What is a roof window flashing kit?
It is the set of preformed metal sections that joins a roof window to the covering around it. Not sealant, not a strip of lead dressed by eye on the day, but a manufactured assembly of aprons, side pieces, a head flashing and foam gaskets, made for one frame size and one type of roof covering.
The kit is bought separately from the window, it costs a fraction of what the window costs, and it decides whether the installation stays dry. That is the whole subject in a sentence. The rest of this page names the parts, explains what each one is doing, and sets out how the code on the box is chosen. The window it weathers is covered under VELUX window installation.
The parts that come in the box
Open a kit and there are four families of component, plus fixings.
- The apron, a single pressed piece that goes at the foot of the window and takes water from the frame out onto the covering below.
- Two side sections, supplied as lengths that lap together, running up each side of the frame.
- The head flashing, which goes at the top and disappears under the course of tiles above the window.
- Cover profiles and gaskets, the clip-on pieces that trim the frame edge and the compressible foam that closes the gap between metal and a profiled covering.
Nothing in the box is cut to length on site except at the head on some kits. Everything is dimensioned to a specific frame size, which is why the kit is ordered against the size code rather than measured.
The apron at the foot, and the water it hands on
Everything that lands on the sash runs down and arrives at the bottom rail, so the apron carries the entire flow of the window plus whatever the roof above has already delivered to it. It is the hardest working component in the kit.
The apron is pressed with a return that tucks under the bottom of the frame and a skirt that dresses down onto the covering. On a plain tile or slate roof that skirt is a flat aluminium sheet lying on the course below. On a deep profiled tile it is a malleable lead or lead-substitute skirt, and it has to be dressed by hand into every trough and over every roll, because a skirt that bridges the troughs leaves an open channel straight under the window. That dressing is the moment on the job where an experienced pair of hands shows.
The side sections, and how they sit into the covering
Down each side of the frame the kit forms a small gutter, roughly forty millimetres wide, running the full height of the window. Water coming down the slope beside the window falls into it and is carried past.
The tiles or slates alongside are cut to land into that channel rather than over it, leaving a consistent margin. The sections themselves lap one over the next in the direction of flow, always upper over lower, and they are fixed only at the top of each piece so the metal can move with temperature without tearing at the fixing. That freedom to move is why the kit is a set of overlapping pieces rather than one long tray.
The head piece, and the drainage behind it
At the top of the window the head flashing runs under the tiles above and over the top of the frame, so the covering above sheds onto metal and the metal sheds onto the sides.
The head is the point of highest risk on the whole installation, because it is where water arriving from the entire slope above meets an obstruction. Two details handle it. The head flashing itself, which is generously deep so the lap under the course above is long. And, separately, the drainage gutter formed in the roof underlay behind the frame, which catches anything that has passed the tiles and returns it onto the felt below the window. The first is in the kit. The second is not, which is the subject of the collars further down this page.
Foam gaskets, and the profile they are cut to
The strips of closed cell foam in the box look like packaging and they are not. They sit between the metal flashing and a profiled covering, filling the void that a corrugated tile leaves under a flat sheet of metal.
Without them, the space under the head flashing at each tile roll is an open tunnel, and wind will drive rain and dry snow straight into it. The foam is cut to a specific profile depth, which is one of the reasons kits are covering specific. It is also why the foam supplied must be used rather than substituted, since a general purpose strip compressed into the wrong depth either leaves a gap or holds the flashing off the tile.
Reading the code printed on the kit
The code names the covering it suits, not the window. Four letters cover most domestic work in Essex.
| Code | Covering it is made for | Where it lands locally |
|---|---|---|
| EDW | Profiled coverings up to about 120mm profile depth | Concrete interlocking tile on interwar and post-war housing |
| EDL | Flat coverings up to about 8mm: slate, plain tile | Victorian terraces and older cottages |
| EDJ | Recessed installation into a profiled covering | Where a flush external appearance is asked for |
| EDN | Recessed installation into slate | Conservation sensitive slate slopes |
Fakro, Keylite and Roto each run their own lettering for the same four situations. The codes do not translate between manufacturers and the kits do not physically interchange, so a window from one maker takes its own maker’s kit.
Lead, aluminium and two metals in one kit
Most kits combine a rigid aluminium section with a malleable skirt, and the two metals are doing different jobs. The aluminium holds a shape, spans between fixings and does not sag. The soft skirt takes the shape of whatever it is dressed onto.
Traditional kits used milled lead for the skirt. Current kits mostly use a lead-aluminium laminate or a lead free alloy, which dresses almost as well, weighs a good deal less and does not stain the tiles beneath it with the white run-off that new lead produces in its first year. On a listed roof the specification may still call for lead, and it is available. On everything else the laminate is the better material for the position.
The collars ordered alongside it
Three components sit under the flashing kit and come in their own boxes, and they are the ones most often left off a cheap quotation because none of them is visible afterwards.
The underlay collar with its integral drainage gutter ties the roof membrane to the window frame and returns stray water onto the felt below. The insulation collar is a moulded mineral wool jacket that closes the gap between the frame and the trimmed timber, without which there is a cold ring around the window that condenses every January. The vapour barrier collar links the internal vapour control layer to the frame so warm moist air from the room cannot travel into the roof build-up. A quotation that names a window and a flashing kit and nothing else is missing three of the parts that decide how the installation ages.
Why the nearest kit in the van is the wrong kit
Kits are chosen by measuring the profile depth of the covering, not by looking at it from the ground. A concrete interlocking tile from one manufacturer can be forty millimetres deep and another eighty, and both look the same from a garden.
Fit a flat-covering kit to a deep profile and the apron bridges the troughs, leaving open channels under the window that a foam gasket was supposed to close. Fit a deep profile kit to slate and the flashing stands proud, the tiles cannot bed down onto it and the whole window reads as an object stuck on the roof. Either way the result is the same: an installation that looks finished, passes its first summer, and produces a mark on a ceiling three winters later. That is the reason the survey identifies and measures the covering before anything at all is ordered.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements