Can a rooflight be fitted between rafters?

Why standard sizes exist

Manufacturers dimension the narrower standard sizes to suit common rafter and truss spacings, which are usually 400mm or 600mm centres. A window sized to fit within one bay slots in.

That is the whole reason the size codes look the way they do.

How to check

From inside the loft, measure between the inside faces of two adjacent rafters. That clear dimension, less a working tolerance, is the widest frame that fits without cutting anything.

While you are there, look at the roof type. Diagonal webs forming Ws between the rafters and the ceiling tie means a trussed roof. Clear space with purlins and struts means a traditional cut roof.

Why it matters more on a trussed roof

On a cut roof, cutting a rafter and trimming is routine. On a trussed roof it compromises an engineered component and needs a structural engineer’s replacement design, which changes the cost and the timescale materially.

On a trussed roof it is almost always worth taking the size that fits between the trusses rather than the size first pictured. The light difference is small; the cost difference is not.

Getting more light without going wider

Make it taller. Extending the opening up or down the slope stays between the same two rafters, needs no structural work, and throws light further into the room than extra width does.

Or fit two windows in separate bays, spaced along the room. Two narrower units light a long space more evenly than one wide one, and neither needs a rafter cut.

Yes, and it is the cheapest way to do it

A rooflight narrow enough to sit between two rafters or trusses, touching neither, needs no structural work at all. No trimmer, no engineer, no Part A structural design. It is the quick, inexpensive job and it is what a great many enquiries turn into.

What the spacing allows

Modern trussed rafters are typically at 600mm centres, so a unit sized to fit clear between them is a fairly narrow window. That is why so many roof windows on modern estates are tall rather than wide.

Traditional cut roofs are more variable. Rafter spacings on older buildings are whatever the carpenter felt was right, and they are frequently irregular across a single slope. That means the clear span at the position you actually want has to be measured rather than assumed.

Why it matters most on a trussed roof

A trussed rafter is a single engineered component in which the rafter, the ceiling tie and the internal webs work together, connected with pressed steel plates. Cutting any member sends the load it carried somewhere it was not designed to go.

Staying between trusses avoids that entirely. Going wider means a structural trimmer designed by an engineer, and that is the point at which the price steps up noticeably.

On a traditional cut roof, trimming is ordinary carpentry, so the between rafters constraint is less binding.

Going taller instead of wider

Go taller. Lengthening an opening up the slope does not cut rafters, so a tall narrow window is frequently the answer.

Fit two. Two units in adjacent bays, spaced apart, give more even daylight than one wide one, each sits clear of the structure, and they fail independently.

Splay the reveal. Cutting the lining back at an angle at head and sill rather than boxing it square opens the beam out and can make a modest window feel considerably larger. It is the best value upgrade available on any installation.

How to check your own roof

Look up in the loft with a torch. Rafters running eaves to ridge with a purlin partway up and clear space between is a cut roof. A repeating pattern of triangular frames with diagonal webs joined by flat steel plates is trussed.

Then measure the clear distance between two of them at the position you want. A photograph and that measurement answer most of the pricing question without a visit.

Why this is the cheapest way to do it

A unit narrow enough to sit between two rafters, touching neither, requires no structural intervention at all. Nothing is cut, no load is redistributed, and no engineer is needed.

Trimmers still go in at the head and sill to close the opening and support the frame, but that is carpentry rather than structural design.

The saving is substantial, and on a trussed rafter roof it is the difference between a straightforward job and a designed one.

What the spacing gives you

Modern trussed rafter roofs are typically built at 600mm centres, which leaves a clear span between trusses of somewhat less than that once the truss thickness is deducted. That sets the maximum width of a between the rafters unit on such a roof.

Older cut roofs are irregular. Hand cut timber, generations of repairs, and no dimensional standard, so the clear span at the position you want has to be measured rather than assumed. It may differ by a hundred millimetres between adjacent bays.

Going tall instead of wide

This is the move that gets people what they actually wanted. Lengthening the opening up the slope cuts no rafters, so a tall narrow unit delivers a large glass area and a real gain in headroom at between the rafters cost.

Two tall units in adjacent bays, with the rafter left between them, frequently reads better than one wide one and costs a fraction as much.

The one thing to check

That the unit genuinely clears both rafters. Forcing a frame against a rafter to gain a few millimetres puts it out of square, and a sash that does not meet its gasket evenly will draught and eventually leak.