Can two roof windows share one flashing?

What a combination kit is, and what it is not

A combination kit is not two single kits in one box. The side sections that would normally run up the outer edges of each window are replaced, on the shared edges, by a single valley section that collects water from both frames and carries it down between them onto the apron. The apron itself is one continuous piece across the full width of the group, and the head is one continuous piece too.

That continuity is the whole point. Every junction it removes is a junction that cannot leak. VELUX sell theirs as the EKW range and the other manufacturers have equivalents; each is specific to the window sizes, the number of units, the arrangement and the covering type, exactly as a single kit is.

The gap is a fixed dimension, and it is not negotiable

The distance between the outer frames is set by the kit you buy, not by where the rafters happen to be. On the common ranges you can order a gap of 18mm, 100mm or 250mm, and the 100mm gap is the everyday choice because it gives room for a rafter or a trimmer between the two openings.

Order the kit for a 100mm gap and build the openings 140mm apart and it will not close. Build them 60mm apart and the valley section has nowhere to sit. There is no adjustment in these parts and no legitimate way to make one up. The sequence is: decide the arrangement, order the kit, then set out the roof to the kit’s dimension.

Side by side, stacked, and the arrangements in between

Two windows next to each other across the slope is the common case and gives a wide band of light with a mullion in the middle. Two windows one above the other up the slope is less common in a house and very useful in a tall room or a high pitch, because the lower unit gives the view and the upper one throws light deep into the back of the room.

Larger groups follow the same logic. Three across, four in a two by two block, and mixed groups where a fixed unit sits above an opening one are all catalogue items. Once you get past four units the setting out has to be very accurate, because a cumulative error of three millimetres per opening becomes twelve millimetres by the far side and the kit will not take it up.

What actually sits in the gap

The space between the frames is not empty. In the usual 100mm arrangement it contains a rafter or a trimmer doing structural work, the insulation collar returns of both windows, and the valley section of the flashing over the top of them.

Inside the room that gap reads as a slim vertical or horizontal mullion in the ceiling, typically a little wider than 100mm once it is lined and plastered. Some people want it narrower, which is what the 18mm option is for, and that arrangement puts the two frames almost touching with no structural member between them. It looks better and it constrains the structure considerably, because all the load then has to be carried around the outside of the pair.

The structure decides more than the flashing does

Two openings side by side is not twice one opening. It is one wide opening with a post in the middle, and the trimmers above and below carry the load of every rafter you have interrupted, which may now be two or three rather than one. Those trimmers get deeper as the span grows, and the wall plate and the ridge have to accept what they are handed.

This is the part that catches people who have priced a pair as two singles. The flashing kit costs less than two separate kits. The carpentry costs considerably more. On anything above a straightforward two-unit group in a domestic rafter roof, the trimming needs sizing properly rather than assuming, and on a trussed roof it needs a structural engineer, because you cannot cut a truss and hope.

When two units are set close without the right kit

You see this on older work. Two windows a few hundred millimetres apart, each with its own flashing, and a narrow strip of tile or slate between them that is too narrow to lay properly. The tiles in that strip are cut down, badly supported, and often bedded rather than fixed. Water leaving the first window’s side channel arrives on that strip at speed, and the strip is not wide enough to shed it.

The result is a persistent wet mark on the ceiling between the two windows that nobody can account for, because both windows look sound. The answer is not more sealant. It is the correct combination kit and a strip that is either wide enough to take a full course of covering or replaced by a proper valley section.

You order the arrangement, not the windows

A combination order has more moving parts than a single one. You are specifying the number of units, their individual sizes, the arrangement, the gap dimension, the covering type and the recessed or above-plane option, and every one of those has to agree with the others.

Lead times run longer than for a single unit, because combination kits are ordered rather than stocked in most sizes. Two to three weeks is normal and it needs to be in the programme rather than discovered at the end of it.

Blinds, motors and the cable between a pair

Two windows in a group usually want two blinds that behave as one, and if they are motorised they want to be on the same control. That means a cable route between the frames, which is straightforward if it is planned and awkward if the plasterboard is already on.

The practical rule is that all cabling within a group is run before the internal lining goes up, with a spare draw cord left in the gap. Retrofitting a control cable across a finished ceiling between two windows means opening the mullion, which is the one part of the assembly nobody wants to touch afterwards.

What we look at before quoting a pair

We measure the rafter centres and depths, work out how many rafters are interrupted, check the slope length available above and below the group, look at the covering and its condition either side, and check whether the roof is plane enough across the full width of the group for a recessed kit to sit properly. Then we set the gap and order to it.

We install and replace roof windows and rooflights, nothing else, across Oxford and up to twenty five miles by road. VELUX, Fakro, Keylite and Roto units, fifteen years in the trade, Building Control notification handled, the roof never left open overnight, and ten years on the workmanship. If you are still deciding between a group and a single wide unit, what size roof window should I get works through it, and what is a roof window flashing kit covers the single-unit version. Call 01865 704245 for a range, then a fixed price after a survey.