Guide · VELUX Window Installation
Roof window flashing kits by roof covering
The flashing kit is the set of formed metal sections that joins a roof window to the covering around it. It is chosen by what is on the roof, not by what the window is, and getting that choice wrong is the most common reason a correctly installed window still lets water in three winters later.
Every maker publishes a kit for each family of covering: flat coverings such as slate, profiled coverings such as concrete interlocking tiles, and deeper profiles such as pantiles. This page works through what is in a kit, how the covering picks it, and the variations that turn up on Essex roofs. The wider installation sequence sits on the VELUX window installation page.
What is actually in the box
A kit has four working parts and a handful of fixings. Along the bottom of the window there is an apron, a single formed section dressed down onto the covering below. Up each side there are either continuous side sections or a stack of individual soakers, one per course, which interleave with the tiles or slates. Across the top there is a head section, usually with a drainage tray built into it.
Alongside those, most kits ship with a foam or brush strip that closes the gap under a profiled tile, and separately packed collars for the insulation and the underlay. Nothing in the kit is optional, and each piece has one correct position.
Why the covering chooses the kit
Water leaving a roof window has to be delivered back onto the covering and then away down the slope, without being able to travel sideways under the covering on the way. That handover happens at the edge of the flashing, and its geometry has to match the shape of what it is landing on.
Land a flat apron designed for slate onto a deep concrete tile and it bridges the troughs. The metal touches the high points and leaves an open channel under every trough, straight into the window opening. Land a deep-profile apron onto slate and it sits proud, flexes underfoot and works its fixings loose. The kit is a shape-matching exercise before it is anything else.
The covering picks the kit before the window is even chosen, and the survey is where that gets established.
Profile depth, and the number that governs the choice
Kits are specified by the maximum profile depth they will sit on, measured from the lowest point of a trough to the highest point of the roll or crown next to it.
Flat-covering kits are made for materials up to around sixteen millimetres of total thickness, which covers natural slate, fibre cement slate, shingles and the thinner flat interlocking tiles. Profiled kits are made for coverings up to around one hundred and twenty millimetres of profile, which covers almost every concrete interlocking tile and most pantiles. Between those two families sits a low-profile kit for flat interlocking tiles with a small roll. Measuring the depth with a straight edge and a rule at survey is a thirty second job that settles which family applies.
Plain clay tiles, and the older Essex stock
Plain tiles are small, flat, double-lapped and laid in a lot of courses. They take a flat-covering kit with individual soakers, one interleaved with every course up each side of the window, dressed against the frame.
The work is slow because the courses are close together and each soaker has to be set with the tile above overlapping it correctly. Handmade clay peg tiles, common on the older cores of Maldon, Coggeshall and the villages north of Chelmsford, add camber and thickness variation, so the soakers are dressed to a surface that is not flat. Tiles are lifted and reset rather than levered, because a nailed or pegged clay tile that is forced usually breaks.
Concrete interlocking tiles across the post-war estates
This is the default covering on the housing built from the fifties onwards across Basildon, Harlow, Chelmsford’s outer estates and the commuter towns. The tiles are large, they have a deep roll and an interlocking side lap, and they sit on battens at a wide gauge.
They take a profiled kit with a corrugated apron that follows the tile shape, plus a foam closure that fills the gap under each trough. The side sections run continuously rather than as individual soakers, because the tile courses are far apart. The detail to get right is the tile immediately below the apron: it is often cut short, and it wants supporting properly rather than left resting on the metal.
Natural and fibre cement slate
Slate is thin, flat and brittle, and it is laid double-lapped with head nailing or centre nailing. It takes a flat-covering kit with soakers, and the fit is tight and low-profile, which is why a slate roof gives such a clean finished line around a window.
The complication is that slates cannot be lifted like tiles. A nailed slate has to be released with a slate ripper, and the replacement slate is held with a clip or a tingle rather than a nail, because the nail hole is now covered. Welsh slate is harder and more consistent than the cheaper imported stone found on later work, and it survives the process better. On a slate roof the finished appearance depends almost entirely on whether the perimeter slates have been cut to line or simply snapped.
| Covering | Kit family | Side detail |
|---|---|---|
| Natural or fibre cement slate | Flat, up to about 16 mm | Individual soakers per course |
| Plain clay tile | Flat, up to about 16 mm | Individual soakers per course |
| Flat interlocking tile | Low profile | Continuous sections |
| Concrete interlocking tile | Profiled, to about 120 mm | Continuous sections plus closures |
| Clay pantile | Profiled, to about 120 mm | Continuous sections plus closures |
| Profiled metal sheet | Profiled, checked against the sheet | Continuous, sealed to the crown |
Pantiles and the deeper clay profiles
Clay pantiles turn up across north Essex and increase towards the Suffolk border. Their single-lap S profile is deep, the tiles are handmade or handmade-effect on older roofs, and the profile varies from tile to tile.
They take a profiled kit, but the closure work is more involved than on a uniform concrete tile because the gaps under the tiles are not identical. On a genuinely irregular old pantile roof the honest answer is a profiled kit plus hand-dressed lead where the apron meets the tiles below, which gives a detail that follows the actual roof rather than an idealised one. It takes longer and it is the difference between a window that looks like it belongs and one that looks bolted on.
