Can you add a roof lantern to an existing flat roof?

A flat roof built in the 1960s or 1970s over a hall or a side wing often has timber joists running one way and a thin layer of insulation, or none, between them and the deck. The lantern needs a rectangular hole through all of that, so the first job is to find which way the joists run and how far apart they are. A lantern whose long side runs across the joists means cutting through more of them than one that runs along them, and each cut joist needs a beam or doubled timber at either end to carry its load.

The covering matters just as much. Felt that has blistered, a membrane that has split along a seam or a deck that feels soft underfoot all point to wet timber below. Patching a new kerb into a roof in that state means building on a problem, so any work on the roof covering is agreed and priced before any glass is ordered.

Roof falls are the third check. Water should run away from the kerb, and an old roof that has settled can hold a puddle against the very spot where the lantern will stand. We can often correct that with tapered insulation laid round the new upstand.

The room below stays in use up to the day, so the sequence is built to keep it dry. We bring the lantern, the upstand timbers and the new covering to site first. On the day we protect the floor and furniture, open the roof, trim the joists, build the kerb and fit a temporary weather cover over the hole at every break, then glaze it and make the junction watertight before we leave.

Where a ceiling already exists, it comes down inside the lined opening and the plasterer or decorator finishes the reveal. Old wiring or a pendant light in the way is something to know about early, so tell us if a circuit runs across the middle of the room.

Why the existing covering changes the job

A modern single-ply membrane can be lifted and re-welded to the new upstand. An old felt roof is harder, because new felt does not bond reliably to weathered felt. In that case we may re-cover a larger area, or the whole roof, so that the lantern is not the only watertight part of an ageing surface. That is the biggest swing in the final figure, and we can only price it once we have seen the roof.

A lantern is a rooflight in the Approved Document L sense, and it usually stands more than 150 mm proud of the flat roof, which normally takes it outside Class C permitted development. Unless the council confirms otherwise, you should expect to apply for planning permission, and owners of flats and maisonettes start with no householder rights at all. Our guide to lantern planning and Building Regulations sets the detail out.

A new opening is also building work. The lantern, glass and frame together, is held to 2.2 W/m²K under Part L for existing dwellings, and the structure round the opening is checked as well. We handle the Building Control notification where the work is notifiable, and the council or a registered building control approver signs it off.

A flat-roofed hallway between the old house and a garage is a good candidate: it is long, narrow and windowless, and the roof above it is often the only thing between the room and the sky. A single-storey boot room or utility that has become a study fits as well, and so does a flat-roofed side extension on a bungalow where the lounge behind it has lost its side window.

Where the roof is small, a shallow flat rooflight may leave more of the roof clear. Our page on whether a lantern is worth it weighs the two, and the comparison with a roof window helps if part of your roof is pitched. On a pitched roof, see the skylight version of this question. To add a lantern to your own roof, book a free survey and we will give you a fixed written quote.