What is a roof window flashing kit?

The parts that arrive when you order one

Open a flashing kit and you find flat-packed sections of coated aluminium or aluminium and lead composite, a folded set of fixing clips, a small bag of screws, and a printed sheet with a fixed sequence on it. There is no window in there and no glass. The kit is entirely about the perimeter.

The sections come as an apron for the bottom, two side pieces, a head assembly for the top, and on most kits a set of corner covers or filler pieces that close the junctions. On the profiled-tile kits the apron is pleated so it can be dressed down into the corrugations of a tile without splitting.

The four pieces, and what each one is doing

The apron sits at the bottom of the window and takes everything the glass has shed. It lies over the covering below, so water leaves the window and lands on top of tiles rather than behind them. It is the most heavily loaded piece and the one that gets damaged first if anyone walks on the roof carelessly.

The side sections run up the slope. They tuck under the covering on the outside edge and sit over the frame on the inside, forming a channel down each side. Water running down the roof from above is caught in that channel and carried past the window rather than around it.

The head is the piece that keeps the ceiling dry in wind. It goes under the covering above the window by a defined amount, so that water arriving from up the slope passes over it and into the side channels. Get the lap at the head short and driving rain will find the back of it, which is the single most common origin of a stain on a ceiling below a roof window.

The corner and filler pieces close the two junctions where the apron meets the sides. Those two corners are where three planes meet and are the hardest points on the whole assembly to weather by hand. The kit does it with a pressed part.

Why the kit is matched to what the roof is covered in

A kit is not universal because coverings are not similar. A plain slate is flat and a few millimetres thick. A concrete pantile is 60 to 120mm deep with a large repeating profile. A stone slate is thick, irregular and heavy. A metal sheet has ribs at fixed centres. Each of those needs a different apron shape, a different side profile depth and a different set of clip positions.

Manufacturers therefore sell the same window with a choice of kits: one for flat coverings such as slate and plain tile, one for profiled tiles up to a stated profile depth, one for deeper profiles, and specials for metal and other surfaces. Which one your roof takes is a survey question and it is set out by covering on roof window flashing kits by roof covering.

Above the plane, or recessed into it

The other choice in the kit range is how proud the window sits. A standard kit sets the frame above the plane of the covering, which is the normal arrangement and the easiest to weather.

A recessed kit drops the frame down so the glass sits closer to the level of the slate or tile, with a deeper side channel and a different head detail to cope. It looks considerably better from the ground and it is what conservation areas and older Oxford roofs generally want. It costs more, it takes longer, and it is less forgiving of a roof that is out of plane, so it wants a surveyor’s eye on the roof before it is ordered rather than after.

The two collars that are not in the flashing kit

Two items sit under the flashing and are sold separately, and they are the ones most often left out of a cheap price.

The insulation collar is a moulded jacket that fits around the outside of the frame, filling the gap between the window and the trimmed rafters with insulation to the same continuity as the roof. Without it there is a cold ring around the window, and the first cold morning shows it as condensation running down the inside face of the frame.

The underfelt collar connects the roof’s breather membrane to the window frame and forms a small gutter above the head. Any water that gets past the covering, and on a windy day some always does, is caught by that gutter and taken out onto the felt below rather than into the insulation. On VELUX these are the BDX and BFX; the other makers have their own equivalents under different names.

Why the box carries the window’s size code

Flashing kits are sold by size as well as by covering, because the sections have to match the frame exactly. A kit for a 780 by 1180 frame will not adapt to a 780 by 1400. There is no cutting to fit: the fold lines, the clip positions and the corner pieces are all dimensioned to one frame.

This is why an order for a roof window is really an order for three or four line items, and why swapping the window size after ordering means swapping the kit too. It is also why a leftover kit from a previous job is almost never useful.

What goes wrong when somebody improvises

Improvised flashing is easy to spot and it fails in predictable ways. A window bedded in mortar cracks away from it within two winters, because the frame moves with temperature and the mortar does not. A window weathered with sealant instead of a lap works until the sealant hardens, then lets water in at exactly the point everyone stopped looking. Lead dressed by hand around a window without the underfelt collar behind it will keep the bulk of the water out and let the rest into the insulation, where it does not announce itself until the plasterboard stains.

All three of those are cheaper on the day. All three of them cost a ceiling later. The unit is the small half of this job; the perimeter detailing is what decides whether it stays dry, and the flashing kit is the manufactured answer to the hardest part of that.

Lead, and why the kit is not made of it

People with older properties often ask for lead because it is what they see elsewhere on the roof. Lead is excellent at dressing to an irregular surface and it lasts a very long time, which is why several kits include a lead or lead-substitute apron for exactly that reason.

What lead cannot easily do is form the repeatable, dimensionally exact side channels and head lap that the frame needs, at the tolerances a factory can hold. So the sensible kits use both: coated aluminium for the engineered channels, and a malleable apron where the assembly has to marry into a profiled or irregular covering.

What we include as standard

Every quotation we issue names the window, the flashing kit by type, the insulation collar and the underfelt collar as separate lines, so you can see that all four are there. If a competing quotation is cheaper and does not list the collars, that is usually where the difference has gone.

We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road, into Bucks, Berks, Northants and Warwickshire. VELUX, Fakro, Keylite and Roto units, fifteen years in the trade, Building Control notification handled, the roof never left open overnight, and a ten-year workmanship guarantee. Where two units sit together you may want a combination flashing instead. Call 01865 704245 for a range, then a fixed price after a survey.