Profiled metal sheet and outbuilding roofs
Garages, workshops and garden rooms across Essex are often covered in profiled steel or fibre cement sheet. Roof windows go into them, and the kit choice depends on the sheet profile depth against the kit limit.
Where the profile is within range, a standard profiled kit works with the closures matched to the sheet. Where it is deeper than the kit allows, the flashing is formed in sheet to match, terminated on the crowns rather than in the troughs, and sealed at the fixings. The other thing to establish first is what the sheet actually is: some older corrugated sheeting on Essex outbuildings contains asbestos, and if it does, the covering is surveyed and handled by a licensed specialist before any window work starts.
Standard depth against a recessed installation
Every kit family is made in two depths. In the standard version the window sits proud of the covering by a modest amount, which is how the great majority of roof windows are fitted. In the recessed version the frame is set deeper into the roof structure so the glass sits closer to the plane of the tiles or slates.
Recessed kits use different sections with a deeper upstand at the head and taller side pieces. They look better, particularly on a slate roof or a front elevation, and they cost more in both parts and labour. They also demand more depth in the rafter zone, which on a shallow rafter roof with insulation between and under can be the constraint that settles it.
Where the water goes, section by section
Follow a raindrop. It lands on the glass, runs down the pane onto the sash, over the bottom rail and onto the apron, which delivers it onto the covering below the window. That is the easy half.
Water running down the slope towards the window meets the head section first. Inside the head there is a drainage tray that collects it and directs it sideways to both sides, where it hands over to the side sections or soakers. Those carry it down past the frame, under the tiles either side, and release it below the apron. Nothing is ever asked to run uphill, and no joint relies on sealant to keep water out. If a kit is fitted so that any of that path is interrupted, water finds the interruption within a season.
Laps, fixing lines and the mistakes that show up later
Three details do most of the damage when they are rushed. Soakers laid with too little lap, so that wind-driven rain travels up between two pieces of metal. Fixings driven through the visible face of a flashing rather than into the concealed upstand, which puts a hole in the water path. And the apron dressed onto tiles that have not been reset properly, so the metal is holding the tile rather than the batten holding it.
None of the three is visible once the last tile goes back. All three are cheap to get right on the day, which is exactly why the workmanship on this trade carries a ten-year guarantee rather than the sales conversation resting on the brand on the glass.
Collars, and what sits behind the metal
The visible kit is only the outer layer. Behind it, a preformed insulation collar wraps the frame and closes the gap between the window and the trimmed timber, and an underlay collar connects the roofing membrane to the frame so that any water in the ventilated void behind the tiles is carried out over the flashing rather than into the ceiling.
Those two are supplied separately from the kit and are the ones most often left in the van. The membrane side of that detail is set out in more depth on roof window installation, and it matters as much as the metal outside it.
Fitting a kit into a covering that is already old
A kit assumes tiles or slates that can be lifted and put back. On a roof at the end of its own life, that assumption is optimistic. Brittle clay, delaminating slate and concrete tiles that have lost their surface all break when they are handled, and each broken one has to be matched.
The survey establishes this before anything is priced, including whether matching material exists and whether there are spares stacked behind the chimney, as there often are on older Essex houses. Where a roof is genuinely at the end of its service life, setting a window into it and then recovering the roof a year later means doing the flashing twice, so the sensible order of work is worth discussing before an order is placed.
Replacement, and whether the old kit comes out
When a window is swapped for a new one, the kit is renewed with it. Formed aluminium sections that have been dressed into a covering, walked on and weathered for two decades do not come out flat, and the small dimensional changes between window generations mean an old kit rarely lines up with a new frame anyway.
The exception is a genuinely recent installation where the same maker’s same generation frame is going back in. Even then the head drainage tray and the closures are renewed. The wider question of what a like-for-like swap involves is covered on skylight replacement.
Reading the kit line on a quote
A quote worth comparing names the kit by its code, not just by the word flashing. The code tells you the covering family it is made for, the size it matches, and whether it is a standard or recessed depth. Against that, check three things: that the covering family matches what is actually on your roof, that the size code matches the window on the line above, and that the insulation and underlay collars appear somewhere on the quote.
Where two windows are going into the same slope near each other, the joint between them has its own arrangement, and that is set out on roof window pairs and triples on one slope.
Keeping the flashing doing its job
Flashing kits are made from mill-finish or coated aluminium with lead or a lead substitute at the dressed edges, and left alone they last decades. What shortens that is debris.
Leaves and moss collect above the head section, particularly under a tree or on a north-facing slope, and once the drainage tray is packed the water goes sideways rather than round. Clearing that once each autumn, from a safe position, is the whole of the ongoing work. Where a roof is not safely reachable, it becomes a job for whoever is already up there doing the gutters.
Keep reading
More on velux window installation
Roof window escape openings and the rules
Read the guide →Roof window blinds and when to specify them
Read the guide →Roof window sizes and how they are coded
Read the guide →Roof window ventilation flaps and filters
Read the guide →Roof window cill height and the view out
Read the guide →
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