Trimming rafters for a pitched roof window

Traditional cut roof

Built on site from individual rafters bearing on a wall plate and a ridge. Rafters carry load individually, so removing one and transferring its load to its neighbours is standard carpentry.

The cut rafter is supported by trimmers above and below the opening, which span between the two rafters either side. Those flanking rafters now carry the extra load, and on a wide opening they are doubled.

Routine work, and it is why period properties in Lexden, Dovercourt or the Colne Valley villages take wider windows without difficulty.

Trussed rafter roof

Standard on almost everything built since the 1970s. A trussed rafter is an engineered component: rafters, ceiling tie and internal webs forming a triangulated frame, connected with pressed steel plates and designed as a system.

Cutting any member of a truss compromises the whole truss, not just that timber. The load paths are designed to be continuous, and interrupting one redistributes forces in ways the design never allowed for.

That does not make it impossible. It makes it a structural engineer’s job: the truss has to be supported off new structure, and a replacement design produced. It changes the cost of the job materially, and it changes the timescale.

Between the trusses

A window that fits between existing trusses needs no structural work at all. Truss centres are usually 600mm, which suits the narrower standard sizes.

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

Establishing which you have

Visible from inside the loft: a trussed roof has diagonal webs forming Ws between the rafters and the ceiling tie. A cut roof has clear space with purlins and struts.

We check at survey, because finding out at installation is far too late.

What trimming is

Cutting a rafter to form an opening removes something that was carrying load. Trimming is the process of transferring that load to the members either side, so the roof is as strong afterwards as it was before.

It is ordinary structural carpentry rather than a compromise, and a correctly trimmed opening leaves a roof entirely sound. An untrimmed one does not, and that is a real fault rather than a theoretical one.

How it works

Two trimmers run horizontally across the opening, one at the head and one at the sill, fixed between the rafters either side.

The cut rafter bears onto those trimmers. The trimmers transfer its load into the neighbouring rafters, which are doubled up where the calculation requires it, and those carry it down to the wall plate and the purlin as before.

The load has not gone anywhere. It has been routed around the hole.

Cut roofs, where this is straightforward

In a traditional cut roof, rafters, purlin and ceiling joists each do a separate job. Rafters carry the covering down to the wall plate, the purlin supports them partway, and the ceiling joists act as ties holding the feet of the rafters together.

Cutting one rafter and trimming the opening is normal work. A competent installer does it as a matter of course, and the roof is unaffected.

Wide openings are possible, several rafters can be trimmed where the design supports it, and lofts convert properly because the space between rafters is clear.

The one caution is the purlin. It is a structural member running horizontally through the slope and it is not something to cut. An opening goes above it or below it, which frequently decides the position of a rooflight on an older roof.

Trussed rafter roofs, where it changes entirely

A trussed rafter is not a rafter. It is a single engineered component in which the rafter, the ceiling tie and the internal webs are connected with pressed steel plates and calculated as a whole.

Cut any member and the truss stops behaving as designed. The load it was carrying goes somewhere it was never meant to go, and the effect is not confined to the truss you cut: adjacent trusses take up load they were not sized for.

That does not mean you cannot have a rooflight. It means two things.

A unit narrow enough to sit between two trusses, touching neither, needs no structural work at all. Trusses are typically at 600mm centres, so that is a fairly narrow window. It is the quick, inexpensive job and it is what most enquiries on modern estates turn into.

It is also why so many rooflights on modern housing are tall rather than wide.

Anything wider needs a structural trimmer designed by an engineer rather than judged on site. That is routine work and it is the point at which the price steps up noticeably.

It is not a cost worth avoiding. Cutting a truss without a designed trimmer is one of the genuinely serious things that happens on unqualified work.

How to tell which you have

Two minutes with a torch in the loft.

Rafters running from eaves to ridge, with a horizontal purlin partway up and clear open space between them: a traditional cut roof.

A repeating pattern of triangular frames with diagonal members crossing the middle of the space, joined by flat pressed steel plates: trussed rafters.

