A flashing is the strip of metal or flexible weathering that closes the gap wherever a roof covering stops against something else: a chimney, a wall, a roof window. Around a roof window it has to do four jobs at once. It has to carry water that arrives at the top of the frame round the sides, stop wind-driven rain being pushed in under the sash edges, let the covering lie as it was meant to, and allow the frame to move a little as the timber and the weather work on it.
The frame does not change; the roof does. A window fitted into smooth concrete tiles sits in a different world from one fitted into natural slate or a thick stone slate roof, and the kit that suits one can be wrong for another. This guide is about the installation detail and the choices behind it. Where water is already getting in, our guide to why roof windows leak deals with finding and curing the cause. For the wider picture of fitting, pricing and planning, start at roof windows and skylights.
What a flashing kit is made of, and what it does at each edge
A manufactured kit is a set of formed pieces cut to the window size and matched to the type of covering. Most kits include a bottom apron that lies over the covering below the window, side gutters or flashings that carry water down the sides, a top piece that sheds water over the head of the frame, and fixings and sealing strips to hold them. They are usually in aluminium finished to look like a roof, or in lead-coated material, with the pieces designed to interlock so that water meets a lapped joint wherever it goes.
Top, sides and bottom do different work
At the top of the opening, the job is to divert water round the window without letting it dam up. The pieces here slope away from the frame so that water running down the roof splits and flows round it. Down each side the flashing has to keep pace with the covering, rising with each course or profile. At the bottom, the apron lies over the covering below, so that water coming off the window drops onto the roof and carries on down.
Why a kit is only part of the answer
The kit comes from the manufacturer for a stated range of coverings, and each is tested against a stated type and a stated height of covering. Where the real roof falls outside that range, the manufacturer's range stops being enough and the flashing has to be made on site. That happens regularly on older Stroud roofs, which were not laid to any catalogue. The sections below look at each covering in turn.
Plain tiles and flat concrete tiles
Plain tiles, whether clay or concrete, lie flat and close, with a small overlap between courses. A flat-tile roof is the easiest covering to flash against, because the water path is predictable and the kit's own side pieces sit tight to the tiles beside the frame. The standard kit for flat tiles uses a short, low-profile side flashing that lies beneath the edge tiles and above the lower courses.
The details that matter on a flat-tile roof are the cuts. The tiles that meet the window frame are cut neatly so that the kit's side flashing can lie under them without lifting, and the courses either side are checked for slipped or cracked tiles before the frame goes in. Slipped tiles beside an opening are a regular source of trouble, and a new window is the moment to deal with them. Cracked clay tiles beside a new frame will let water onto the kit from above, so we replace them as part of the job.
Minimum pitch still applies
A roof window does not change the pitch limits of the covering around it. Marley's data gives its concrete plain tile a minimum pitch of 35 degrees, and its clay plain tiles 30 to 40 degrees by range, so a window in a shallow plain-tile roof may sit in a covering already close to its limit. Read the tile data sheet before settling the position.
Pantiles, double Romans and other profiled tiles
Profiled tiles have a wave or roll in the section, and the sections below the window and beside it are not flat. The flashing has to follow the profile, which is where standard kits show their limits. The tile makers' own ranges publish minimum pitches of 15 degrees for the Marley Lincoln clay pantile and 30 degrees for the Sandtoft Old English Pantile, so the covering and the roof pitch vary a good deal, and the kit has to follow.
Manufacturers supply a profiled-tile kit with a deformable or soft edge that is pressed down into the troughs on each side, sometimes with a separate infill piece. The principle is that the edge fills the gap in the profile, so water cannot run sideways under it. The lead or aluminium is dressed into the shape of the tile, called dressing, and it has to be done carefully so that the metal is not stretched thin or split at the high points.
Where the profile and the frame disagree
Profiled tiles do not cut cleanly the way flat tiles do. A tile cut through the middle of a roll leaves a ragged section that no flashing can seal. The better solution is usually to adjust the position of the window, within the rafters, so that the frame lands between rolls and the cuts are minimal. Sometimes a half-tile or a purpose-made fitting is used to finish the junction. This is another reason the window position is set on site against the actual tiles, and not drawn from a plan alone.
Natural slate: soakers, lead and the double-lap
Slate lies flat, like plain tiles, but in a different way: it is thin, hard and cut square, and it is laid with a double lap, so every part of the covering sits under at least two layers. A window kit built for slate has a thin flexible edge to sit in that tight stack, and it relies on the slate being sound and the laps being correct.
