Guide · Roof Window Installation
Two rooflights side by side on one slope
Two units on one slope is very often a better answer than one wide unit, and on a trussed roof it is frequently the only answer. Light arrives from two positions instead of one, the structural work is lighter, and the elevation usually reads better from the garden. What it demands is that the spacing between them is decided deliberately, because the flashing kit, the rafters and the internal lining all have opinions about that dimension and they are not the same opinion.
This page covers how a pair is set out, what fixes the gap, how the two openings are aligned, and where a pair genuinely beats a single opening in a roof window installation. It applies to a matched pair on a rear slope, to a run of three across a long extension, and to the narrow units that trussed roofs push you towards.
Why a pair rather than one wide unit
Three reasons come up repeatedly, and only one of them is about daylight.
The first is structural. A single 1340mm unit cuts two or three rafters and needs substantial trimming, while two 780mm units in separate bays may cut one each. The second is availability: standard sizes stop at a certain width, and beyond that a unit becomes a special with a lead time and a price to match. The third is the trussed roof, where the clear gap between trusses caps the width at around 550mm and one wide opening means a designed structural modification. That constraint is covered on rooflights in a trussed roof.
How two openings light a room differently
The same glass area split into two openings does not produce the same room. A single opening produces one bright zone with light falling away in every direction from it. Two openings produce two zones that overlap in the middle, and the overlap is where the improvement lives.
Shadows are the clearest evidence. Under one rooflight, a person standing at a worktop casts a single hard shadow onto what they are doing. Under two, the shadow from each is filled by light from the other, and the room feels softer even though a light meter reads a similar figure. In a kitchen or a workspace that difference is worth more than the extra hundred lux a single larger unit would have delivered.
Two openings light a room; one opening lights a spot in a room.
What actually fixes the gap between them
Four things want a say in the spacing, and they rarely agree without being asked in the right order.
- The flashing. Combination kits are made for specific gaps, and those are the only gaps that can be weathered with a standard component.
- The structure. Rafter or truss positions decide where openings can start and stop.
- The covering. A gap of a whole number of tile widths lays out cleanly; an awkward gap means cut tiles down both sides of the strip.
- The room. The internal ceiling strip between the two reveals has to be wide enough to read as ceiling or narrow enough to disappear.
The order to resolve them in is structure first, then flashing, then covering, then the internal reading. Reversing that order produces a spacing that looks right on a sketch and cannot be built.
Combination flashings, and the dimensions they impose
Where two units sit close together, the strip between them cannot be weathered with two separate kits. The soakers from each side would meet in a channel with nothing beneath it. A combination kit closes that strip with a purpose made section that collects water from both frames and delivers it down the slope.
Those kits come in fixed gap dimensions, commonly around 100mm and around 18mm for a tight coupled pair, depending on the manufacturer and the covering. Choosing a gap outside the available range means either a bespoke lead detail between the two, which is legitimate work and is covered on lead flashing and weathering detail, or moving the units far enough apart that each gets its own complete kit with full courses of covering between them. The three flashing components themselves are explained on soakers, aprons and head flashings.
Coupled, spaced and separated: the three arrangements
In practice a pair falls into one of three arrangements, and they behave differently in every respect.
| Arrangement | Gap between frames | External reading | Internal reveal |
|---|---|---|---|
| Coupled | Around 18 to 25mm | Reads as one wide opening | One lining, flat soffit between |
| Spaced | Around 100mm | Twin openings, clearly paired | Narrow strip, usually flat |
| Separated | Full courses of covering | Two independent windows | Two complete splayed reveals |
Coupled units are the way to achieve the appearance of a wide opening on a roof that will not take one. Separated units give better light distribution across a wide room. The spaced arrangement in the middle is the most common and the least considered.
The rafter or truss that sits between them
In a coupled or spaced pair, there is a timber running up the slope between the two openings, and it is doing real work. In a cut roof that member is doubled on both sides, because it is now the trimming rafter for two openings at once and it carries the load from both.
In a trussed roof, a coupled pair is not achievable in adjacent bays at all, because the truss occupies the space the combination flashing needs. Trussed pairs are therefore separated, sitting in bays with at least one whole bay between them, and that gap is fixed at a multiple of the truss spacing. It is a constraint that makes the design decision for you, which is not the worst outcome.
Aligning the head and the sill
A pair of units on one slope wants its heads on a common line and its sills on a common line. This sounds obvious and it is the detail most often missed when the two units are different heights.
The head line matters more than the sill line, for two reasons. From outside, the eye reads the top edge against the ridge and any misalignment is obvious across the whole elevation. From inside, the head governs how much sky is visible from standing height, and two different head heights produce two visibly different views from the same room. Where sizes have to differ, align the heads and let the sills fall where they fall.
Matching sizes, and when to break the match
A matched pair is the default and it is right most of the time. Symmetry on a roof slope is restful, it aligns with the way most elevations are composed, and it keeps the ordering simple.
There are cases for breaking it. A room that is not symmetrical, a slope interrupted by a chimney breast, or a situation where one unit needs to be opening and reachable while the other sits higher up the slope as a fixed light. In those cases the deliberate difference reads better than a forced match. What reads badly is a near match: two units 100mm different in width look like a mistake, while two units of visibly different sizes look like a decision.
