Forming the Structural Opening in a Pitched Roof
Every pitched rooflight needs an opening cut in the roof structure, and cutting a rafter means its load has to be carried somewhere else. The standard solution is a pair of trimmers, horizontal timbers of the same section as the rafters, fixed across the top and bottom of the opening between the two rafters that stay, with the cut rafter then supported off those trimmers. On a normal domestic roof where one rafter is cut, that detail is routine and does not usually need an engineer. Cut two or more, touch a trussed roof, or land near a valley, hip, ridge or eaves, and you want a structural engineer’s calculation and Building Control involved. The order of work matters: the trimmers and any props go in before the rafter is cut, never after.
This page is the structural detail. Choosing a size that lands well on your rafters is on rooflight rafter spacing, fitting a unit narrow enough to avoid cutting anything is on a rooflight without cutting rafters, and the full fitting sequence is on the pitched roof skylight installation hub.
How load moves when you cut a rafter
A rafter carries roof load from the ridge down to the wall plate. Cut it in the middle and the two ends have nothing spanning between them, so the load that was travelling down that rafter has to find another path. The trimmer provides it. A trimmer fixed horizontally between the two adjacent uncut rafters picks up the end of the cut rafter and carries its load sideways into those neighbours, which then take it down to the wall plate as normal. The neighbours are doing a bit more work than before, which is why on a bigger opening, or a marginal roof, an engineer checks that they can.
The two trimmers, one across the top of the opening and one across the bottom, are sometimes called headers. They must be:
- The same depth as the rafters, so they sit in the same plane and carry properly.
- Fixed into the full depth of the retained rafters either side, not just skew-nailed to the face. Proper connection is what transfers the load.
- Doubled up where the span or load calls for it, on the engineer’s instruction.
- Fixed before the rafter is cut, so the structure is never unsupported.
The order of work
This is the part that separates a safe opening from a dangerous one, and it is not negotiable.
- Prop first. Where the span or the load warrants it, temporary props go in under the rafters either side of the opening, taking the roof load onto a firm bearing before anything is cut. On a small single-rafter opening in a sound roof the trimmers alone may suffice, but the props go in whenever there is any doubt.
- Fix the trimmers. Top and bottom trimmers are fixed into the retained rafters, forming three sides of the opening with the rafter that will be cut running through the middle.
- Then cut the rafter. Only once the trimmers are carrying the structure is the rafter cut through, top and bottom, and the cut section removed. The load it was carrying is now travelling through the trimmers into the neighbours.
- Square and true the opening. The opening is packed and trued to the frame dimensions, because an old roof is rarely square and a frame forced into an out-of-square hole binds and flashes unevenly.
Cut the rafter before the trimmers are in and you have, for however long it takes, an unsupported roof over your head. It is the difference that does not show in the finished job and matters more than anything that does.
When you need a structural engineer
A single rafter cut in a sound, traditionally built domestic roof, trimmed with matching sections either side, is standard practice and does not usually need a calculation. Any of the following changes that, and we want an engineer’s design before we start:
| Situation | Why | What we do |
|---|---|---|
| Two or more rafters cut | Load on the retained rafters and trimmers rises sharply | Engineer sizes the trimmers and checks the neighbours |
| Trussed rafter roof | Trusses are engineered as whole units and must never be cut without design | Engineer designs the trimming, or we fit between trusses uncut |
| Opening near a valley, hip, ridge or eaves | Load paths are more complex at those points | Engineer checks the local structure |
| Timber-framed or historic roof | Irregular sizes, spans and past alterations | Survey from inside, engineer with conservation experience |
| Part of a loft conversion | The whole roof is being redesigned anyway | Openings are part of the conversion structural design |
The trussed roof point is the one people most want to argue with, so it is worth being blunt. Trussed rafters, the thin engineered triangulated frames used in most housing built since about 1970, including all the new-build stock at Grange Farm, Sproughton, Martlesham and Kesgrave, are designed as complete units. Cut any member and you change how the whole truss behaves. You do not cut a truss without a structural design that says how the load is carried instead. This is not a grey area and it is not something experience lets you eyeball. If your roof is trussed, either the unit drops between trusses without cutting anything, or an engineer designs the trimming.
An engineer’s calculation on a domestic opening runs to a few hundred pounds. It is the cheapest insurance on the job, Building Control will normally want to see it, and we build to the engineer’s design. We do not design structure ourselves and we will not pretend to.
Building Control touchpoints
Forming a structural opening in a roof is building work, and Building Control has an interest in it. The touchpoints on a typical rooflight opening:
- Structural adequacy. Building Control will want to be satisfied the opening is properly trimmed and, where required, will want the engineer’s calculations.
- Thermal performance, Part L. The glazing you install has to meet the U-value requirement, which is stated on the unit and on our energy efficient skylight installation page.
- Fire escape, Part B. If the rooflight serves a habitable loft room, it may need to meet escape window dimensions. This is a hard requirement, not a guideline, and it can constrain the size before anything structural does.
- Guarding, Part K. Relevant where the opening and any low sill create a fall risk.
A straightforward like-for-like replacement in an existing opening generally does not open the structural questions, because no new load path is being created. A new opening does. We flag which category your job falls into at survey.
We never advise anyone up onto their own roof, and we never cut structure by eye. The opening is set out and surveyed from inside the loft, the structural question is answered before we order the unit, and where it needs an engineer, we wait for the drawing. If a quote skips the structural check on a two-rafter opening or a trussed roof, that is the corner being cut, and it is the one that matters.
How this affects your window size
The structural picture and the daylight picture pull against each other, and the survey reconciles them. A wider window lets in more light and may need more rafters cut and an engineer involved. A narrower window that drops between rafters needs no cutting at all and is the cheapest, quickest install available, but it may not give the room the glazed area it wants. We measure the actual rafter positions, show you the sizes that land well against the ones that land badly, and price both so you can weigh cost against daylight. The daylight side of that trade is on loft rooflight daylight levels, and how long the opening takes to form is on how long it takes to fit a pitched rooflight.
Common questions about rooflight openings
Do you fix the trimmers before or after cutting the rafter?
Before, always. The trimmers and any props go in first, so the roof load has a path before the rafter is cut. Only once the structure is supported is the rafter cut through and the section removed. Cutting first leaves the roof unsupported, which is unsafe and can let the structure move. The order is not a preference, it is the method.
Do I need a structural engineer for one rooflight?
Not usually, if it is a single rafter cut in a sound, traditionally built roof, trimmed with matching sections either side. You do need one if two or more rafters are cut, if the roof is trussed, if the opening lands near a valley, hip, ridge or eaves, or if the roof is historic or timber-framed. An engineer’s calculation runs to a few hundred pounds and Building Control will normally want it.
Can I cut a truss to fit a bigger rooflight?
Not without a structural design. Trussed rafters are engineered as complete units and cutting any member changes how the whole truss carries load. Most housing built since about 1970 is trussed, including the new-build estates around Ipswich. The usual answers are a narrow unit that drops between trusses uncut, or an engineer’s design for trimming and supporting the cut truss. We check whether your roof is trussed or traditionally cut at survey.
Get a rooflight opening formed safely
Tell us what you want the window to do and roughly what the roof is, and we will survey the structure from inside the loft, tell you honestly whether it needs an engineer, and give you one written fixed price with the structural work identified. Where a calculation is needed we build to it. Every installation carries a 10-year workmanship guarantee and the installers have 25 years fitting rooflights behind them.
Request a fixed quote for a pitched rooflight in Ipswich, Suffolk or north Essex, or read our published prices first.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.