A roof window leaves a rectangular hole in a surface that was built to shed water. Every tile, slate or concrete interlocking tile on that slope relies on the one above it to throw rain clear, and the window frame interrupts that chain. The flashing kit is the set of formed pieces that repairs the chain around the frame, so water meets metal, runs down it and returns to the roof covering below the window. If you are planning a window for a loft bedroom, a landing or a stair void, the covering on your roof decides which kit goes in.
Roofs around Maidstone carry clay peg tiles, natural slate and concrete interlocking tiles, and a house of any age may have been re-covered in a different one. The hub page on roof windows covers the windows themselves.
What the kit is made to do
Rain does not fall politely down a roof slope. It is driven sideways by wind, it creeps upslope by capillary action between layers, and it collects in channels wherever the surface changes level. A window frame stands slightly proud of the covering, and each of its four sides meets the rain in a different way.
At the top, water arriving from the slope above has to be steered round the frame so that it does not pool against it. Along each side, water that runs down the slope meets the frame's edge and needs a channel to carry it past. At the bottom, the covering below the window has to receive that water and keep it on the outside. A kit therefore has up to four distinct pieces, one for each edge, and the pieces overlap one another in a set order so that each sheds onto the next.
Where the sides and the bottom meet the covering, the kit has to follow the shape of the tiles or slates. A flat piece of metal pressed against a roof of curved clay tiles leaves gaps under each curve. That is why one roof window frame can be sold with several different kits, and why the one we order depends on what we measure on your roof.
Terms on a flashing line
- Apron
- The piece across the bottom edge of the frame that lies over the covering below.
- Side channel
- The piece along each upright of the frame that carries water down past it.
- Head upstand
- The piece at the top edge that turns water away from the frame.
- Lap
- The overlap between one tile or slate and the one above, which keeps rain from reaching the battens.
Clay peg tiles, course by course
Peg tiles are small. A single course is a long line of narrow, slightly curved clay tiles, and the next course overlaps the one beneath by a margin that the roofer set when the house was covered. The result is a roof with many layers and many joints, and a window cuts across a stack of those courses.
Fitting a kit here means lifting the tiles on either side of the opening and relaying them against the new side channels, a few courses above the window as well as below it. Each lifted tile goes back in its original position. Clay that has been on a roof for a long time can crack when it is disturbed, and clay tiles of the same age are hard to match by colour, so we look at the tiles from the ladder at the survey and tell you if spare tiles are likely to be needed.
Thickness and camber
Hand-made peg tiles are not uniform. Thickness and camber change from tile to tile, and they change more across a roof that has been patched over the decades. A kit made for a thicker, flatter covering sits wrongly on a thin, cambered one, and the gap under the edge becomes a path for water. The side channels have to be bedded to follow the tiles as they actually lie.
Tile-hung gables and mixed roofs
Some Kent houses have a hipped main roof and a tile-hung gable or bay, and some have two ages of tile on one slope, where a section was re-covered. When the window is near the join, we work out which covering the kit has to sit on, since the two may differ in thickness. A window can sometimes be shifted along the slope by a rafter width to leave the old and new tiles each undisturbed.
Where a roof is close to needing a new covering, the window and the re-roofing can be timed together, and the flashing then only gets done once. Our guide to the conservation rooflight on peg tiles adds the low-profile option for visible slopes.
Peg tiles on one slope can differ in thickness from the eaves to the ridge
Natural slate and its generous overlaps
Slate is thin, hard and nailed through two holes. Each slate hangs over two below it, and the lap between courses is generous, so the roof is several slates deep at every point. That depth helps. It means water that gets under one slate usually meets another before it reaches the felt.
The kit on a slate roof is formed to lie beneath the slates beside the window. The slates on either side are slid out, the side channel is fixed, and the slates are fitted back over its edge at the same lap as before. Slate cannot be bent to fit, so a slate that breaks in the process is replaced by one of the right size and thickness, and any slates we need to source are listed at the survey.
Where a terrace is slated, the roof window can be seen from the pavement, so we stand across the road and look at where the window will sit against the slate courses and the chimney stacks. A window set in line with a neighbour's window, and sized to the rafter spacing, looks like part of the roof from below.
Nail positions and old fixings
Slate roofs can be fixed with copper nails, which last, or with iron nails, which rust and let slates slip. If a slate near the opening slips as we lift its neighbour, that points to the fixings across a wider part of the slope. We tell you what we have found before we go on, since it can affect how much of the slope needs attention.
Where the lap sits
On slate the side channel goes beneath the slates beside the frame; on plain clay tiles it is bedded to the tiles' own curve.
Concrete interlocking tiles and their profiles
Concrete interlocking tiles are large and have a rib or channel along each side so that neighbours lock together. Each tile does the work of many small clay ones, so there are fewer joints on the slope, and the tiles come in profiles from flat to deeply waved.