Setting the pair out against the elevation
The roof slope is not a blank surface. It sits above a wall with windows and doors in it, and from the garden the eye reads the whole elevation at once.
The useful discipline is to set the pair out against what is below. Aligning the outer edges of the two rooflights with the outer edges of the windows below, or centring the pair on the same axis as a set of bifold doors, makes the roof look designed rather than populated. Where the slope has a chimney, the pair usually sits to one side rather than straddling it, since a chimney between two rooflights concentrates a great deal of detail into one part of the roof.
The ceiling strip between two reveals
Internally, the strip of ceiling between the two openings is the thing that decides whether a pair looks generous or awkward. There are two good answers and a bad one in between.
Either close the gap and line the pair as one opening with a flat soffit running between them, which suits coupled and spaced arrangements, or open the gap enough that each unit gets a complete splayed reveal with a comfortable width of flat ceiling between. What fails is the strip of around 200 to 400mm, which is too narrow to read as ceiling and too wide to disappear, and which forces a plasterer to form two splays meeting at a ridge. Reveal geometry generally is covered on rooflight reveals and why splaying them matters.
Spacing for daylight rather than for the roof
If light distribution is the priority and the structure allows freedom, there is a working rule from daylight design: spacing units at roughly the same distance apart as the height from the floor to the glass gives reasonably even coverage without hot spots.
In a room in the roof with the glass around 2.4m above the floor, that suggests something around 2.4m between unit centres. Closer than that and the two pools of light merge into one and you have effectively built a single wide opening. Further apart and the middle of the room falls between them. The rule breaks in narrow rooms and in rooms with a single external wall, where the units want to be biased away from that wall to push light into the far side.
One opening unit and one fixed
Where a pair is fitted, both do not need to open. Ventilation requirements are usually satisfied by one, and the second can be a fixed light at lower cost.
The practical question is reach. An opening unit is only useful if somebody actually opens it, which means the handle wants to be within reach or the unit wants an electric opener. On a pair where one sits lower down the slope, making the lower one the opener and the upper one fixed is usually the sensible split. Where both are high, wiring both for electric operation at first fix costs very little and retrofitting either costs a great deal.
Operating a pair together
Two electric units on one slope are normally run from a single controller so they open and close together, which matters more than it sounds. A pair where one is open and one is shut looks careless from the garden, and a rain sensor closing one but not the other is the sort of thing that irritates for years.
The wiring is planned as a pair from the start: one supply, one control position, both units addressed together, with the option to run them individually if somebody wants that later. Deciding this at first fix rather than at second fix is the difference between running a cable through an open ceiling and lifting a finished one.
Three or more across a long slope
A run of three or four narrow units down a long slope is a common and effective arrangement, particularly on the rear of a Victorian outrigger or across a long single storey extension. The rhythm reads well and each unit is structurally trivial.
Two things change with a run. The setting out has to be exact, because three units with slightly unequal gaps is far more obvious than two, and the eye picks up a 20mm discrepancy across a run instantly. And the cumulative glazing area needs checking against the room, since four units that each seemed modest can add up to a summer overheating problem on a south facing slope. Solar control glass and orientation belong in that conversation, and glazing specification is where it gets settled.
Stacking one above the other
Units can also be arranged vertically up the slope, one above the other, which is a different exercise. The combination flashing between them is a horizontal detail carrying the water from the upper unit past the lower one, and it is more demanding than the side by side version because all the water from above arrives at it.
Vertical stacking suits tall rooms and steep slopes, particularly on a converted loft where a low unit gives a view out and a high one brings light deep into the room. It is worth noting that the vertical arrangement is more sensitive to pitch than the horizontal one, and on anything below about 20 degrees the head detail on the lower unit needs particular care. Shallow slopes generally are covered on rooflights on a shallow pitch below 15 degrees.
What a second unit adds to the price
Less than double and more than half. The unit itself and its flashing kit are close to full price again. The access, the scaffold, the protection of the room below, the site set-up and the notification are already paid for by the first.
The internal work is where the difference varies most. A coupled pair lined as a single opening adds very little plastering. Two separated units with full splayed reveals add a second complete lining. Where the two sit in adjacent bays of a trussed roof, the carpentry for the second is close to trivial because nothing is being cut. It is worth asking for both figures at quotation stage, since the difference between one wide unit and two narrow ones is sometimes smaller than expected once the structural work is priced honestly.
What the survey settles for a pair
Five things, and they are settled together rather than one at a time. The structural bay positions, which decide where openings can go at all. The available combination flashing gaps for the covering on that roof. The head line, set against the ridge and against the openings below. The internal ceiling strip, and whether it is being lined as one opening or two. And the operation, meaning which unit opens and how it is driven.
Photographs of the slope from the garden and of the loft interior are usually enough to give a sensible range by email. The survey then fixes the spacing, and once the spacing is fixed everything downstream of it, flashing, tiling, plastering, follows from a single dimension.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →Underlay and sarking around a pitched rooflight
Read the guide →
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