Trimming rafters for a pitched roof rooflight

Cutting one rafter changes three, and the arithmetic behind that

A rafter carries the strip of roof half way to its neighbour on each side. At 400mm centres that strip is 400mm wide; at 600mm centres it is 600mm. Cut one rafter and the strip it was carrying does not disappear, it transfers into the head and cill trimmers, and from there into the two rafters flanking the opening. Each of those now carries its own strip plus roughly half of the orphaned one, so a flanking rafter picks up around a 50 per cent increase in the load it was designed for over the length of the opening.

That is why cutting one rafter is routine and cutting two is not. Take out two and each flanking rafter picks up a whole extra strip, a 100 per cent increase, and on a heavy covering the flanking members themselves need doubling as well as the trimmers. Three is engineer territory every time. The honest version of that at survey is that the width of the window, the rafter centres and the weight of the covering together decide whether the job is a day’s carpentry or a structural submission.

Common rafters, principal rafters and the members that stay whole

Not every sloping timber in a roof is a common rafter. In Gloucester’s Victorian terraces and the interwar semis across Longlevens and Hucclecote the roof is usually a simple cut roof: common rafters at 400 or 450mm centres, a ridge board, and a purlin propped by struts. The commons are what get trimmed.

Older and larger roofs, including a good many Cotswold farmhouses and the Forest of Dean stone terraces, are principal rafter roofs. A principal is a heavier member landing on a truss or a masonry cross wall, carrying purlins that in turn carry the commons. A principal is never cut for a rooflight. Neither is a purlin, a wall plate, a ridge in a roof that relies on it, nor a tie at the foot of the rafters holding the walls in. In a trussed roof the whole triangulated frame stays whole, which is a different job entirely and is set out on rooflights in a trussed roof.

The head trimmer and the cill trimmer do two different jobs

Both trimmers span horizontally across the opening between the flanking rafters, and people assume they are a matched pair. They are not doing the same work.

The head trimmer, at the top of the opening, carries the cut rafter above it. That upper stub of rafter runs on up to the ridge and everything on it, including the roof covering, any snow load and any wind uplift, arrives at the trimmer as a point load near mid-span. The head trimmer is usually the more heavily loaded of the two.

The cill trimmer, at the bottom, carries the lower stub running down to the eaves, which is generally shorter, but it also takes the weight of the window frame itself and it is the member the internal reveal and often a radiator get fixed to. On roofs where the opening sits high in the slope, the head trimmer is doing nearly all the structural work and the cill trimmer is doing most of the practical work.

Timber grade and section: what a trimmer is actually made from

Structural softwood in a British roof is graded C16 or C24. C24 is roughly a fifth stiffer and stronger for the same section, and on a marginal opening specifying C24 for the trimmers is the cheapest structural upgrade available. It arrives on the van as the same piece of wood with a different stamp on it, and the stamp is what an inspector reads.

The section is matched to the existing rafters in depth wherever it can be, so the insulation zone stays continuous and the plasterboard has a flat plane to land on. Old cut roofs are usually 50mm thick sawn stock rather than 47mm regularised, so a modern trimmer sits 3mm proud, and the fix is a planed packer rather than an accepted step. Where the roof is 100mm deep, which plenty of pre-war rafters are, the trimmer often has to go deeper than the rafters and the reveal is designed around the projection.

Notching and drilling a rafter that is carrying an opening

A cable route or a pipe run through a flanking rafter can undo the trimming quietly. The general rule for joists carries across: notches only in the top edge, within the outer quarter of the span, no deeper than an eighth of the depth. Holes on the neutral axis at mid-depth, no bigger than a quarter of the depth, spaced at least three diameters apart.

Around a rooflight opening the sensible answer is stricter than that. The flanking rafters and the trimmers are carrying more than the roof was designed for, so nothing gets notched or drilled through them at all inside the length of the opening. Cable for an electrically operated unit is run in the void beside the frame or over the top of the trimmer, and that route is decided while the roof is open rather than improvised by an electrician six weeks later.

Cutting the opening oversize, and the packing gap that follows

The structural opening is set out 40 to 45mm wider and taller than the frame. That gap is not slack, it is the adjustment the whole installation depends on. Old rafters are not parallel, they are not plumb, and the tile courses are not square to anything. The frame goes in level, square and true within that gap, packed at the bracket positions with rigid plastic or plywood packers rather than being pulled tight to a rafter that happens to be leaning.

Cut it to a tight fit and the frame gets forced. A frame racked by even a few millimetres across its diagonal will still take its sash, and the sash will bind on one corner and hold its gasket open on the other for the rest of its life. Cut it too generous and the packers become stacks, which compress and settle. The 40 to 45mm figure exists because it is enough to correct a bad roof and small enough to pack in one piece.

Purlins across the line of the window, and the two zones they leave

The purlin is the reason most Gloucestershire openings end up where they do. In a typical interwar or Victorian cut roof it runs horizontally at about mid-height on the slope, propped by struts down to a load bearing wall or a binder in the ceiling. A purlin is not cut and it is not notched.

What it leaves is two clear zones: one from the purlin up to the ridge and one from the purlin down to the eaves. On a rear slope of 4 metres along the pitch, that is often around 1.6 metres of clear run above and 1.9 metres below, and the flashing laps eat into both ends of whichever one gets used. A 1180 tall unit fits the lower zone with room for the apron to land on whole courses. A 1400 does not, and finding that out at survey costs nothing while finding it out with the covering off costs a day.

