A builder is stripping the front slope of a 1930s semi ready for two new roof windows. The windows are on order, the openings are trimmed between the rafters, and someone asks which flashing goes with them. Nobody on site is quite sure, because the box the window comes in does not contain it. The flashing is a separate part, chosen for the roof covering rather than for the window, and it governs whether the finished joint sits neatly or looks like an afterthought.
Getting it right is mostly a matter of asking early. The kit has to suit the tile or slate that is actually on the roof, the window size, and the depth at which the frame will sit. We settle all three at survey and put them in the written quote, so the kit is ordered with the window and arrives with it rather than being sourced on the day the tiles come off.
What a flashing kit does
A roof window is a frame set into a hole in the roof. Water running down the slope meets that frame, and something has to carry it round the sides and over the bottom without letting it find a way underneath. That is the job of the flashing kit: a set of formed metal pieces, with seals and fixings, that overlaps the roof covering at the sides and head and laps over it again at the sill.
Each side of the frame has its own piece. The bottom apron sheds water on to the tiles below. The side pieces, sometimes called soakers or step flashings depending on the covering, tuck under the tiles on one edge and lap the frame on the other. The top piece takes the water that comes down from higher up the slope and turns it left and right around the window. Because the roof covering sits above and below each piece in turn, the overlaps work with gravity and do not depend on sealant.
Makers design a kit for one family of coverings, and they are not interchangeable. A kit for deep profiled tiles has a soft, shaped edge that follows the roll of the tile. A kit for slate has a flat, stepped profile that interleaves with each course. Put the wrong one on and the metal either lifts clear of the covering or has to be bent into shape on the roof, which is where most badly finished roof windows start. The quote lists the kit beside the window, and the two are matched by the maker's own chart.
If you are choosing a window as part of a larger scheme, the roof windows hub sets out the full range, and the guide to roof windows in a loft conversion covers how many to specify and where to put them. Whatever you choose, the kit goes on the list at the same moment.
Profiled tiles
Profiled tiles are the ones with a pronounced wave or roll across their width: interlocking concrete or clay tiles, double pantiles and similar. They are common on postwar estates and many newer builds around Southampton, and they need the most generous flashing of the three coverings because the gaps under each roll are large.
The side pieces for profiled tiles have a flexible edge that can be pressed down into the troughs and over the crowns of the tiles beside the window, and the fitter moulds it to the actual profile on the roof. The bottom apron is deeper than on other coverings, because it has to sit over the tile roll below and still deliver water cleanly on to it.
What the roofer prepares
Before the kit goes on, the tiles around the opening are lifted and the battens re-spaced to suit the frame. Interlocking tiles make this slower than it looks, because each tile locks into its neighbours, so the roofer cuts back and re-lays the courses on either side of the opening until the cut tiles seat flat against the metal.
We note the tile type at survey and photograph it. If the roof is old and the tiles are brittle, tell the builder before the quote is signed, so spare tiles are on site before anyone needs them. Where a roof is being re-covered at the same time, the new tiles can be ordered to suit the flashing and the opening, and the roofer simply works to the written sizes.
Flat tiles and slate
Plain clay tiles, flat concrete tiles and natural or fibre-cement slate are all low-profile coverings. They lie in overlapping courses with very little relief, so the flashing for them is a stepped arrangement: one small piece of metal, or a continuous run with steps cut in, for every course the frame crosses.
On flat tiles the kit is typically supplied as individual soakers or a stepped side flashing. Each piece is laid over the tile edge and under the next course above, so it builds up in the same way the tiling does. That gives a neat result, at the cost of time, because every course has to be dealt with.
Slate behaves in a similar way but the material is less forgiving. Slates are thin and brittle at their edges, so the roofer cuts with a slate cutter or a ripper rather than a grinder, and tidy cutting matters more than it does with tile. The kit for slate is flatter than the others and relies on the overlap of the slates to hold the metal in place. Where the slates are old, the roofer will often strip back further than the flashing needs, because weathered nail holes can let a slate slip once it is disturbed.
- Plain clay tiles: stepped soakers, laid course by course.
- Flat concrete tiles: stepped soakers or a continuous stepped flashing, according to the kit.
- Natural slate: a flat stepped kit; the roofer should expect to disturb surrounding slates.
- Fibre-cement slate: as natural slate, with extra care at cut edges because the material chips.
