Can two rooflights be joined together?

Why it exists

Every glazing system has a maximum pane size, governed by glass thickness, wind load and snow load. Where a client wants a larger continuous expanse than a single pane allows, two panes share a common structural bar.

The bar carries the edge load of both panes and transfers it to the kerb, and the weathering is designed across the joint so water sheds off rather than collecting in it.

What it looks like

A single visible line across the glazed area, typically 50mm to 100mm wide depending on the system and the span. From inside it reads as one large rooflight with a bar rather than as two rooflights.

What it is not

Two separate units set close together on one kerb. The gap between two independent frames has no designed drainage, and it will collect water and eventually leak. If somebody proposes that, it is the wrong answer.

Two separate units instead

Frequently better. Two rooflights spaced apart in the roof light a long room far more evenly than one large joined expanse in the middle, and each pane is lighter to handle and cheaper to replace.

The joined arrangement is for when the continuous expanse of glass is the point architecturally.

The structural consequence

A joined unit means one large opening, which usually means a steel where two smaller openings might need none. That is often the largest cost difference between the two approaches, and it is worth pricing both.

Yes, and there are two quite different ways

Coupled units. Most manufacturers make purpose designed coupling kits that join two or more roof windows side by side or one above another, with a common flashing running around the group. From the ground it reads as a single wide opening divided by a slim structural bar.

Separate units with a gap. Two individual rooflights fitted a short distance apart, each with its own flashing. From below the room is lit by two apertures rather than one.

Why couple rather than fit one large unit

Structure. Two narrower units coupled between rafters or trusses may need no structural trimmer at all, where one wide unit would. On a trussed rafter roof that is frequently the difference between a straightforward job and an engineered one.

Handling. Two moderate units go up a ladder or a tower. One very large unit may need a lift.

Cost. Glass price rises steeply rather than proportionally with size, so two moderate panes are usually cheaper than one large one of the same total area.

Failure. They fail independently. A seal failure in twenty years means replacing one modest unit rather than a very large and expensive pane.

Why fit separate units instead

Light distribution. Two apertures spaced across a slope give ambient daylight across a room; one wide coupled opening, or one large unit, gives a bright band with the ends darker by contrast.

In most rooms, spaced units are the better result. Coupling is the answer where the effect wanted is a single generous opening.

What coupling requires

The manufacturer’s own kit. Two windows butted together with a flashing improvised between them is where water gets in, because the junction between two units is the hardest detail on the roof and it is not something to invent on site.

Coupled units also need consistent spacing, which means the structure has to accommodate the specified gap exactly. That is measured off the roof rather than assumed from a drawing.

Mixing types

A coupled pair can combine a fixed unit with an opening one, which is worth considering: you get the glazed area without paying for two mechanisms, and the opening unit provides the ventilation and, where required, the means of escape.

On a loft conversion that combination is frequently the sensible specification.

How to decide between coupled and separate

Stand in the room and think about where you want the light to land.

If you want one generous opening to the sky, and the room is broad enough for a wide band of light to work, couple them with the manufacturer’s kit.

If you want the whole room evenly lit, fit two separate units spaced apart. Two apertures read as ambient daylight where one wide opening gives a bright band with the ends darker by contrast.

In most rooms, spaced units give the better result, and they have the practical advantages too: each can frequently sit clear of the structure without trimming, and they fail independently.

Send us a photograph of the room and one looking up inside the loft, and we will tell you what the structure allows before you decide on appearance.

Two arrangements, and they are different products

Two separate units with a combination flashing. The units sit side by side or one above the other with a rafter between them, and a purpose made kit weathers the whole group as one continuous system.

This is the common and cost effective answer. No rafter is cut, so there is no structural work, and from the ground the pair reads as a single wide element.

One large unit. A single frame spanning what would have been two openings, which means cutting rafters, trimming, and on a trussed roof an engineer.

Visually cleaner inside, because there is no rafter dividing the view. Considerably more expensive.

The dimension that governs a combination

Combination kits specify a fixed gap between the units, and it is not adjustable. It corresponds to the rafter left between the two openings.

That means the opening positions are determined by the kit before any carpentry starts. Setting the openings by eye and ordering the kit afterwards leaves a gap that no kit fits, and the result is lead dressed by hand.

Why two separate kits butted together fails

Where two individual kits meet there is a junction neither was designed to make, and between them a narrow strip of covering that is frequently too small to lay properly.

That strip cannot shed water reliably, collects debris, and on a shallow pitch holds water against both flashings.

The design point

Two tall units in adjacent bays, joined by a combination flashing, deliver a large glazed area at between the rafters cost. It is one of the best value moves available on a pitched roof.