Do roof lanterns need a steel beam?
Usually not. A roof lantern over an ordinary rear extension sits on an opening formed in the flat roof structure, and where that opening is under roughly a metre and a half in its shorter direction, timber trimming carries it perfectly well. A steel appears when the opening gets wide, when it runs across the joists rather than with them, or when a wall underneath was taken out at the same time.
That is the short version, and it is worth knowing which case you are in before anything is priced. A beam changes the programme, the paperwork and the sequencing of the whole job. The wider view on roof lantern installation always starts with what is holding the roof up, and this is the question where that matters most.
The honest answer for a typical rear extension
A three by four metre rear extension with a lantern in the middle of it is the most common job in this trade, and the majority of those are trimmed in timber. The joists either side of the opening are doubled up, trimmers go in at the head and the foot, and the kerb is built off that frame. No calculation, no engineer, no beam.
The reason is that a lantern is not carrying anything above it. It replaces roof, and roof is light. What the trimming has to do is pick up the joists that were cut and hand their load sideways into the joists that were not, over a short distance.
Why a lantern weighs less than people assume
Homeowners hear the words “glass roof” and picture something heavy. A sealed unit in a domestic lantern is typically toughened outer and laminated inner, and per square metre it lands in the same territory as a tiled pitched roof, sometimes below it. The aluminium sections are hollow extrusions.
So the weight of the lantern itself is rarely the deciding factor. What decides it is the geometry of the hole, not the mass of the thing sitting in it. A narrow opening in a deep joist is trivial. A wide opening in a shallow one is not.
The three situations that usually bring a beam in
First, width. Once the trimmer has to span much beyond a metre and a half while carrying two or three cut joists, the timber section needed becomes deeper than the roof build-up allows, and steel is simply the thinner solution.
Second, direction. An opening running across the joists cuts more of them than one running with them, and each cut joist adds load to the trimmer. Third, coincidence with other structural work. If a rear wall is coming out for bifolds in the same project, there is already a beam in the design and the lantern opening usually gets built into that scheme rather than treated separately. The detail of all three is set out on structural openings and steel for a roof lantern.
Reading your own roof before anyone visits
You can get most of the way to the answer yourself. Stand in the room and look at the ceiling: downlights, plasterboard joints and any visible sag usually run with the joists. If the extension has a loft hatch or an open eaves detail, better still, because you can see the direction and the depth.
The two facts worth having ready are the direction of span and the rough depth of the joists. On a modern extension, 200mm or 220mm engineered joists at 400mm centres are common. On a nineteen seventies flat roof, 150mm sawn timber at 450mm is more likely, and shallower timber runs out of capacity sooner.
New extensions, where the beam is already drawn
On a new build extension the question is answered on paper months before anyone orders glass. The structural engineer sizes the roof for the openings shown on the drawing, so if the lantern is on that drawing, whatever it needs is already in the calculation.
The trap is changing your mind afterwards. Deciding to enlarge a lantern once the steel schedule has been fabricated means going back to the engineer, and that is a delay measured in weeks rather than days. Settle the lantern size at design stage and the structural question never becomes a problem.
Essex flat roofs, and what tends to be underneath
The stock varies more than people expect. Purpose built flat roof extensions on the Harlow and Basildon estates often have shallow sawn joists spanning the short way, which limits how wide an opening can be trimmed without help. Later rear extensions on Victorian terraces in Chelmsford and Colchester frequently span the other way, from the original rear wall out to a new blockwork one.
Bungalows on Canvey and the reclaimed ground around it bring a different consideration, because the original roof was often designed light and any added load gets checked more carefully. Trussed roof estates like Great Notley and Beaulieu Park rarely present this question at all, since lanterns there go on flat roof extensions built to take them.
What a beam changes about the job
It adds three things. A structural calculation, which takes a few days and is submitted to Building Control. A lead time for fabrication, usually a week or two. And a day or so of extra work on site for propping, setting the beam onto its bearings and building the padstones.
It also changes the insulation detail, because steel conducts heat readily and a beam sitting in the roof line has to be wrapped so it does not become a cold surface inside the room. None of this is unusual. It is simply worth knowing about at the point you are choosing a size rather than the week before installation.
What to ask, and when to ask it
Ask the structural question at first contact. Send a photograph of the ceiling below, a rough size for the extension, and the size of lantern you have in mind. That is generally enough for a sensible view on whether a beam is likely, and it costs nothing to establish.
Sunspire settles this at survey, before any unit is ordered, and where a calculation is needed the Building Control notification is handled as part of the job. Two figures follow: a range by email once the basics are known, then a fixed price once the roof has actually been opened up to inspection or reliably assessed from below.
Where a smaller lantern is the better answer
Sometimes the sensible move is to size the lantern to the structure rather than the structure to the lantern. A lantern trimmed comfortably in timber, set slightly off centre so it lands between the joists that are already there, delivers nearly the same daylight for meaningfully less work.
Two lanterns down a long extension can also sidestep the problem entirely, since each opening is small enough to trim conventionally. Whether that reads well in the room is a separate question, and it is one the survey is there to answer honestly rather than a decision to make from a brochure.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements