Can a Skylight Be Replaced Without a New Roof?

A roof window sits in a framed opening: a rafter on each side, a trimmer above and below. Those timbers carry the load, and the tiles or slates are laid up to the frame with a flashing kit. When the window is replaced, the opening stays as it is.

1. The sash and frame come out, and the old flashing is lifted away.
2. A few courses of tiles or slates beside and above the opening are taken up and set aside.
3. The new frame is fixed to the same rafters and trimmers, levelled and checked for square.
4. The new flashing and underfelt collar are dressed in, and the tiles or slates go back over them.
5. The sash is hung and the lining inside is finished.

Only a strip of the roof around the unit is disturbed. The rest of the covering, which may be decades old, is not touched. That is the whole answer to the question in most pitched roofs.

A flat rooflight or lantern stands on a kerb, which is a raised upstand of timber or a manufactured base built into the roof. Each maker states how high the kerb must stand, and about 150 mm is typical, so water running across the roof stays clear of the joint.

If the existing kerb is sound, level and the right size, a new rooflight can sit on it, with the roof covering dressed up the sides again where needed. If the kerb is too low, out of square or not the size of the new unit, it is rebuilt or raised. In both cases the main roof covering stays where it is.

For this reason the flat roof survey takes in the kerb itself: its height, its fall and the condition of the covering that meets it. A kerb that looks fine from below can be a different shape once it is uncovered, and the survey sheet records what we measured. Our flat roof rooflight installation page describes the units that fit onto them.

Replacing a unit does not need a new roof, but it does need the roof around the unit to be fit to receive it. Three things are checked at the survey.

• The timbers. The rafters and trimmers round the opening must be sound and fixed firmly. A new frame is only as secure as the timber it is screwed into.
• The covering at the edge. Tiles, slates or membrane near the opening have to be intact enough to be lifted and relaid. Brittle slates that crack when moved, or a flat roof membrane that will not bond to a new dressing, change the plan.
• The old flashing's route. Where the previous installer has run lead, felt or sealant over the covering, that is cleared back so the new flashing sits on clean material.

When one of these fails, the work needed is local: new timber at the opening, a few replaced slates, a fresh section of membrane around the kerb. It becomes a new roof only when the covering across the whole slope is past lifting at all, and that is a separate conversation with the homeowner.

A like-for-like replacement keeps the opening. A larger unit does not. Going from a narrow window to a wider one means cutting a rafter and fitting trimmers to carry the load, and that is a structural alteration requiring Building Regulations approval.

VELUX size codes show how much leeway there is. The first letter is the width: a window coded C is 55 cm wide, M is 78 cm and P is 94 cm. Rafters at 600 mm centres leave a clear gap of 600 mm less the thickness of one rafter, about 550 mm with 50 mm timbers. That is close to the 55 cm frame of the narrowest code, so even a C window is checked against the measured gap and the maker's fitting allowance, while a 78 cm window needs one rafter trimmed. The page on fitting a bigger VELUX covers that case in detail.

A smaller or equal replacement can usually be made to fit the existing gap. Even so, the new unit's flashing kit has its own dimensions, and the survey checks that it sits correctly against the neighbouring tiles.

Some jobs do not need the frame replaced at all. If the frame is sound and only the sealed glass unit has failed, a new glass unit can be fitted into the existing frame. That is the lightest option, and it is not a notifiable job under the Building Regulations because the frame stays.

Replacing the whole rooflight or roof window, frame and glass, is notifiable as a controlled fitting. The Part L figure for the replacement rooflight is 2.2 W/m²K. A new roof window has two routes: reach 1.4 W/m²K, or earn an energy rating of Band B. Our misted and failed rooflight replacement page sets out when a glass-only swap is possible and when the frame has to go too.

The survey answers the question for your own roof. We measure the opening, rafter centres, pitch and the state of the timber, lift one or two tiles to see the flashing route where access allows, and record it all on a written sheet. The unit is ordered from that sheet, so the size chosen is the one the measured opening takes.

If the sheet shows a sound opening, the job is a replacement into it. If it shows a rotten rafter end or a kerb that cannot be reused, the sheet says that too, with the extra work listed before you agree to anything.