Can rooflights be replaced without stripping the roof?

A rooflight is not built into the roof the way a chimney is. It sits in an opening, on a kerb or upstand, with the roof covering dressed up and over the edge of that kerb to keep water out. When the glazing unit or the whole rooflight comes out, that structure stays put. The installer lifts out the old unit, checks the kerb, renews the weatherproofing detail at the edge, and sets the new unit down onto the same base.

What gets disturbed is a band a few centimetres wide around the opening, where the flashing or the membrane laps onto the kerb. The felt, the single-ply, the tiles or the slates across the rest of the roof are never touched. That is the difference between a rooflight swap and the sort of job people picture when they hear the word: scaffolding up, half the covering off, days of exposure. A same-size replacement is closer to changing a window than re-laying a roof.

The size match is what makes it clean. If the new unit is built to the same external kerb dimensions as the old one, or the glass is the same size in a glass-only change, the covering does not need to move. Most manufacturers hold to standard sizes, so a swap for the same nominal dimension usually drops straight in. Where the old rooflight was an odd or discontinued size, a made-to-measure replacement can still be built to fit the existing opening rather than the other way round.

There are jobs where you cannot keep it contained, and it is worth knowing them before anyone quotes you.

• You are changing the size. Going from a small roof window to a wider rooflight, or turning two separate units into one lantern, means enlarging the opening. That involves cutting back the covering, moving or adding rafters and trimmers, and forming a new kerb. The roof around the new opening has to be made good afterwards.
• The kerb or upstand has failed. If water has been getting past the old detail for a long time, the timber it sits on can soften. A replacement unit needs sound timber underneath it, so a rotten kerb has to be rebuilt before the new rooflight goes on, and that pulls back a little more of the covering.
• You are switching systems. Moving from an old dome or a kerb-mounted unit to a frameless flush-glazed rooflight, or from a pitched roof window to a flat-roof design, changes how the covering has to be dressed at the edge. The upstand detail is not interchangeable, so more of the surround comes into play.

None of these means the whole roof comes off. They mean the working area grows from a narrow band at the edge to a patch around the opening. It is still local work, not a re-roof, and it is still a replacement rather than anything a roofer would be called for. If the roof covering itself is at the end of its life across the whole slope, that is a separate question, and re-covering is a roofer’s job, not ours.

On a good many flat roofs, especially larger commercial ones, a rooflight can be replaced with barely any external work at all. Where the ceiling below is open or can be dropped, the old unit is released, lifted, and the new one bedded onto the existing kerb from inside and above, with only the perimeter seal renewed on top. That keeps the membrane across the field of the roof untouched, which matters when the roof is a warm deck build-up that is awkward and slow to reinstate.

This is common on flat-roofed extensions, single-storey commercial units and rooflit walkways where the existing kerb was built to last and the only tired part is the glazing. Our commercial skylight installation work leans on exactly this: reusing sound upstands so a building stays weathertight and in use while the units are changed. On a pitched roof it works differently, because the covering laps over the top edge of a roof window and has to be lifted locally to free it, but even there the disturbed area is small.

A replacement is the one time the opening is already there and the only decision left is what goes back into it. That makes it the cheapest moment you will ever get to upgrade the glass specification, because you are paying for the access anyway.

It matters most on a south-facing roof. Southend faces south across the Thames Estuary, so a rear extension or loft rooflight here can take sun on the glass from mid-morning until the sun swings west, with reflected light off the water adding to it. If the old unit is clear double glazing at a G-value near 0.6, roughly sixty per cent of the sun’s energy is coming straight into the room. Swapping to a solar-control unit at around 0.3 halves that at a stroke, and the room still reads bright. Doing it during a replacement costs you the glass upgrade and nothing more, where doing it later would mean opening the roof again. Our solar-control glazing pages set out the numbers by orientation, and it is worth reading them before you sign off on a like-for-like unit that keeps the same tired glass.

Start by working out which kind of job you have. If the new unit is the same size as the old one and the kerb underneath looks sound, expect a contained swap with the roof left alone. If you want it bigger, or the timber around it is soft, expect a patch of the surround to be opened and made good, still short of a re-roof.

Then decide on the glass before anyone fits anything, because the access is free while the opening is open. Tell us the orientation, whether the roof is flat or pitched, and the rough size of the existing unit, and we will come back with a specification and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the unit that is coming out.