing.

Reusing an old flashing. A flashing salvaged from a previous, smaller window will be the wrong size and often the wrong covering, and it voids the new window's warranty.

Forgetting it entirely. The classic. The window goes in, the flashing was never on the order, and the roof sits open while the right kit is chased. Order the flashing with the window, every time.

Where you'll need this

• Roof windows and rooflights - the flashing kit is ordered to match the chosen roof window and its covering, and is the part of the order most often forgotten
• Roof covering - the kit is woven into the tile or slate covering as the roof is laid, lapping onto the tiles on all four sides of the window

A roof window flashing kit is needed wherever a roof window or rooflight goes into a pitched tiled or slate roof, on any extension, loft conversion, or re-roof. It is ordered to match both the roof covering and the exact window size, fitted as the covering is laid, and works together with the insulation and underfelt collars below it whatever the project type.