The profile is what the kit has to match. A flat tile needs a flat-bedded side channel. A waved or double-roll tile needs a channel that rises and falls with the wave, or filler pieces that close each gap beneath the flashing. Putting a flat kit on a waved tile leaves a hollow at every trough, and wind-blown rain gets into it. We identify the tile, and the maker's profile where it can be read from the roof, at the survey.
Cutting tiles to the frame
A window opening seldom lands on a tile boundary. The tiles beside it are cut to the frame with a disc cutter, and the cut edge is hidden under the side channel. Cutting leaves dust and sharp edges, so we protect the roof and the garden below. Whole tiles on either side of the cut should interlock with the courses above, and tiles near the opening are fixed with clips so that they hold in strong wind.
Roofs with a small pitch
A low pitch changes what a window needs. Water runs slower and wind can drive it upslope more easily, so we check what the instructions for the window ask of the head and apron at that pitch. For very low slopes, see how shallow a roof can be before a standard window stops suiting it.
| Covering | What is lifted or cut | Checked at the survey |
|---|---|---|
| Clay peg tile | Courses either side, relaid in original positions | Tile condition, thickness, spare tiles |
| Natural slate | Slates slid out and refitted at the same lap | Fixings, slate size, any slates to source |
| Concrete interlocking tile | Tiles cut to the frame, edges hidden by the channel | Profile, pitch, clips |
Windows near a chimney, valley or neighbour
A flashing kit assumes a clear stretch of ordinary covering round the frame. Some windows go where that does not hold. A loft room built at the back of a terrace may have the chimney stack on one side and a valley gutter on the other, and the only sensible spot for the glass is between them.
Beside a chimney, the kit and the stack's own lead flashing both have to shed water, and the two must not funnel it into each other. We keep a gap wide enough for each to lap properly and check the stack's flashing while we are on the roof. Beside a valley, the water running down it is concentrated, so the window is set clear of the channel and the side channel finishes above the flow. We tell you if the best place for the room means moving the window up or along by a rafter.
Two windows side by side
A landing and a loft bedroom can sometimes use two windows set together. Sets that share a frame joint need a coupling or a combined kit, and the two sashes must sit at one height so that the apron runs straight across. A pair may suit a long slope better than a single large window, since the rafters may allow a smaller unit more easily.
Timber behind the metal
When tiles are lifted, we see the battens and the rafters beside the opening. Rotten battens are replaced there and then, and a damp or soft rafter is something you hear about the same day. A frame fixed into sound timber stays in line, and the flashing stays tight against it.
Dressing the kit on a tiled slope
- Frame fixed The window frame is secured to the rafters at the height set at the survey.
- Apron laid The bottom piece goes over the covering below the opening.
- Side channels These go on beside the frame, tucked under the covering or bedded to it.
- Head piece The top piece turns water round the frame.
- Covering relaid Lifted tiles or slates are put back at the original lap.
How the kit meets the frame and the lining
The kit sits outside the roof covering, and the lining of the opening sits inside it, but the two are connected by the frame. A thick covering such as double-roll concrete tile raises the point where the kit meets the frame. If the kit sits higher on the frame, the insulation round the opening has to follow, and the lining inside has to be cut to the right depth.
This is why the flashing is decided before the frame goes in. We note the covering thickness and the build-up under it at the survey, then order the kit and the lining pieces together. Inside, the frame meets the insulation at a second joint, which is sealed against air so the loft room has no draught round the glass.
Replacement windows
When an old window comes out to make way for a new one, the old kit comes out with it. The covering round the opening is lifted again, and the new kit is fitted to the new frame. Old pieces would join a new frame to metal that has spent years under weather. The same-size case has its own write-up in swapping an old VELUX window.
Planning and Building Regulations
A frame and flashing add to how far a window stands off the slope, and the planning rules for a roof window count the frame in the 150 mm limit beyond the original roof plane. The full set of conditions is in the skylight planning permission guide. A replacement roof window must meet the window limit of 1.4 W/m²K, or sit in Window Energy Rating band B at minimum, and we handle the Building Control notification where the work is notifiable. The wider Building Regulations picture sits in skylight building regulations.
How the flashing should read on a quote
A written quote that mentions only the window leaves the weathering unclear. The flashing should be its own line, naming the covering it is for, whether that is clay peg tile, natural slate or a named interlocking tile profile. If the line says only "flashing", ask what it is for.
The same quote should state who relays the lifted tiles or slates, whether replacement tiles are included, and what happens if a lifted piece breaks. Our guide to what a roof window costs to fit explains the other lines.
We survey free of charge before we price, measure the covering on the roof itself, and give a fixed written quote that names the kit for your tile or slate. The 10-year workmanship guarantee is a written document and covers the weathering of the joint as part of our work. To start, send a photo of the slope on WhatsApp at 07880 284133, or email info@orchardskylightsmaidstone.co.uk, and book a survey.
