Can two rooflights be joined together?
Not joined in the sense of two frames pushed together to make one big one. Two roof windows are installed as a pair with a fixed gap between them, weathered by a combination flashing kit that is designed for exactly that spacing, and the gap is a dimension the manufacturer sets rather than something the installer chooses. On a flat roof the equivalent is two kerbs with a strip of finished roof between them. In both cases the pair reads as one composition from inside the room while every weathering detail stays standard. If what you want is a single expanse of glass, the answer is one larger unit rather than two joined ones, and that is a different conversation about structure. The installation sequence for either is on the roof window installation page.
Why the frames are not butted together
Every roof window is a self-contained weathering object. It has drainage channels moulded into the outer frame, a flashing kit that wraps it on all four sides, and an underlay dressing that runs around it. Push two of them together and there is no route for water arriving between them to get out.
The gap exists so that the space between the two frames stays a piece of working roof. Covering runs through it, the underlay runs through it, and water landing there behaves exactly as it does anywhere else on the slope. Instead of a sealed joint holding water back, you get a drained channel letting it past, which is the same principle as everything else on a roof.
The combination kit, and the dimension it fixes
Manufactured combination flashings come in defined arrangements: side by side, one above the other, or a grid of both. Each has a set spacing. Side by side, the usual figure is 100mm between the frames, which leaves room for a purpose-made section of flashing to run down the join and shed water out to both sides. Vertically, the gap is larger, commonly around 250mm, because water arriving from above has to be carried clear across the width of the lower window before it can pass either side of it.
Those figures are not adjustable on site. The kit is pressed and folded to them. Order a kit for a 100mm gap and set the frames 140mm apart and the flashing will not reach, which is the point at which somebody starts inventing a detail. The spacing is therefore fixed at the setting-out stage, before the first tile comes off.
Side by side against one above the other
| Arrangement | Typical gap | What it gives the room | What the roof has to allow |
|---|---|---|---|
| Two side by side | About 100mm | A wide band of light across the room, even shadows, a panoramic view out | Width across the slope, and a rafter or truss surviving between the two openings |
| One above the other | About 250mm | Light deep into the room and a taller view; the lower sash gives the view out, the upper one gives sky | Length up the slope, clear of purlin, ridge and eaves |
| Two by two, four units | Both figures apply | A near-wall of glass in a loft room | Both dimensions, and normally structural design |
| Two separate kerbs, flat roof | Set by the design | Two distinct pools of light with a solid band between | Falls that still work between and around both kerbs |
Side by side is the arrangement people picture and it is the more common of the two. One above the other is underrated: on a long rear slope it puts daylight at the back of a room that a single unit never reaches, and on a steep pitch it gives a view out at standing height from the lower sash while the upper one lights the ceiling.
The rafter that survives between two openings
The structural side follows directly from the gap. Side by side with a 100mm gap, the two openings are close but the timber between them still has to be real. On a cut roof at 400 or 450mm centres, two adjacent openings each in their own bay will leave a single rafter between them, and that rafter now carries a trimmer from each side. It is doing more work than its neighbours and on a longer span it may need doubling.
Where each unit is wider than one bay, you are cutting rafters twice with a narrow strip of structure in between, and that is a designed member rather than a rule of thumb. On the trussed estates at Quedgeley, Abbeydale and Bishops Cleeve the arithmetic is easier: two narrow units in adjacent bays leave one whole truss between them, nothing is cut, and the pair goes in with head and cill trimmers only.
Why a pair often beats one wide unit
On a Gloucestershire roof the pair is frequently the better technical answer as well as the cheaper one. Two openings in existing bays need no calculation. The total glass area is usually greater than the single wide unit the same slope would take. Light arrives from two positions, which softens the shadows across a room in a way a single source does not. And if one sash is the escape opening, the other can be fixed or can carry different glass.
The exception is the view. A single wide pane with no frame down the middle gives an uninterrupted view of sky and treeline, and on a loft room facing out over the Severn Vale or across a Stroud valley that is worth the structural work. Both are correct. The choice is made with the clear widths measured, not from a brochure.
Lining the pair up with the windows below
From the garden, two roof windows are a much more prominent feature than one, and how they sit relative to the elevation below is what decides whether they look intended. The convention is to align them with the windows on the wall underneath: two roof windows over two bedroom windows, centred on the same vertical lines, reads correctly from the street even though nobody consciously registers why.
Equally, a pair set symmetrically about the centre of a rear slope looks deliberate. A pair pushed to one side because that is where the bays happened to be does not. On a hipped rear slope at Longlevens or Charlton Kings there is often only one place a pair will physically go, and it is worth checking what that looks like from the garden before committing. In a conservation area the alignment question becomes a consent question, and the officer will usually have a view on it.
Two kerbs on a flat roof, and the band of roof between
On a flat roof the equivalent job is two separate made-to-size units on two separate kerbs. There is no combination kit because there is nothing to combine: each kerb is built, insulated on its outer face, and the waterproofing is dressed up and over it independently.
What has to be designed is the roof between and around them. Two kerbs sitting in the middle of a deck create a dead area between them where the fall has to work in two directions to get water out, and if the band between the kerbs is narrow the falls have nowhere to go. A minimum of 300 to 400mm of finished roof between kerbs is a sensible working figure, and more where the outlet is on the far side. That detail is covered on the flat roof rooflight installation page.
Three or four in a row on one slope
Rows of three and four do get built, most often in a loft conversion across a long rear slope, and manufactured kits cover the arrangement. The considerations scale rather than change: more rafters or trusses involved, more chance of a designed member, more weight on the scaffold, and a much longer run of covering to strip and re-lay.
What does change is the character of the roof. Beyond about three units in a row the slope stops reading as a roof with windows in it and starts reading as a glazed roof. That is a legitimate design decision on a modern extension and an odd one on an interwar semi. It also has a permitted development dimension, since each unit must project no more than 150mm beyond the plane of the existing slope, and a long run makes any deviation obvious from the pavement.
Setting the pair out before anything is ordered
The order of work is the same as for a single unit with one extra step. Clear widths are measured in every candidate bay, the surviving rafter or truss between the two openings is identified, and the combination kit is chosen for the covering and the spacing at the same time as the units. The setting-out is marked from inside so the pair lands where the room wants it, then transferred outside by drilling through at the corners.
Send a photograph of the slope from the garden, a photograph of the room, and the rafter centres if you have been able to measure them. From that, an indicative range by email will usually say whether the pair fits in existing bays or whether the arrangement you want needs a designed member, which is the fork in the road on price.
Next on this subject.
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