Can two roof windows share one flashing?
Yes, and the product that does it is called a combination flashing. Where two roof windows sit next to each other in the same slope, or one directly above the other, a combination kit replaces the two individual side flashings that would otherwise face each other across the gap with a single shared component running between the frames. It is a boxed product ordered against both window codes and the covering profile, exactly like a single kit. The one thing it takes away is choice about the gap: the dimension between the two frames is fixed by the kit and cannot be adjusted on site. Everything else about setting out a pair on a slope, including the structural side, is on the two rooflights side by side on one slope page.
What a combination kit replaces between the two frames
Fit two windows independently and each gets its own complete set: apron, two side flashings, head flashing, corner pieces. Between them you end up with two side flashings back to back, two separate strips of covering, and a narrow channel of tile or slate that has to be cut on both edges and still drain.
A combination kit removes that arrangement entirely. In its place goes a single shared section, sometimes called a link or centre profile, that spans between the two frames and is fixed to both. There is no covering in the gap at all. The head flashings and aprons are supplied as continuous pieces running the full width of the pair rather than as two separate ones, so the whole assembly weathers as one opening with a bar across the middle. The gap is metal from top to bottom and water landing in it runs straight down onto the covering below the pair.
The gap dimension is chosen by the manufacturer, not by you
This is the point people are most often surprised by, and it needs settling before an order goes anywhere. The distance between the two frames is a fixed value determined by the kit, and it comes in a small number of options rather than as a free measurement.
Typical values sit at around 18 to 20mm for the tight version, roughly 100mm for the standard, and around 250mm for the wide version. Some ranges offer a fourth at about 60mm. Those are the numbers, and a request for 150mm between two frames has no kit behind it. What that means practically is that the overall width of the pair is arithmetic rather than judgement: two 780mm frames with a 100mm gap occupy 1,660mm of frame width, plus the structural tolerance either side. Set that number out on the roof before deciding anything about the room, because it is the figure the rafters have to accommodate.
Tight, standard and wide gaps do different things
The three options are not interchangeable and each has a use.
| Gap | How it reads | Structural consequence | Where it suits |
|---|---|---|---|
| About 18 to 20mm | Almost a single wide opening with a thin dark line across it | No rafter survives in the gap, so a designed trimmer is normal | Where the largest possible unbroken daylight is the point |
| About 100mm | Two clearly separate windows, evenly paired | Rarely lands over a rafter, usually still a trimmed member | The general-purpose choice on a rear slope |
| About 250mm | Two windows with a band of roof reading between them | Can be set out so a rafter survives whole in the gap | Trussed estates and any roof where structure must stay intact |
The 250mm option is the one that most often solves a real problem. On a roof at 400mm rafter centres, a wide gap can be positioned so that one full rafter runs uncut between the two openings, which turns a designed trimmer into two ordinary single openings with a shared flashing. That is a materially cheaper and quicker job.
One above the other, and the water the upper unit sends down
Vertical combinations exist too and behave differently. Here the shared piece runs horizontally between the head of the lower window and the cill of the upper one, and it is doing more work than a side link because it is catching everything shed by the whole area of the upper unit.
The gap in a vertical combination is larger for that reason, typically around 100mm as a minimum, and the shared section is formed as a small gutter with a fall across it and an outlet at each end. Anything narrower would concentrate the flow into a channel too shallow to clear it in a downpour. Vertical pairs turn up on steeper roofs where the run of slope allows two heights, which in this county means the Forest of Dean cottages at Coleford and Cinderford at 40 to 45 degrees, and the taller Cotswold roofs. On a shallow vale lean-to at 22 degrees there is rarely the slope length to consider one.
Mixing sizes, and mixing brands
Two windows in a combination must be the same width if they sit one above the other, and the same height if they sit side by side. That is a hard constraint of the shared piece, which is a straight component with a fixed length. A 780 by 1,180 next to a 780 by 980 has no combination kit, because the head and cill lines do not line up and there is nothing for the continuous apron to be continuous across.
They do not have to be the same window, though. Pairing an opening unit with a fixed one is common and sensible: the opening sash serves ventilation and escape, the fixed unit adds daylight at lower cost with no hardware to reach. Pairing a centre pivot with a top hung is also possible where the pitch and the knee wall suit each differently, and that choice is set out on the centre pivot and top hung compared page. What cannot be mixed is brands. Combination kits are dimensioned to one manufacturer’s frame profiles, so a VELUX and a Fakro side by side need two separate kits and a strip of covering between them, which defeats the object. Severn fits VELUX, Fakro, Keylite and Roto, and a pair is specified as a pair from one range.
Three or more under one combination
The kits extend. A row of three or four windows across a slope is ordered as a single combination with two or three shared links, and the aprons and head flashings run continuously across the whole set. The same gap options apply between each pair, and the gaps must all be the same.
What changes at three is the structure and the setting out. Three 780mm frames with 100mm gaps occupy over 2.5 metres of slope width, which crosses four rafter bays at 600mm centres and means a designed trimmer with the load carried out to rafters either side. That is an engineer’s calculation and a Building Control submission rather than a routine trimming exercise. The other change is that the whole row now has to be set out square and true across its full width in one go, because any error accumulates across the links and shows as a gap that tapers. It is measured, marked and checked before the covering comes off.
Adding a second window alongside one already in the roof
This comes up regularly and the answer is usually yes with a condition attached. A combination kit can be fitted to an existing window, but the existing window has to be a current profile from a range still in production, because the shared link is dimensioned to a specific frame section.
A unit fitted in the 1990s is generally not that. Frame profiles have been revised, some codes have been discontinued, and a shared link made for a current frame will not sit on an older one. Where that is the case there are two honest routes: fit the second window independently with its own kit and a strip of covering between the two, which works perfectly well and is what most older pairs on Gloucestershire roofs actually are, or replace both units at the same time with a matched pair under one combination while the scaffold is up. The second is often the better value, because a unit past twenty years old has gaskets, hinges and a sealed unit all at the same stage, and a second scaffold visit costs more than the difference. What is found under an old frame is set out on the replacing a failed or misted rooflight page.
Where a combination earns its place on a Gloucestershire roof
The pattern is consistent across the county and it comes down to run of slope. Interwar semis at Longlevens, Hucclecote, Warden Hill and Charlton Kings have hipped concrete tile roofs where the slope narrows towards the corner and a purlin crosses at mid-height. The clear zone on a typical rear hip is often around 1.2 metres of vertical run across two rafter bays, so a tall single window will not go in. Two shorter units side by side under one combination deliver more glass in total, light the room more evenly, and stay inside the run of slope the roof actually has.
On the trussed estates at Quedgeley, Abbeydale, Bishops Cleeve and Yate the argument is structural. Trusses at 600mm centres give around 550 to 570mm of clear width between them, and the manufacturers make 550mm wide units precisely for that. Two of those in adjacent bays with a wide combination gap sitting over the truss between them needs only head and cill trimmers in each bay, with no truss touched. On the Victorian rear additions through Gloucester, Longford and Tewkesbury, where the roof gives only 1.8 to 2.4 metres from gutter to abutment, a horizontal pair set high is often the only arrangement that leaves room for the flashing laps above and below. The combination and the two window codes go on one order, and the process is described on the VELUX window installation page.
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