Trimming rafters for a pitched roof rooflight

What a trimmed opening is

A trimmed opening has four sides. Two of them are the rafters left standing either side of the gap, called trimming rafters. The other two are horizontal members, called trimmers, spanning between those rafters at the head and the sill of the opening.

The cut rafters, now shortened, are called trimmed rafters and they stop at the trimmers. They no longer run continuously from wall plate to ridge. Their load is transferred into the trimmer, along the trimmer, into the trimming rafters at each end, and from there down to the wall plate and up to the ridge as before.

Every part of that chain has to be sized and connected for the job. A trimmer nailed between two rafters with four wire nails is not a load path. It is a shelf.

When nothing needs cutting at all

The cheapest trimmed opening is the one you do not form.

Rafters in most Oxford roofs sit at 400mm or 600mm centres. Subtract the rafter thickness and you are left with a clear gap of roughly 350mm or 550mm. Manufacturers make roof windows in structural widths that drop straight into those gaps, typically 550mm and 780mm for the common cases, though the 780mm still needs one rafter removed at 400mm centres.

If a unit fits the existing gap, the work reduces to two short trimmers at head and sill, no structural alteration, and a considerably shorter day. We cover the sizing question in detail on can a rooflight be fitted between rafters. It is always worth asking before you settle on a window size, because the difference in cost between fitting between rafters and cutting two of them is real.

Where the load goes when a rafter is cut

A rafter carries the tiles, the battens, the underlay, snow, and wind load on its share of the roof, and it delivers that load in two directions: down the slope to the wall plate and up the slope to the ridge or purlin.

Cut it, and the section above the cut has lost its lower support while the section below has lost its upper one. Both now bear onto the trimmers instead. The trimmer therefore takes a point load from every rafter that stops at it, and those point loads accumulate towards the middle of its span.

This is why an opening two rafters wide is not twice the problem of one rafter wide. The trimmer span goes up, the number of point loads goes up, and the bending moment rises faster than either. Three rafters cut is a different conversation again, and usually one involving a steel or an engineered timber section rather than a doubled softwood.

Trimmers, and why they are doubled

Standard practice is to double the trimmers and the trimming rafters where more than one rafter has been cut. Two pieces of the same section, bolted or nailed together in a specified pattern, act very nearly as one piece of twice the width.

Doubling is not decorative. A single 47 by 100mm trimmer spanning 900mm with two point loads on it will deflect visibly over a decade, and deflection at the head of a rooflight shows up as a frame going out of square, then as a sash that catches, then as a seal that no longer compresses evenly along its length.

The doubling has to be fastened properly for the two pieces to share load. Nails at 300mm centres in two rows, or bolts where the engineer says so. Two pieces of timber standing beside each other, fixed only at the ends, are two separate members and each is carrying half of what it was meant to.

Sizing the trimmer against what it carries

Sizing depends on span, on the load coming off the cut rafters, and on the depth available. Depth does most of the work: the stiffness of a beam goes up with the cube of its depth, so a 150mm deep trimmer is far more than half again as stiff as a 100mm one.

In practice we prefer to match the trimmer depth to the existing rafter depth, which keeps the ceiling line and the insulation zone intact. Where the rafters are shallow, 100mm being common in older Oxford roofs, and the opening is wide, matching depth is not enough and something has to change: a deeper trimmer dropping below the rafter line, an engineered joist, or a narrower window.

Nobody should be sizing a trimmer by eye at a width above about a metre. Below that, standard details cover it. Above it, a calculation is cheap and settles the argument.

Hangers rather than skew nails

The connection between a trimmed rafter and the trimmer, and between the trimmer and the trimming rafter, is the part most often skimped.

A skew nailed joint transfers load through a couple of nails loaded across their shanks. It works until it does not, and it slackens with every cycle of thermal and moisture movement in the timber. The correct detail is a joist hanger sized for the timber, fixed with the specified square twist nails in every hole provided, not in half of them.

Hangers must sit hard against the supporting timber with no gap behind the back plate, and the supported timber must bear fully in the shoe. A hanger with a 5mm gap behind it is transferring load through its own fixings rather than by bearing, which is precisely the arrangement it exists to avoid.

Where a hanger cannot be used because of the geometry, the alternative is a proper housed and bolted connection, not more nails.

One rafter cut, two rafters cut

These are different jobs and it is worth being blunt about the difference.

One rafter cut at 600mm centres gives roughly a 1,150mm structural opening. Doubled trimmers in matching section handle it comfortably in most domestic roofs, and the work sits inside standard details.

Two rafters cut takes the opening past 1,700mm. Now the trimmer is spanning further, carrying twice the point loads, and delivering a much larger reaction into each trimming rafter. Those trimming rafters need doubling too, and the load they deliver to the wall plate needs somewhere sensible to go, which may mean checking what is under the plate.

Our general position is that two narrower rooflights, each fitting a single rafter bay, will usually cost less and perform better than one wide unit needing two rafters removed. Where the wide unit is the right answer visually, it is worth paying for properly rather than fudging.

Position along the slope changes the answer

An opening low on the slope near the eaves behaves differently from one high near the ridge.