Where kits are not enough, the traditional answer is lead. A soaker is a small L-shaped lead sheet, laid in with each course of slate or tile beside a vertical obstruction, so that it slides under the slates above and turns up against the frame. A soaker per course is the classic detail beside a window, and it is cut and bent on site. Lead sheet comes in numbered codes; Leadworx calls Code 4 the usual pick for general flashing, and soakers use Code 3 or 4, thin enough to dress and thick enough to last. MyJobQuote gives over 80 years for lead flashing that is installed correctly, and over 100 years in the higher codes.
Exposure and headlap
Cupa Pizarras treats anywhere within 30 miles of a west-facing coastline as severe exposure for slate, and the headlap needed grows with exposure and shrinks as the pitch steepens. A window cuts through the slates, so the laps either side must be as good as the field of the roof; a thin lap beside a frame is a weakness that the kit cannot hide.
Thick stone slates: why catalogue kits rarely fit
A Cotswold stone slate roof is a different problem. Stone slates are thick and uneven, and Cotswold District Council's technical guidance reckons a stone slate roof at about 1000 kg per 10 m2. The slates run in diminishing courses, shortening from eaves to ridge, so thickness and lap alter up the slope. Our guide to diminishing courses on a stone roof explains why.
A standard kit is built for a covering of limited thickness and a regular surface. Thick stone breaks both assumptions. The kit may not close down onto the surface, it may sit proud of it, and the gaps under the apron can be wide enough to let water run up and in. On a stone roof the flashing is usually made on site in lead, dressed over the stone and into the gaps, with soakers or step pieces cut to the course heights.
Matching what the roof already has
On a listed or conservation-area building, the choice of flashing is also part of the look of the roof. Lead is the traditional metal on stone and is generally what the planning team expects to see. A window that is permitted development may still need a conversation about the finish, so ask the planning team before ordering.
Pegs, battens and loading
Stone slates hang on pegs, not nails. Cutting the opening interrupts the pegged courses, so the slates around the frame have to be re-hung and re-pegged, and sometimes the battens have to be made up to carry the extra points of load. A heavy covering also means the trimmed rafters must carry the weight round the opening, so the structural side of the cut is more than a formality.
Recessed fitting: sitting the window lower in the roof
Most roof windows are fitted so the sash stands above the roof plane, held in place by the flashing. A recessed installation sets the frame lower, so the glass sits closer to the level of the covering. The result looks neater from the street and sits better in a roof with a historic character, and it is the usual choice for a conservation-style window on a listed roof.
The detail changes with the position. A frame sunk into the depth of the rafters needs a recessed or low-profile flashing kit made for that depth, and the insulation and lining around the frame have to be built to match. Because the frame is lower, water sitting round it has less fall to leave the opening, so the side pieces need extra care and the top has to shed water properly.
The Class C protrusion limit
Class C of the General Permitted Development Order caps how far a roof window can stand proud of the existing slope at 0.15 m. Recessing helps stay inside that cap on a thick roof, but it is measured on the finished roof, so confirm it on a drawing. For your own address, ask Stroud District Council's planning team.
Recessing a window changes the flashing, the lining and the insulation in one go. Ask for the flashing kit by name on the quote, so you know a recessed product is being priced.
Costs, the order of work and checking the finished joint
In MyBuilder's 2026 pricing a standard roof window costs about £350 to £600 to fit, with scaffold near £500 if needed. Lead flashing costs more, and a stepped flashing in Code 5 runs at about £80 to £120 per metre, ex VAT, according to Checkatrade's roof repair guide. Hand-dressed lead around a window on a stone roof sits at the upper end of that kind of figure, since the work is skilled and slow. After the survey the quote takes in the kit, any lead, scaffold and making good.
The order of work
The sequence matters. The opening is cut and trimmed, the frame is fixed and the underlay is lapped up round it as a collar or sleeve, so that any water getting under the covering is carried to the surface. Then the covering is laid back up to the frame, and the flashing pieces go on from the bottom upwards, each lapping the one above. Work from the top down and the laps run the wrong way.
What we check at the end
Before the scaffold comes down we check that the laps all run downhill, that no fixing goes through a part that has to shed water, that the tiles or slates beside the frame are firm, and that the sash opens and closes without catching the flashing. We photograph the joint. Our 10-year workmanship guarantee is in writing. Winter jobs bring their own problems, set out in our guide to fitting roof windows in winter, and a window that mists inside after fitting is covered in the guide to condensation around roof windows.