Can a rooflight be fitted between rafters?

What fitting between rafters actually means

A pitched roof carries its covering on rafters running from the eaves up to the ridge, spaced out across the slope. Between any two of them there is a clear channel of roof with nothing structural in it apart from battens and underlay. A rooflight fitted between rafters sits inside one of those channels. The opening is formed by cutting the battens and the underlay, adding timbers across the top and bottom of the opening to close it and carry the frame, and leaving both flanking rafters entirely alone.

Those short timbers at head and sill are not structural alterations in the sense that matters. They span a single bay and they carry the frame and the covering immediately around it. Nothing that was holding the roof up has been interrupted, so nothing has to be replaced with something stronger.

From below, once the reveal is lined, you cannot tell which route was taken. From the point of view of cost, programme and paperwork the difference is considerable, which is why it is the first thing we look at in a loft.

Why avoiding the cut keeps the cost down

Cutting a rafter changes the problem completely. Load that was travelling down that rafter has to go somewhere else, which means a trimmed opening: heavier timbers across the head and sill, sized to pick up the cut member and carry its load out to the rafters either side, which in turn take more than they were built to take. That is a structural design question, not a carpentry preference.

So a trimmed opening brings in design input, larger sections of timber, more labour in the roof, and a longer period with the roof open. None of that is a reason to avoid it where the room genuinely needs a wider unit, but all of it costs money and time.

An opening that sits between two untouched rafters skips every part of that. The structure is undisturbed, the timber is light, the roof is open for less time, and the price reflects it. Where the daylight you want can be delivered without alteration, that is the sensible route.

The gap you already have caps the unit

You do not choose a size and then find a roof for it. The roof is already built, the clear span between adjacent rafters is already settled, and that span puts a hard ceiling on the width of any unit going between them. The frame sits within the gap with the covering detailed around it, so the widest unit available without alteration is narrower than the gap itself.

Manufacturers make widths intended to suit the spacings common in British roofs, so there is usually a stock size that fits neatly. But the only spacing that matters is the one over your head. Roofs vary by period and by builder, and older roofs in particular are irregular, because they were set out by eye rather than to a standard.

Height is a different matter. Length up the slope is not constrained the same way, so a tall narrow unit can deliver considerably more glass than the width alone suggests.

Going wider means a designed alteration

If the room genuinely wants something wider than a single bay allows, the route is a trimmed opening, and that is a designed structural alteration worked out properly rather than judged on site. We are rooflight installers rather than structural engineers, so where that route is taken we coordinate with one and install to their design rather than improvising around a cut rafter.

It is worth being clear about what triggers this. It is not the size of the glass in itself. It is whether the opening crosses a structural member. A wide unit sitting in a wide bay needs no alteration at all; a narrow unit placed carelessly so that it clips a rafter does. Position on the slope therefore matters as much as size, and moving a proposed opening sideways by a bay sometimes removes the whole question.

Where a wider opening is genuinely wanted we say so at survey, with the cost implication attached, so you can weigh it up rather than discover it after the roof is open.

Trussed roofs are a stricter case

A great deal of post-war housing across Oxfordshire has a trussed roof rather than a cut one, and in a loft the difference is unmistakable. Trussed rafters are light timbers forming a triangle with diagonal webs across the middle, joined at every intersection by a toothed metal plate, and braced to one another.

A truss is an engineered system rather than a set of independent rafters. Every member carries a calculated load, the plated joints are part of that calculation, and the bracing makes the roof behave as a whole. Cutting a web or a bottom chord changes how load travels through that truss and, through the bracing, how its neighbours behave. A truss cannot be trimmed the way a rafter can.

In practice an opening in a trussed roof almost always has to fit within the clear space between two adjacent trusses, and going wider means a designed alteration by an engineer. Which kind of roof you have is established in the loft, and it settles a great deal.

Why a pair often beats one wide unit

When the width cap lands short of what somebody had in mind, the useful move is generally sideways rather than upwards in cost. Two units in separate bays, spaced apart along the slope, give more usable daylight in most rooms than one wide unit in the middle, because light arrives from two positions and washes the room more evenly.

They also tend to look better from outside. Openings set in a rhythm read as part of the roof, while one large opening reads as an interruption in it. On an older building, or one in a conservation area, that difference is part of what an authority assesses.

So the sequence is the room and the roof first, the product afterwards. We establish what the roof is, what the clear bays allow and where the light is needed, then specify VELUX, Fakro, Keylite or Roto units to suit. Our workmanship carries a ten-year guarantee, we handle the Building Control notification, and we have been installing rooflights for fifteen years.