Can a roof lantern be fitted to an existing extension?

What has to be true for it to work

Four conditions decide it, and all four are checkable in an hour on site.

• The roof structure can give up an opening of the size you want, with new trimmers or a beam where needed.
• The deck is sound enough to build a kerb off and to walk on while the work happens.
• The falls can still run to the outlets once the kerb interrupts them.
• There is a way of getting the frame and the glass to the roof.

If all four hold, the job is ordinary. If one fails, it usually becomes a different job rather than an impossible one: a smaller lantern, a different position on the roof, or a new covering laid at the same time.

What the deck is, and how we find out

Extensions built at different times have different roofs, and you cannot tell which you have by looking at the ceiling.

A 1970s or 1980s addition is very often chipboard or softwood boarding over joists with three layers of felt on top, sometimes with no insulation at all. A 1990s one may have plywood and a single-ply membrane. Anything since the mid 2000s is likely to be a warm roof with rigid insulation above the deck and a membrane over that. Each behaves differently when you cut into it.

We open a small area of the covering on the survey where the owner agrees to it, or read the build-up from an existing outlet or edge detail. What we are looking for is the deck material, its thickness, whether it is dry, and where the insulation and vapour control layer sit in the sandwich. Boarding that is soft underfoot or delaminating means the covering has been letting water through for a while, and that has to be dealt with before anything is built on it.

The joists you are about to cut

Joist direction decides how the opening is formed, and it decides the shape of the lantern as often as taste does. A lantern whose long axis runs parallel with the joists crosses fewer of them and needs less structural work than the same lantern turned ninety degrees.

Cutting one or two joists is routine: doubled trimmers either side, trimming members across the ends, joist hangers, and the load finds its way back into what is left. Cutting four or more and the trimmer becomes a designed beam. That is an engineer’s calculation and it goes to Building Control with the notification. Structural openings and steel for a roof lantern covers the detail.

One thing worth knowing early: on many extensions the joists also tie the walls together. Cutting them without replacing that restraint is a structural change, not a carpentry one, and it is a place where a cheap installation causes trouble years later.

Warm roof, cold roof, and the one that causes trouble

In a warm roof the insulation sits above the deck and the deck stays at room temperature. In a cold roof the insulation sits between the joists with a ventilated void above it. Both work. What does not work is a hybrid created by accident, where somebody has stuffed insulation between the joists of a roof that was designed to be ventilated and then blocked the ventilation.

A new lantern is an opportunity to correct this, and sometimes it is a requirement. Building Regulations bring thermal upgrades into play when you disturb a significant area of a roof, and the kerb has to be insulated regardless, so the two decisions get taken together. What matters practically is that the insulation line and the vapour control line both continue up the kerb and meet the lantern frame without a break. A gap there produces a cold reveal and condensation running down the plaster, which people then report as a leak.

Falls, outlets and where the water already goes

A flat roof is not flat. It has a designed fall, typically around 1:80 laid to achieve 1:80 after deflection, taking water to an outlet or a gutter. Drop a kerb into the middle of it and you have built a wall across that route.

Water then has to go round the kerb, which means the falls need adjusting either side of it, usually with tapered insulation or firrings, so there is no ponding against the upstand. Standing water sitting permanently against a kerb is the single most common cause of trouble on retrofitted lanterns. On a roof that already ponds before we start, the fall gets rebuilt as part of the job, and that goes in the quotation rather than appearing later.

The covering has to end somewhere

Every roof covering has a rule for how it terminates against an upstand, and it is not optional. Felt is dressed up the kerb and mechanically fixed under a drip. Single-ply membrane is welded up the kerb and finished with a termination bar. EPDM is bonded and trimmed. GRP is laid up in one piece over the kerb angle.

Sometimes the existing covering is in good enough condition and long enough at the edges to dress into a new kerb. Often it is not, and the sensible answer is a new covering across the whole roof while the access is there. That is a cost, and we would rather it appeared in the first quotation than as a variation halfway through.

Ceiling, plaster and the room below

The inside is where the job is judged, and it is the part people underestimate.

The opening has to be lined and plastered, the reveal formed either square or splayed, and the junction between new plaster and old ceiling made without a visible line. A splayed reveal throws more light into the room and reads better on a low ceiling. Any downlights, extract ducting, pipework or cables in the ceiling void have to be traced and diverted before the joists are cut. Kitchen ceilings are full of them, so this is the question we ask on the survey rather than on the morning of the work.

A realistic sequence for an occupied house

The frame is made to the surveyed opening, so nothing happens on the roof until it is in production. While it is being made, the kerb can be built and weathered, which means the roof is opened, formed and closed within a working day or two rather than left waiting.

We do not leave a roof open overnight. The opening is either closed with the frame and a temporary cover or not cut that day. In a kitchen that stays in use throughout, that matters more than any other promise on the quotation. Timings across the whole job are set out in how long a roof lantern takes to fit.

Fifteen years of retrofitting rooflights onto Oxford extensions, Building Control notified by us, and ten years on the workmanship. A range by email, a fixed price after the survey.