Near the eaves, the trimmed rafter below the opening is short and stiff and the load in it is modest. Near the ridge, the situation reverses and the short section is above. Both are manageable. What is not manageable without more thought is an opening that lands directly on top of a structural interruption, such as a purlin bearing, a strut, or the head of a supporting wall below.

The other constraint is head and sill clearance. A trimmer needs somewhere to sit, and an opening set 100mm off the ridge board leaves no room for a doubled member. There is a workable minimum, and where the position matters we set the window out from the ridge downwards rather than from the ceiling upwards.

Purlins and the propped rafter

Older Oxford roofs, anything cut on site rather than trussed, are almost always propped part way up the slope by a purlin running horizontally, itself supported on struts down to a load bearing wall or on the gable.

The purlin is what makes a 100mm rafter work over a long slope. It halves the effective span. So an opening that requires notching, cutting or otherwise interfering with a purlin is a structural alteration of a different order, and it takes the roof from a standard detail to a designed one.

Sometimes the answer is simply to move the rooflight up or down 300mm so the purlin passes cleanly below the sill or above the head. That costs nothing and it is one of the first things we look for from inside.

The two ends: ridge and wall plate

Load leaving the trimming rafters has to arrive somewhere that can take it.

At the bottom, that is the wall plate, and the plate is usually fine because it is continuous and bedded on masonry. The thing worth checking is whether the plate is sound where the reaction lands, since older plates near eaves can be soft.

At the top, the ridge board in a traditional cut roof is not a beam. It is a spacer that the rafter feet bear against in pairs, with the opposing rafter providing the balancing thrust. Cut a rafter near the ridge and its opposite number loses its pair. That is generally accommodated by the trimmer, but it is exactly the kind of detail that gets missed when somebody treats a roof as a set of independent sticks.

Timber sizes in older Oxford roofs

What you find varies by decade, and it changes the plan.

Victorian and Edwardian roofs around Jericho, Cowley Road and Summertown typically run 50 by 100mm rafters at irregular centres, with a purlin. The timber is often dense old softwood in good condition, and the centres are rarely consistent enough to design from a drawing.

Inter-war and 1950s roofs move to more regular centres with similar depths. From the 1960s onward, trussed rafters take over, usually 35 or 47mm thick and shallower, and the rules change completely.

Stone built cottages out towards the Cotswolds sometimes carry heavier, hand cut timbers with no consistency at all, and occasionally sarking boards over them. Every one of those roofs is set out on site rather than from a table.

Building regulations and when a calculation appears

Forming an opening in a roof structure is building work. Whether it needs a formal application depends on the scale of the alteration and how the work is being carried out, and a homeowner is entitled to a straight answer rather than a shrug.

As a rough guide: a unit dropped between existing rafters with short trimmers, altering nothing structural, sits at the light end. One rafter cut with doubled trimmers is routine and usually documented with standard details. Two or more rafters cut, or anything touching a purlin, a truss or a strut, is where a structural calculation earns its money.

We would rather tell you a calculation is needed and be right than skip it and leave you with an opening nobody will sign off when the house is sold.

Propping, and the order of cutting

Nothing is cut until the load it carries has somewhere else to go.

In practice that means temporary propping under the rafters either side of the intended cut, taken down to something that can carry it, before the saw comes out. On a loft floor that may mean spreading the prop across joists rather than bearing on one.

The sequence is: prop, expose, fix the trimming rafter doubling, cut the rafter, install the head trimmer, install the sill trimmer, check the diagonal of the opening, then release the props. Cutting first and building the frame afterwards puts the roof through a period of being unsupported, and that is the moment when a rafter moves 3mm and never quite goes back.

What the trimmer costs you in insulation depth

A doubled trimmer running horizontally across the slope occupies the full depth of the rafter zone at the head and sill of the opening. That is exactly where insulation would otherwise sit.

In a warm roof conversion this creates a cold bridge along two edges of the window unless it is dealt with. The fix is straightforward and it has to be planned rather than discovered: insulation returned into the reveal, or a thin high performance board applied over the trimmer face before the plasterboard goes on.

Skipping it produces condensation on the plasterboard immediately above and below the frame in cold weather, and the homeowner reasonably concludes the window is leaking when it is not.

Opening width against approach

Trussed roofs do not follow these rules

Everything above applies to a cut roof, meaning one built stick by stick on site with rafters, purlins and a ridge board.

A trussed rafter roof is a manufactured structure in which every member is in calculated tension or compression, and cutting any of them changes the behaviour of the whole truss rather than just the piece you cut. The trimming rules are not the same, the timber sizes look deceptively similar, and the consequences of guessing are worse.

That case has its own page: rooflights in a trussed roof and what cannot be cut. If your roof was built after about 1965, read that one first.

What we check before a saw comes out

From inside, we measure the actual rafter centres in the area concerned rather than assuming 400 or 600, because older roofs wander. We record rafter depth and thickness, and note the timber condition where it will be cut.

We find the purlins, struts and any binder, and we mark where they cross the intended opening. We look at what is under the wall plate at the point the reaction will land. And we work out whether the window you want can sit in a single bay, because if it can the whole structural discussion goes away.

The carpentry around a rooflight is not visible when the job is finished. Neither is the underlay dressing, the flashing lap direction, or the upstand at the head. Those four invisible details decide whether the installation is sound, and they are the reason the workmanship carries a ten year guarantee rather than a photograph.