Broadly, properties from before the 1970s are cut roofs and anything later is likely trussed. A photograph looking up in the loft answers it without a visit and it is the single most useful thing you can send us, because it decides more about the price than the choice of window does.

Going taller instead of wider

Worth knowing, because it frequently avoids the whole question.

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

That is considerably simpler than widening, and where more glass is wanted a tall narrow unit frequently delivers what a wide one was being asked for at a fraction of the cost.

The other alternative is two units in adjacent bays, spaced apart. Each sits clear of the structure, they light a room more evenly than one wide unit, and they fail independently.

Hipped roofs and the hip rafter

On a hipped roof, the hip rafter running from the corner of the wall plate to the ridge carries the corner of the roof. It is a primary structural member and nothing is cut into it.

An opening sits clear of it, which on a modest hipped roof means the usable slope is narrower than it appears from the ground.

That is worth establishing at survey rather than assuming, because it frequently produces two or three genuinely available positions rather than a free choice.

What can genuinely weaken a roof

Four things, and all of them happen on unqualified work.

Cutting a truss member without a designed trimmer.

Cutting a hip rafter or a purlin.

Removing ceiling joists that are acting as ties holding the feet of rafters together, which lets the roof spread outward at the eaves.

And notching or drilling structural members to make a rooflight or its flashing fit.

Building Control

Forming or widening an opening is notifiable under Part A.

Treat that as a benefit rather than a burden. An independent inspection of the structural work while it is open is exactly the check you want, and it produces a certificate that a buyer’s solicitor will ask for.

Where the rooflight forms part of a loft conversion or an extension already going through Building Control, it is covered by that application and nothing separate is needed.

What to ask on a quotation

Whether the roof is cut or trussed. Whether trimming is included in the price. Whether structural design is included where it is needed, and whether an engineer’s fee is in the figure or excluded.

On a trussed roof, whether the proposed unit fits between trusses or requires a designed trimmer, because that single answer accounts for most of the difference between two quotations.

What trimming is doing

A rafter carries load from the roof above down to the wall plate below. Cut it and that path stops, so the load has to be redirected.

Trimming redirects it. A trimmer runs horizontally across the head of the opening and another across the sill, picking up the cut rafter and carrying its load sideways into the rafters either side of the opening. Those neighbouring rafters, called trimming rafters, are frequently doubled because they now carry their own load plus a share of the one that was removed.

Done properly the roof behaves as it did before. This is ordinary, well understood carpentry on a traditional cut roof.

The connections matter as much as the timber sizes

A trimmer is only as good as how it is fixed. The load arrives at its ends, and it has to be transferred into the trimming rafters reliably.

Traditionally that was a tusk tenon or a housed joint. In modern work it is more often a joist hanger sized for the timber and fixed with the correct nails in every hole, or an engineered connection specified by a designer.

A trimmer skew nailed into the side of a rafter is not a connection. It looks solid on the day and it is not carrying anything reliably.

Trussed rafter roofs are different

A trussed rafter is not a beam. It is an engineered triangulated component in which every member and every plated joint works with the others, and it is designed as part of a system with the trusses either side.

Cutting one without a designed repair compromises the whole roof, and it is not something to improvise. It needs an engineer and a specified solution, which usually involves transferring the load into the adjacent trusses through designed timbers.

That is why so many rooflights on modern estates are tall and narrow: sized to sit between the trusses, touching neither, so nothing is cut and no engineer is needed.

The purlin

On a traditional cut roof there is usually a purlin running horizontally partway up the slope, supporting the rafters along their length.

It is not a member to cut. An opening goes above it or below it, and on many older roofs that single fact decides the window position before anything else is considered.

The cheap way round all of it

Extend the opening up the slope rather than across it. Lengthening between the same two rafters moves only the trimmers, cuts nothing, and delivers a large glass area and real headroom at ordinary carpentry cost.

Two tall units in adjacent bays with a combination flashing frequently beats one wide opening, visually and financially.