Can two rooflights be joined together?
Yes, and there is a product made specifically for it. A combination flashing kit weathers two or more roof windows as a single composition, closing the gap between them to a fixed dimension and dressing the covering around the group rather than around each unit. On a flat roof the equivalent is a shared kerb with a structural bar between the panes. What does not happen in either case is the two units becoming one piece of glass. They stay separate frames with something deliberate between them.
This page covers what joining actually means, the gap dimensions the kits are built around, the arrangements available, and what a quote for a grouped installation should name. The wider process is on roof window installation.
Yes, using a kit designed for the arrangement
A standard flashing kit weathers one frame. It has an apron at the head, a sill piece, and side sections that dress into the covering left and right. Fit two units close together with two standard kits and the side flashings meet in the middle with nowhere sensible to go.
A combination kit solves that by supplying a single continuous piece for the space between the frames, usually a channel or a capping that carries water from between the units out to the sides and down. Every manufacturer publishes them, they are ordered by the specific arrangement and the specific gap, and they are not interchangeable between window ranges.
What joins and what stays separate
The frames remain individual. Each has its own sash, its own hinges, its own handle and its own glazing unit. What is joined is the weathering and the appearance.
That distinction matters practically. Two units joined by a combination kit still open independently, which is usually what you want, and a failure in one glazing unit years later means one unit is dealt with rather than the whole group. It also means that a group of three does not need a structural opening three frames wide: it needs three openings with timber between them, which is a much smaller intervention in the roof.
The gaps the kits are built around
The gap between frames is not a free choice. Each manufacturer publishes a small number of fixed dimensions and the kit is ordered to match one of them exactly.
The common figures are around eighteen millimetres for a tight coupled look, one hundred millimetres as the standard spacing, and a wider figure of a hundred and fifty or more where the covering runs between the units. The narrower the gap, the more the arrangement reads as a single window from the garden, and the more the roof timbers have to be planned to suit, because eighteen millimetres between frames leaves almost nothing between the trimmed openings.
| Gap between frames | How it reads outside | What it asks of the roof |
|---|---|---|
| Around 18mm | One continuous glazed band | Openings almost adjacent, careful trimming |
| Around 100mm | Clearly a pair, visually grouped | A single rafter or truss can sit between |
| 150mm and above | Two separate windows in a rhythm | Standard trimming, no combination kit needed |
Side by side, stacked and cornered
Three arrangements are in general use and they do quite different things to a room.
- Side by side, across the slope, which widens the band of light and is the usual answer where a room is wide and shallow.
- Stacked, one above the other up the slope, which is used where a tall opening is wanted but a single unit that long is not made or will not fit past a purlin. The horizontal joint between them lands on a solid trimmer.
- Grouped around a corner or a hip, less common, where two planes meet and the units follow the geometry.
Side by side is by far the most frequent, and on a trussed roof it is often the only route to a wide glazed opening, since each unit sits in its own bay between trusses.
The timber between two frames
Whatever the gap, something structural sits between the two openings, and it is doing more work than it looks.
Where the gap is a hundred millimetres and a rafter or truss is left standing in the middle, the load path is barely disturbed and the carpentry is straightforward. Where the gap is eighteen millimetres, there is no room for an existing rafter between the frames, so a rafter is trimmed out and the two openings are framed as one structural opening with a light mullion between them for fixing. That is a bigger piece of work and it wants designing, because the load from the removed rafter has to go into the trimmers and out to the doubled rafters at the edges of the group.
What this does to the light inside
Two units close together do not deliver the same light as one wide one, and the difference is the bar in the middle.
Where the gap is narrow, the shadow line from the mullion falls across the room and is barely noticed, and the light behaves as though it came from a single opening. Where the gap is a hundred millimetres or more with a rafter and a lining behind it, the internal reveal has a solid pier in the middle, and that pier can be turned into an advantage by splaying the reveals on both sides of it so the light opens outwards rather than dropping straight down. A pair of splayed openings with a narrow pier between them throws far more light onto the ceiling than a single square reveal of the same total area.
Handling, weight and getting a group onto the roof
Two or three units are two or three separate lifts, which is one of the quiet advantages of joining rather than going wide. A single very large roof window becomes a special order, a heavy awkward object and a lifting problem. Three standard units are three manageable lifts by two people.
That difference shows up in the price and in the access arrangement. It is also why a combination arrangement is often cheaper than the single wide unit it imitates, despite involving more frames, more flashing and more fixing.
What a quote for a joined pair should name
More than a quote for a single unit, because the arrangement is as important as the products. It should name each unit by manufacturer and model code with its external size and glazing specification. It should name the combination flashing kit by its own code, and state the gap dimension it is built for.
It should say whether the covering runs between the frames or whether the kit caps the gap. It should state what structural work sits between the openings and who is doing it. And it should confirm that the Building Control notification is included, which Sunspire handles as part of the job. Sunspire fits new rooflights in groups and singly, and replaces units that have reached the end of their working life, with a range by email and a fixed written figure after the survey.
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