Can you replace a roof window with a bigger one?

What has to happen

Tiles removed over a wider area, the membrane cut back, the opening enlarged, rafters cut and trimmed, new trimmers installed, the membrane made good, the new window fitted with a new flashing kit, tiles cut and reinstated, and the internal reveal enlarged, boarded and finished.

That is a two to three day job against one day for a like for like swap.

The roof type decides the cost

Traditional cut roof: cutting a rafter and trimming is routine carpentry. Straightforward and predictable.

Trussed roof: cutting a trussed rafter compromises the truss and needs a structural engineer’s replacement design. That changes the cost materially and adds weeks.

This is the question to settle before pricing anything.

Getting taller instead of wider

Often overlooked and usually cheaper. Increasing height means extending the opening up or down the slope between the same two rafters, so no rafter is cut and no truss touched.

A taller window also throws light further into the room than a wider one, so it is frequently the better answer as well as the cheaper one.

Building Regulations

Enlarging an opening is structural work and is notifiable. Where the work is notifiable we handle the notification and the certificate comes to you.

The tiles

A wider opening means cutting more tiles, and on an older roof with obsolete or fragile tiles, sourcing matching replacements can be its own problem. We check availability at survey rather than discovering it with the roof open.

Yes, and it becomes a structural job

Swapping like for like into an existing opening is the cheapest job in this trade. Making the opening bigger is a different order of work, and how much bigger decides how different.

Widening: the difficult direction

Roof openings are bounded by rafters or trusses at the sides, so widening means cutting one or more of them.

In a traditional cut roof, where rafters, purlin and ceiling joists each do a separate job, cutting a rafter and installing trimmers is ordinary carpentry. It still needs designing rather than guessing, and it is notifiable under Part A, but it is routine.

In a trussed rafter roof, each truss is a single engineered component in which the rafter, the ceiling tie and the webs work together. Cutting any member sends the load it carried somewhere it was not designed to go, and a structural trimmer has to be designed by an engineer. This is the point at which the price steps up noticeably.

A unit narrow enough to sit between two trusses at 600mm centres needs no structural work at all, which is why so many roof windows on modern estates are tall rather than wide.

Lengthening: the easier direction

Making an opening taller, up the slope, does not cut rafters. It means removing battens, extending the trimmers at head and sill, and reworking the covering above and below.

That is considerably simpler than widening, and where a bigger window is wanted it is worth asking whether a taller one of the same width would do. Frequently it would, and it costs a fraction as much.

What else changes

The flashing kit is size specific, so a new one is needed regardless.

The covering has to be reworked over a larger area, and on a fragile or handmade tile roof a proportion will break.

The lining is reformed and replastered, which is where most of the internal cost sits.

Permission. A significantly larger rooflight in a visible slope may need permission where the original did not, and on a listed building consent is required regardless.

Two smaller instead of one bigger

Worth considering seriously. Two units spaced across a slope give more even daylight than one large one, each can frequently sit clear of the structure without trimming, and they fail independently.

On a trussed roof that is often the difference between a straightforward job and an engineered one, and the result in the room is usually better rather than merely cheaper.

Wider costs more than taller

This is the single most useful thing to know before asking the question.

Making an opening taller means extending it up or down the slope between the same two rafters. No rafter is cut, so the structural work is limited to moving the trimmers. It is ordinary carpentry.

Making it wider means cutting through at least one rafter and transferring its load into trimmers on either side. That needs designing, and on a trussed rafter roof it needs an engineer because a truss is an engineered component that cannot be cut without a designed repair.

So a window that grows by 500mm up the slope may cost very little more than a like for like replacement. The same 500mm across the slope can be several times the price.

What else changes when the opening grows

The flashing kit is size specific, so a new one is needed regardless.

The reveal is being rebuilt on all four sides rather than made good, which means more plastering and a longer decorating tail.

More tiles or slates come off and go back, and on a roof of age a proportion of them will break, so a matching stock needs sourcing.

And where the enlarged window is in a loft room, check whether it now needs to meet escape window requirements, because the answer may change what you are allowed to fit.