Struts, collars, ceiling binders and what turns up in the loft

Struts land where the original carpenter put them, which is frequently in the middle of the bay somebody wants the window in. A strut can sometimes be moved one bay along and refixed to the same binder or wall, which is a designed change rather than an improvised one, and it needs recording. Sometimes it cannot move at all, and the window shifts instead.

Collars tie opposite rafters at high level and reduce the head height available inside. Ceiling binders sit in the loft floor tying the ceiling joists, and they interfere with the internal reveal rather than with the opening itself. Where a roof has a hip, as most of the interwar stock across Warden Hill and Charlton Kings does, the hip rafter and its jack rafters change the geometry again: the jacks get progressively shorter towards the corner and cutting one near the hip removes a member that was helping to stabilise the hip itself.

Propping the slope while the rafter is out

Between cutting the rafter and fixing the trimmers there is a window of time when the roof above the opening is holding itself up on nothing but the covering and habit. On a small opening in a sound roof with a light covering that window is short enough not to matter. On anything wider, or on a heavy covering, the slope gets propped first.

Propping on a pitch means an adjustable prop or two off a sole plate spreading the load onto the ceiling joists near a wall, with a head plate under the rafters either side of the cut, taking the weight before the saw goes in. The props stay until the trimmers are in, bolted and hung. On a Cotswold roof carrying stone slate the props go in as a matter of course, because the weight overhead is several times what a concrete tile roof puts on the same rafter.

Sarking boards, and the cut edge that needs its own support

A boarded roof changes the sequence. Forest of Dean terraces around Coleford and Mitcheldean, and a number of Cotswold roofs, are sarked: close boarded over the rafters with the slates nailed to the boards or to counter battens over them. The boards are structural in the sense that they brace the roof plane and spread point loads, and cutting a rectangle out of them leaves four unsupported board ends.

The boards get cut back to land on the trimmers and the flanking rafters, so every cut end has bearing, and where a board end falls in mid air a noggin goes in to catch it. On old riven or feather edged boarding, which is not uncommon on a stone roof, the board thickness varies across the width of the opening and the trimmer tops get packed to bring the plane true before any batten goes back on. Underlay detailing over a sarked deck is its own subject, covered under underlay and sarking around a pitched rooflight.

Where timber runs out and a flitch or a steel takes over

There is a point on every roof at which a deeper timber stops being available, usually because the rafter depth plus the insulation zone has used the space. The next move is a flitch: a steel plate, commonly 8 or 10mm, bolted between two timbers so the steel does the bending and the timber gives something to nail to and keeps the section the same depth as the rafters. It comes up on Gloucester terraces where the rafters are 100mm deep and the customer wants a wide unit.

Beyond a flitch it is a rolled steel section, sized by an engineer, with its bearings designed at both ends. On a pitched roof that usually means the steel sits horizontally across the slope at the head of the opening with the cut rafters hung off it, and its ends land on the flanking rafters doubled up, or on masonry where the geometry allows. That is a bigger piece of work with its own programme, and it gets described as such at survey rather than discovered halfway through.

What Building Control looks at on a trimmed pitched opening

Forming an opening in a roof is notifiable, and the structural element is the part an inspector wants to see before it is covered up. The notification is submitted as part of the job and the inspections are booked around the programme.

What gets looked at is the trimmer sections and grade, the connections at each end, whether the flanking rafters have been strengthened where the calculation called for it, and whether anything that should not have been cut has been. Where an engineer’s calculation was needed it goes in with the application, before the covering comes off rather than after. The inspection happens with the opening formed and the frame in but before the insulation and plasterboard close it, which is why the sequence puts a day between the outside work and the internal finish rather than running them together.

Fixing the frame into the trimmed opening

The frame goes in on brackets screwed to the flanking rafters and the trimmers, usually four, occasionally six on a tall unit. The brackets have slotted holes so the frame can be moved after it is loosely fixed, and it gets brought level across the cill, plumb in the plane of the roof, and equal on both diagonals before anything is driven home.

Fixings go into solid timber, not into a packer and not into the edge of a hanger. Where a bracket lands on a doubled trimmer the screw wants to reach the second timber, so the length is checked rather than assumed from the box. Once the frame is fixed the sash goes in and gets operated through its full travel before the flashing starts, because a frame that is 2mm out shows up as a sash that catches, and correcting it takes ten minutes at that stage and a strip-back at any later one. The perimeter weathering that follows is set out on soakers, aprons and head flashings, and the insulated lining that closes the job is on rooflight reveals.

Getting a structural answer before a unit is ordered

The order of the decisions matters more here than on any other part of a rooflight job, because the structure decides the size and the size decides the product. A survey measures the rafter depth and centres across several bays, records the covering and its condition, finds the purlin, the struts and any collar, checks the flanking rafters for historic notching, and photographs the loft properly. From that a width is either achievable as carpentry or it needs a calculation, and the difference between those two answers is the largest single step in the price of a roof window. The costs page sets out what else moves the figure.

To start, email info@severnskylightsgloucester.co.uk with a photograph of the roof from the garden, a photograph inside the loft looking at the rafters, and the room you want lit. The areas pages cover the thirty miles by road from Gloucester, and the guides go into individual questions in more depth.