Usually not. A roof lantern on a typical flat roof extension needs no steel at all: the frame sits on a timber kerb built off the joists, and the hole in the deck is trimmed with doubled timbers running back to the wall plates. Steel appears in two situations only. Either the opening is wide enough that trimming cannot carry what the cut joists were carrying, or the lantern sits where a wall or a beam already does structural work. Both are questions for a structural engineer, not for us and not for the company selling the lantern.
Where the load actually goes on a flat roof
A flat roof deck spans one way, joist to joist, from wall plate to wall plate. Every joist carries its share of the deck, the insulation, the covering, the snow load and anybody who walks up there. Cut three joists to make a hole and that share has to go somewhere.
It goes sideways. Trimmers run across the cut ends of the shortened joists, and those trimmers hand the load to the two full joists either side of the opening, which are usually doubled up to take it. The result is a rectangular frame of timber in the plane of the roof. The lantern kerb is built on top of that frame. Nothing about a lantern loads the roof unusually: the frame and glass together weigh less per square metre than the deck build-up it replaced.
Trimming an opening in joists, and its limit
Timber trimming works well up to a point, and the point is set by the span of the trimmer rather than by the size of the lantern. A trimmer spanning 1.5 metres across three cut joists is ordinary carpentry. A trimmer spanning 3.5 metres is a beam, and once it is a beam somebody has to size it.
Depth is the other limit. A trimmer has to sit within the joist depth if the ceiling below is to stay flat, so a 200mm joist gives you 200mm of timber to work with. When the sums say you need 300mm of softwood in a 200mm zone, the options are a deeper zone, an engineered timber section, or steel. That is the fork in the road, and it arrives long before the lantern itself becomes unusual.
When a lantern does bring steel with it
Three patterns come up repeatedly on Oxford extensions. The first is the wide rear opening: a lantern proportioned to a 5 metre kitchen, where the trimmer span alone puts it into engineered territory. The second is the lantern positioned over a line that already carries load, typically where an old rear wall was removed and a beam put in, and the new opening wants to sit across it. The third is a lantern on a roof whose joists were undersized to begin with, common on additions built in the 1970s and 1980s, where cutting anything at all leaves too little in reserve.
In all three the answer is the same: a structural engineer calculates the load, specifies the section, and issues a drawing. We build to that drawing. The structural openings page works through how those calculations get commissioned and what the drawing has to show.
Pitched roofs are a different question entirely
A lantern is rarely fitted into a pitched slope, but it does happen on a shallow-pitched rear addition or a mansard flat. There the structural picture changes, because rafters are usually part of a triangulated arrangement with a ridge, purlins and ceiling ties all doing work together. Cutting rafters can release a load path that is holding the walls apart.
Openings in a pitched roof therefore get looked at more carefully, and the trigger for engineering input is a smaller opening than on a flat roof. If your lantern is going into anything other than a flat deck, assume calculations until an engineer says otherwise.
Who decides, and who signs it off
The decision belongs to a structural engineer, and the sign-off belongs to Building Control. Neither role sits with the installer, and any installer telling you a 4 metre opening is fine because they have done one before is guessing on your behalf.
The sequence is straightforward. An engineer visits or works from your drawings, calculates the opening, and issues a beam schedule with sizes, bearing lengths and connection details. That schedule goes to Building Control as part of the notification. The inspector will want to see the steel in place, with its bearings, before anything covers it up.
Padstones, bearings and the wall below
A beam is only as good as what it sits on. Steel concentrates load at two points, so the masonry at each end has to be able to take it. That is what a padstone does: a dense concrete block, sized by the engineer, spreading the end reaction over enough brickwork to keep the stress within what the wall can carry.
On older Oxford properties this is where a job gets more involved than expected. Solid Victorian walls in lime mortar, rubble stone cores, and cavity walls with only a single leaf available to bear on all change what the engineer specifies. Sometimes the padstone is enough. Sometimes a pier or a pad foundation is drawn as well. The lantern is unaffected by any of it, but the programme and the cost are not.
What steel adds to the programme
| Item | Typical effect |
|---|---|
| Engineer’s calculations | Commissioned before ordering, adds days rather than weeks |
| Beam fabrication | Cut to length and drilled to order, a lead time in its own right |
| Lifting on site | Two people for a small section, mechanical handling for anything long |
| Building Control | An inspection at open-beam stage, before covering |
| Fire protection | Boarding or intumescent coating where the beam is exposed internally |
None of that is a reason to avoid a wide lantern. It is a reason to know at the design stage rather than at the point of order, because a beam discovered late reorders the whole job.
What we do here, and what we do not
We install and replace rooflights. We build kerbs, form and weather the opening in the roof covering, and fit the lantern to a structure that is ready for it. We do not size beams, and we do not install them.
What we do at survey is tell you honestly whether your lantern is likely to need an engineer, so that you commission one before you commit to a size. On a modest lantern in a sound deck the answer is usually no, and we will say so. Fifteen years fitting rooflights across Oxford, a ten-year guarantee on our workmanship, and a straight answer on 01865 704245 about whether the opening you want is an ordinary one.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
Tell us about your roof
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