Does fitting a rooflight weaken the roof?

What a rafter is actually carrying

A common rafter spans from the wall plate at the eaves to the ridge, usually with a purlin or a strut somewhere in between taking the middle. It carries the weight of the battens and the covering, plus wind load pushing and pulling, plus snow when there is any. Concrete tiles are heavy: a tiled slope can be carrying well over 50kg for every square metre of roof before anything lands on it.

The load path runs down the slope of each rafter into the wall plate, and outward pressure at the foot is resisted by the ceiling joists acting as ties. Every part of that chain has a job. The reason a cut has to be thought about is that it takes one member out of a system where the members are sharing.

The detour the load takes instead

The standard detail is old, simple and reliable. The rafter is cut, and a horizontal timber called a trimmer is fixed across the opening at the head and another at the foot, spanning between the two rafters either side. The cut rafter ends bear onto those trimmers. The trimmers pass the load sideways into the neighbouring rafters, which are doubled up where the loading warrants it, and the load continues down its normal path to the wall plate.

Nothing is lost. The load takes a short detour. The members doing the detour are sized for it, fixed with proper hangers or bearing housings rather than nails through the end grain, and the whole assembly is a stiffer box than the equivalent area of plain rafters.

Why the frame is often stiffer than what it replaced

Two doubled rafters and two trimmers add timber to a small area of roof. The frame resists racking better than the original spacing did, and the unit itself, screwed into that frame, adds a further rigid element.

This is why the fear behind the question is generally misplaced. Homeowners picture a hole. What is actually built is a reinforced perimeter with a hole inside it. The comparison worth making is with a floor: nobody worries that a stairwell weakens a floor, because everyone can see the trimmers around it.

The three ways a roof does get weakened

Weakness is a real outcome, and it comes from a short list.

• Cutting without trimming. The cut ends transfer their load into the battens and the covering, which were never intended to carry it. The symptom is a sag in the slope above the unit and, eventually, a wave visible in the courses from the ground.
• Undersized or badly fixed trimmers. A trimmer skew-nailed into the side of a rafter is holding on with the nails alone. The detail wants proper hangers or a housed bearing.
• Cutting a truss. Different problem entirely, and the serious one.

Trusses are not rafters, and cannot be treated as them

A trussed rafter roof is an engineered frame. Every member is sized for the forces in that exact geometry and the joints are plated, so a diagonal web that looks like a nuisance may be the member holding the whole triangle in shape. Cutting one changes the forces everywhere in the truss, not just at the cut.

That does not put a rooflight out of reach on a trussed roof. It means the unit is either sized to fit within a single trussed bay untouched, or the alteration is designed by a structural engineer and built to that design. Our page on rooflights in a trussed roof sets out what can and cannot be touched.

Where Building Control comes into it

Forming a new opening in a roof is notifiable work. The Building Regulations are concerned with the structure, with the thermal performance of the new element, and where the room is a habitable one in a loft, with escape and fire separation.

The practical effect is that the trimming detail is not left to opinion. It is declared, inspected, and signed off, and you receive a completion certificate that stays with the house. We handle the notification as part of the job. There is more detail on who notifies Building Control and what the inspector looks at.

How to tell whether an existing rooflight was trimmed

If you have inherited a rooflight and want to know whether it was done properly, the loft answers it in five minutes with a torch.

Look at the head and foot of the opening. You should see a solid horizontal timber the same depth as the rafters running across, with the cut rafter ends landing on it. You should see the rafters immediately either side of the opening either doubled or noticeably heavier than the rest. What you do not want to see is a rafter that simply stops, with nothing under it but the frame of the window, or a thin length of batten spanning the gap.

Also worth a look: the timber immediately around the opening. Old, damp-stained rafter ends near a poorly detailed perimeter are a structural matter as much as a weathering one.

What we find at survey, and what we do about it

Occasionally the roof is not in a state to be opened where the customer wants it opened. Soft rafter feet at the eaves, an earlier alteration that already removed a strut, a purlin that has dropped, or a bay that has been notched to death by an electrician. None of that is a reason to abandon a rooflight, but it does have to be addressed before an opening is formed rather than discovered halfway through a Tuesday.

Where that structural work is beyond forming and trimming the opening itself, we will tell you plainly that it belongs with a carpenter or an engineer first, and we will price our part around it. Our scope is installation and replacement, deliberately narrow.

What we do take responsibility for is the opening we form: trimmed to a detail that satisfies the inspector, the roof closed the same day and never left open overnight, and ten years of guarantee on the workmanship. Fifteen years doing this around Oxford means the awkward roofs are the familiar ones.