Do I need a steel beam for a roof lantern?

It helps to keep them apart in your head, because they get answered by different people and priced as different jobs.

The lantern is a glazed frame that sits on a raised kerb, or upstand, around a hole in your flat roof. The frame is aluminium in most cases, sometimes timber, and it is engineered to hold its own glass and to shed wind and snow load down onto that kerb. The kerb spreads the load into the roof deck and the joists around the opening. For a hole of ordinary size in a sound roof, the timbers already there are enough, trimmed around the opening in the usual way. Nothing in that chain needs steel.

The structure is everything below the roof: the joists that span the room, the ceiling, and any wall you are removing to make the space the lantern is meant to light. That is where beams come in. A lantern often gets fitted as part of a bigger project, a rear extension or a knock-through, and it is the extension roof or the removed wall that calls for the steel, not the piece of glass sitting on top. If you are only swapping an old lantern for a new one on an opening that already exists, there is no new load path to design and no beam to add.

There are a handful of situations where the answer flips to yes, and they are worth knowing before you get a quote so you can tell whether you are being told the truth.

• You are forming a new opening in the roof structure. Cutting through joists to make a hole means the loads those joists were carrying have to go somewhere. Trimming with doubled timbers handles a modest opening. A wide lantern that interrupts several joists over a long span can need a steel trimmer to pick up that load and carry it back to the walls.
• The lantern spans a room where the wall below is coming out. This is the common one. You want a big open kitchen-diner with a lantern over it, which means removing the wall that used to divide two rooms and hold up the floor or roof above. That removal needs a beam, sized by a structural engineer, and the lantern simply sits over the resulting space.
• The opening is genuinely large. A lantern over about two and a half to three metres on its long side starts to interrupt enough structure that a timber trimmer is working hard. Past that, an engineer will often specify steel to keep deflection under control, so the frame is not fighting a roof that sags towards it.
• The existing roof is already marginal. Some flat roofs on older extensions were built with shallow joists at wide spacing and little to spare. Put a heavy glass lantern and its kerb onto that and the numbers can require strengthening, whether that is steel or a doubling-up of timber.

In every one of these the beam is answering a question about spans and loads in your particular roof, not a rule about lanterns in general. That is why nobody can tell you over the phone whether you need one. It depends on the joist depth, the span, the spacing and what the lantern weighs.

The person who signs off a steel beam is a structural engineer, not the installer and not you. They calculate the load coming down from the roof, the glass, the snow and the wind, work out how far it has to span, and specify a section that carries it with an acceptable amount of bend in the middle. That specification is a real document with a beam size on it, and Building Control will want to see it.

This is the point worth understanding: a new or enlarged rooflight is notifiable under Building Regulations, and where you form an opening in the structure the Building Control body needs the engineer’s calculations before the work is signed off. Part L covers the thermal side, the glass and the kerb, and Part K covers safety glazing and any guarding. At Skylights On Sea we make the Building Control notification to Southend-on-Sea City Council, or to whichever local authority covers your address, as part of the installation. If your project needs a beam, the engineer’s design sits alongside that notification.

If you are only replacing a lantern like for like on an existing opening, none of this beam-and-calculation machinery applies, because you are not touching the structure. You are changing the unit that sits in a hole that is already there and already trimmed.

People assume the glass is the heavy part and worry about it crushing the roof. It is heavy, but it is heavy in a way the frame is built to manage. A double-glazed lantern unit runs somewhere around 25 to 40 kilograms per square metre depending on the glass build-up, and triple glazing pushes that higher. The lantern frame collects all of it and delivers it down onto the kerb as a spread load around the perimeter of the opening, which is exactly how you want a load to arrive at a timber roof. Spread loads are kind to joists. Point loads are not.

Where the weight does matter is when you are choosing between double and triple glazing, or adding a walk-on panel, on a roof that is already close to its limit. Those choices change the number the engineer is working with. So does orientation, though not structurally. On a south-facing rear extension, the glass you would specify for summer comfort, a solar-control unit with a low G-value of around 0.3, weighs much the same as a clear one, so choosing the right glass for the estuary sun costs you nothing in structural terms. It is the same kerb and, in most cases, the same trimming either way. You can read how we approach that glazing choice on our energy-efficient skylight installation page, and the broader picture of how a lantern goes in on our roof lantern installation service.

Start by working out which job you actually have. If your old lantern is coming out and a new one is going into the same opening, you are almost certainly beam-free, and the conversation is about glass and kerb, not steel. If you are making the opening bigger, or the lantern is part of an extension or a knock-through, assume an engineer needs to look at it and budget for their calculations early, because that drawing sets the size of everything else.

Do not let a lantern quote quietly leave the structure out. Ask the installer directly whether the opening is new or existing, whether any joists are being cut, and whether the wall below is staying. If the answer to those puts you into beam territory, ask who is doing the structural design and whether the calculations are going to Building Control. A good installer will have a clear answer and will not be doing sums on the back of an envelope.

If you would like us to look at yours, tell us the rough size of the lantern you have in mind and whether it is going onto an existing roof or a new extension, and we will tell you honestly whether a beam is in the picture. Ring 01702 898232 or request a quote, and if the structure needs an engineer we will say so before you spend anything, rather than after.