Can two roof windows share one flashing?
Yes, and the part that does it has a name: a combination flashing kit. It weathers two or more roof windows as a single assembly, running a gutter between the frames and taking the whole group out to the covering with one continuous apron at the foot and one head flashing at the top.
What it does not do is let you choose the gap between the units freely. Combination kits are made for specific spacings, and the one you pick changes the appearance, the carpentry and the structural setting out all at once. The single unit version of the same subject is on what is a roof window flashing kit, and the products themselves sit under VELUX window installation.
What a combination kit is doing that two kits cannot
Two separate windows fitted close together each carry their own side gutter, so between them you get two channels, two rows of cut tiles and a narrow strip of covering in the middle that has to be supported, fixed and kept clear. Below about 250mm that strip is too narrow to lay properly.
A combination kit replaces the whole middle with a single formed gutter that sits between the frames and discharges onto the apron below. There is no covering between the units at all. The result reads as one glazed area with bars across it rather than as two windows that happen to be near each other, and there is a great deal less to go wrong in the middle.
The gaps that are actually available
Three spacings cover almost all domestic work, and they are not interchangeable once the opening is trimmed.
| Gap between frames | How it reads | What it means structurally |
|---|---|---|
| 18mm | Almost a single window with a bar down it | One structural opening, trimmed as one |
| 100mm | A clear division, glass then frame then glass | Timber between the two openings doing work |
| 250mm | Two distinct windows, visually paired | A full rafter or truss can sit between them |
The 18mm spacing gives the most glass for a given area of roof taken out and is the one people want when they see it. It also removes any structural member between the two openings, so the trimming has to carry the whole width in one. The 250mm spacing is the easiest to build and the one that suits a trussed roof, because the thing in the gap can be a truss.
The rafter between two roof window frames
The gap is not empty. At 100mm there is a common trimming rafter between the two openings, cut to a section that carries load from both sides, and the combination gutter sits over it. At 18mm there is no room for anything structural, so the two openings become one and a single pair of trimmers spans the full combined width.
That distinction is where the cost sits. Two MK06 windows at 18mm spacing form an opening roughly 1580mm wide across the rafters. In a cut roof with rafters at 400 centres, that is three rafters cut and a trimmer designed to carry them. In a trussed roof it is a structural engineer’s drawing. The same two windows at 250mm may need nothing cut at all.
Side by side, stacked, or an array
Horizontal pairs are the common arrangement, but combination kits also handle vertical and grouped layouts. A stacked pair puts one window above another up the slope, which suits a tall room where the lower unit gives a view out and the upper one throws light deep onto the ceiling.
Vertical spacing works differently from horizontal. The gap between a lower head and an upper cill is larger, commonly 100mm or 250mm, because the gutter between them has to shed the flow off the upper window sideways before the lower window starts collecting. Arrays of four, two wide by two high, use kits that combine both. In a room with a genuine cathedral ceiling the effect is very good and the setting out has to be exact, since every frame has to align in both directions with no accumulated tolerance.
Where the water goes between two units
The gutter between the frames collects run-off from the facing edges of both sashes plus anything the head flashing above has diverted sideways. It runs the full height of the windows and terminates at the apron.
Because it is fed from two sides and has no fall other than the pitch of the roof, its capacity is the limiting factor on how many units a single kit will handle. That is also why the pitch matters more on a combination than on a single window. On a shallow slope the flow moves slowly and the gutter runs deeper. Below the minimum pitch for the kit, which is often stated a few degrees higher for combinations than for single units, the manufacturer will not stand behind it.
Pairing across a truss rather than through it
On the trussed roofs of Great Notley, Beaulieu Park, South Woodham Ferrers and the Harlow and Basildon estates, the combination question has a particularly satisfying answer. Trusses sit at 600mm centres, leaving a clear gap around 550mm between them, which is exactly the C width.
Put a C width unit in one bay and another in the next bay, with the truss between them, and the spacing lands at roughly 250mm. That is a standard combination kit gap. The result is two windows weathered as one assembly, in a trussed roof, with no web cut, no bottom chord touched and no engineer engaged. It is the neatest outcome available on that kind of roof and it delivers more usable daylight than a single wide unit would, because the light arrives from two positions.
Setting two openings out against the batten gauge
With one window there is usually a workable position within a course either way. With a combination the head and cill of every unit have to align, so the setting out is fixed in both directions at once and there is no adjustment left.
The consequence is more tile cutting, and how much depends on where the combined head lands against the batten gauge. On a plain tile roof at roughly 100mm gauge there is normally a position that works. On a deep interlocking tile with a 300mm plus gauge, or a hand made peg tile roof out towards Thaxted and Coggeshall, the setting out is decided on the roof with the battens exposed rather than from a drawing. Getting it wrong means cut tiles supported on nothing across a wide opening, which is where combinations acquire their reputation for being fussy.
How many units before it stops working
Kits are made for two, three and four unit groups, and beyond that the sensible route changes. Three C width units at 18mm spacing across a bedroom slope form an opening approaching 1750mm, and at that width the trimming, the glass handling and the gutter capacity are all being stretched at once.
Where a room genuinely wants that much glass in a slope, the conversation usually turns to whether the roof plane is the right place for it at all. A wide continuous run reads as a strip of sky and can be excellent. It is also a structural opening with an engineer attached, so it belongs in the design conversation early rather than as an upgrade after the price is agreed.
Separate kits with tiles between, and when that wins
There is a good case for keeping two windows apart. Once the gap reaches about 400mm or more, standard single kits on each unit and a properly laid strip of covering between them is simpler to build, cheaper, and easier to replace one half of in twenty years.
It also suits the elevation on some houses. Two windows spread across a wide rear slope, one over each half of a room, light the space more evenly than a tight pair in the middle. The pair reads as a feature and the spread reads as original, which on a Victorian rear addition in Chelmsford is often the more comfortable result. Where the sizing question is still open, what size roof window should I get works through the room side of the decision.
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