Can a roof lantern go on an existing flat roof?

What has to be true for it to work

A flat roof is a deck laid over joists, covered with a waterproof layer, and often with insulation above or below. To put a lantern in, part of that deck has to come out, which means the joists around the opening have to be cut and supported another way. If the joists are sound and the span allows it, this is routine building work.

Three things decide whether it is straightforward. The first is the structure: joist size, spacing and direction, and what they bear on. The second is the covering, because some can be cut and joined and some are better renewed in the area. The third is the ceiling below, where the opening will become a light well that needs lining and finishing.

We look at all three at the survey and tell you in the quote what we found and what we propose.

Trimming the joists for the opening

When a lantern is wider than the gap between two joists, one or more have to be cut. The ends are carried by new timbers, called trimmers, fixed across the opening and joined to the joists either side with hangers. The result is that the loads that used to pass through the cut joists go round the opening instead.

The extra weight of a lantern and its kerb is taken into account in the same way. A glazed lantern is heavy, and the members that carry it must be sized for it. Where the span is long or the opening large, an engineer's calculation may be needed, and we will say so in the quote and ask your builder or architect to arrange it.

If the opening can be sized to fit between existing joists, less structural work is needed. We can often suggest a lantern size to suit the joist spacing, which keeps the work smaller.

Building the kerb on a covered roof

The kerb is a timber upstand that rises from the deck around the opening and carries the lantern. It is formed from treated timber, tied to the trimmed joists, and sized to the lantern's drawing. Insulation is added to its sides so that the kerb does not become a cold bridge, and the manufacturer's instructions set the final height, typically 150mm or more clear of the finished covering.

On a roof with no builder on site, we form the kerb ourselves. If a builder is working on the house, they can build it from sizes we give in writing, and our answer on who builds the rooflight kerb describes how that split works.

Because the lantern is made to measure, the kerb is built or checked before the order goes in, not after. That avoids a lantern arriving for a kerb that is a few millimetres out.

Making good the existing covering

The covering has to be cut back from the opening and finished against the new upstand. How that is done depends on what is there. A single-ply membrane can be cut and dressed up the kerb with a compatible joint. A bitumen felt or built-up system is cut back and layered into the new upstand. A liquid-applied covering is reinstated around the kerb with the same product.

Where the covering is old, brittle or near the end of its life, it may be more sensible to renew the area around the lantern, or the whole roof, than to join new to old. We tell you what we see and the roofer agrees the approach with us. A covering that carries a guarantee from its installer needs the right product and method, so we check that with the roofer first.

The weather decides the order. The lantern is fitted only when the upstand is covered and dry, which keeps the opening protected throughout.

Inside the room

Once the opening is cut, the ceiling below has a hole in it that forms a shaft up to the lantern. The sides of the shaft, called reveals, are boarded, insulated and plastered, and splayed if the room needs more light to spread. This is a plasterer's job and comes after the lantern is in and weathertight.

If electric lighting or an opening vent is planned, the electrician runs cables at first fix, before the reveals are closed. We agree the sequence with them so the work is done once.

Dust control is worth agreeing before work starts. We lay protection under the opening and keep the area clear as the deck is cut.

Rules, price and next steps

Adding a lantern to an existing roof is different from including one in an extension that is being built. The permission position depends on the house, and the rules for your address are checked by us as part of the survey, and the council has the final say. Our answer on roof lanterns and planning permission explains the difference. The work is also subject to Building Regulations, with the notice to Building Control sent by us where the job is notifiable.

The price depends on the points above, and our page on what a roof lantern costs to fit explains the parts of the quote. Our roof lantern installation service shows how a survey, an order and a fitting day follow one another.