In conservation areas and on listed buildings the choice of covering and the look of the window matter as much as the weather detail. The conservation rooflights page covers the low-profile alternative made for those roofs, which has its own flashing arrangement.
Recessed installation
A standard installation leaves the frame sitting proud of the roof covering by a small amount, with the glass standing a little above the tiles. A recessed installation sets the whole window lower in the roof structure, so the glass lies closer to the plane of the covering and the outside of the window reads as a slim, flat panel instead of a raised box.
Recessing needs a recessed flashing kit made for the purpose, which is a different set of parts from the standard one. It is also a decision for the person building the roof, because it changes the opening. The rafters or trimmers may need to be set a little differently, the insulation has to be cut to suit the lower frame, and the window loses a small part of its clear height from inside. Those points all have to be agreed before the roof is boarded, which is another reason the kit is specified at the survey and not at the end.
Recessing and planning rules
Permitted development for rooflights depends on how far the unit projects beyond the roof slope, and a recessed frame sits closer to the slope than a standard one. The projection is measured for the exact product and kit, report it back to you in writing, the conditions themselves are set out in the planning permission guide, and the council decides in any case where the answer is not clear.
The trade-off is a more careful detail at the head, where water coming down the slope has to be taken round the frame, so the roofer needs a little more room to work it.
Underfelt collars
Below the tiles or slates there is usually a layer of underlay, the sheet or felt that catches any water that gets past the covering. A roof window cuts through that layer, so the opening needs its own second line of defence. The underfelt collar is a shaped membrane that fits around the frame and connects to the underlay on every side, so any water that reaches the underlay is carried down and around the window and out over the lower courses.
The collar is the waterproofing behind the visible flashing, and it works even if wind-driven rain gets behind a tile. The order of laying matters: the collar goes on first and the underlay laps over it at the head so that water can only run down, never back in.
- The collar is supplied for the window size, with a defined drainage channel along the top.
- The underlay at the head overlaps the collar, and the collar overlaps the underlay on both sides and at the sill.
- Battens are fixed back on over the collar flanges, never through the drainage channel.
- Any insulation surround is fitted before the collar is closed in, so there is no gap to bridge later.
An insulation surround is available for most windows and suits a roof that is being insulated, because it stops the joint between frame and roof becoming a cold edge.
The collar is easy to leave out when a window is fitted in a hurry, so if you are comparing quotes, check that each one lists it by name.
Who fits the flashing
On a roof window job, two trades meet at the opening: the person setting the window and the person who works the roof covering. They can be the same person or two different firms, and the arrangement changes who is responsible for what. On a loft conversion, the main roofer often prepares the opening, lifting tiles, cutting the battens and laying the underlay, and the rooflight fitter then sets the frame, fits the flashing and hands the opening back for the roofer to close in. On a retrofit window in an existing roof, our fitters usually do the whole sequence themselves.
Where a roofer is working on the same slope, we agree the order of work with them in advance. The roofer strips and prepares, we fit the window and kit to the written sizes, and the roofer re-lays the covering round the metal. The plasterer comes afterwards for the reveals. Everyone who works on that opening should have the same sheet saying which kit, which window code and what depth, so that nothing is decided separately on the roof.
Fitting a roof window is a Building Regulations matter as well as a roofing one. Every rooflight we fit is fitted to current Building Regulations, with the notification handled where the work needs one, with a 10-year workmanship guarantee on the fitting.
Before the job is lined up, settle a few points. Which covering is on the roof, and is it sound? Is the roofer going to be on site the same week? Has the window code been settled, and does it appear in the written sizes sent to the builder? If those answers are on paper, the kit rarely becomes a problem. The roof window cost page explains what goes into a quote, and fitting between rafters covers the structural side of the opening.
Flashing questions that come up on site
- Is the flashing kit included with the roof window?
- Usually it is a separate item, chosen for the roof covering and the window size. We list the window, the kit and the underfelt collar separately in the written quote so each can be checked against the others, and all three are ordered together.
- Can one kit be used if the roof has two different coverings?
- Each window takes the kit for the covering around it. If a roof has been part re-covered, the windows on each slope may need different kits, and we confirm this from the survey photographs before anything is ordered.
- Does the flashing change how the room looks inside?
- Not directly. The lining and plasterwork inside are set by the window and the roof structure. A recessed fitting does change the depth of the opening, which the plasterer and decorator will want to know.
