Structural openings and steelwork for a roof lantern

A lantern does not float. It carries its own weight, which for a glazed aluminium unit over a typical extension is a real load once you add the glass, the frame and the timber kerb underneath. On top of that sits the imposed load: snow settling in the valleys of the bars, wind trying to lift the whole thing off, and the point load of a person kneeling on the roof to clean or survey it. All of that has to travel down through the kerb, into the roof deck, across to the trimmed edges of the opening, and out to the walls that were carrying the roof in the first place.

The problem is that the roof was not built with a hole in it. Rafters or joists ran continuously from one bearing to another, sharing the load evenly. Cut three or four of them out to make room for a 1.5 by 1.0 metre lantern and that load has nowhere to go until you give it a new path. The members you leave standing on either side of the opening now have to pick up the share that the cut ones used to carry, and the timbers you frame across the ends of the opening, called trimmers, have to collect the cut ends and hand the load sideways to the members that are still full length.

So the real question on every lantern job is not what the glass costs. It is how big the opening is, which way the structure spans, how much load the cut members were carrying, and whether the timbers left around the hole can take the extra without help. When they can, the job is joinery. When they cannot, it needs steel.

Most lanterns in south Essex go on flat or shallow-pitched roofs, because that is where the single-storey rear extension lives. On a flat roof the joists run one way, usually spanning the short dimension between the outer wall and the original back wall of the house. Before you cut anything you need to know which way they run, because a lantern opening behaves completely differently depending on whether its long side lies along the joists or across them.

Cut a hole whose long axis runs along the joists and you interrupt only one or two of them. Cut the same hole across the joists and you might sever four or five. The second case takes far more of the roof out of action and puts a much larger load onto the trimmers. This is why the position and orientation of a lantern is a structural decision, not just a question of where it looks best from the garden. Sometimes moving a lantern 300mm, or turning it so its long side follows the span, halves the steel you need underneath.

Pitched-roof lanterns are less common, and a true lantern on a slope usually means a bespoke unit or a box gutter arrangement. On a pitched roof you are cutting rafters, and rafters are doing more than carrying weight downwards: they also resist spreading at the eaves. Interrupt them and you can introduce a spreading force the original roof never had, which is why a rafter opening for anything sizeable wants a designed solution rather than a guessed one. Where a client wants roof glazing on a slope, a run of in-plane rooflights is often the cleaner answer, and our pitched-roof skylight installation is built around exactly that geometry.

A roof lantern is not fixed straight to the roof deck. It sits on an upstand, also called a kerb, which is a raised timber box built up off the structure around the edge of the opening. The upstand does three jobs at once. It lifts the aluminium frame clear of any standing water so the weathering detail works. It gives the roofing membrane something to turn up against and terminate on. And it transfers the lantern’s load down onto the trimmed opening in a controlled line rather than a random one.

Height matters. A kerb should stand at least 150mm above the finished roof surface, measured after the insulation and the membrane are on, so that driven rain and ponding water cannot reach the frame. On a warm-deck flat roof, where the insulation sits above the structure, the kerb has to be tall enough to clear that build-up and still leave the weathering margin. Build it too low to save a course of timber and you have created a puddle that sits against the frame every time it rains. That is a detail, not a small one.

The upstand also has to be insulated and lined so it does not become a cold bridge. A bare timber kerb standing proud of an insulated deck will run cold on its inner face, and cold surfaces at the edge of a glazed opening are where condensation collects. On a south-facing extension the same detail matters for a different reason: the kerb is part of the thermal line that keeps the summer heat management honest. The glass does the heavy lifting there, which is a subject in its own right, but a leaky, uninsulated kerb undoes some of it.

Not every lantern needs a beam. The dividing line is span and load. For a modest opening in a flat roof where you are cutting one or two joists, doubled-up or trebled timber trimmers, bolted together and sized by a structural engineer, will often carry the interrupted load back to the full-length joists on either side. Timber is cheaper, lighter to lift onto a roof, and easier to fix a kerb to. Where it works, it is the right answer.

Steel enters the picture in three situations. The first is a wide opening: once the trimmer has to span more than timber can sensibly manage for the load it is carrying, a steel section does the job at a fraction of the depth. The second is a heavy lantern, particularly a large glazed unit where the dead load alone is significant. The third, and the most common reason on extension work, is when the wall below the roof is also coming out.

That last case is the one people underestimate. A great many lanterns go into open-plan kitchen-diners where the original rear wall of the house has been removed to knock two rooms into one. That wall was holding up the floor above, or the original roof, or both. Take it away and you need a beam across the opening to carry what the wall used to carry, and that beam then also becomes the bearing for one side of your lantern opening. Now the steel is doing two jobs, and its size is governed by the wall load, not the lantern. This is why lantern jobs and structural openings are so often the same conversation.

Every opening has to be calculated for its own loads, so the figures below are industry-typical starting points, not a specification for your roof. They give you a sense of what a structural engineer is likely to arrive at, and where the thresholds sit. A qualified engineer produces the actual design and the calculations that go to Building Control.

Read the table for the pattern rather than the exact numbers. As soon as a wall comes out below the roof, the beam sizing jumps, because it is no longer holding up a hole in a roof, it is holding up part of a house. And every steel beam bears onto something at each end. That bearing usually lands on a padstone, a spread concrete or high-strength block pad that stops the concentrated end reaction from crushing the masonry below it. Skipping the padstone, or undersizing it, is a quiet way to overload a wall that looked perfectly solid.

New and enlarged roof openings are notifiable under Building Regulations, and the structural work sits inside that. Two parts of the regulations bear directly on a lantern opening. Part A covers structure: the beam and trimmer design, the calculations, the bearings and the way the load finds the ground. This is where the structural engineer’s drawings and figures do their work, and it is why a beam is not a thing you eyeball.

Part K covers safety glazing and guarding, and it governs the glass overhead. Anything glazed above your head has to be laminated on the inner pane so that if it ever fails it holds together rather than falling into the room. Toughened outer panes are standard on a lantern. Part L covers thermal performance, which is where the U-value of the glazing and the insulation of the kerb are held to a standard. Every installation we carry out is built to current Building Regulations and notified to Building Control, and on lantern work that notification includes the structural design for the opening. We make that notification to Southend-on-Sea City Council, or to the relevant local authority for your address, on your behalf.

One thing worth being clear about: forming a structural opening and swapping a tired unit are different scales of job. Where a lantern has failed and simply needs taking out and a new one fitted onto a sound existing kerb, that is a replacement, and the structure may not need touching at all. Where the opening itself is new, or the wall below is coming out, the structural work is the larger part of the project and the glass is the finish on top of it. The full picture of how a lantern goes in, from survey to weathering, sits on our roof lantern installation page.

The housing stock across this patch shapes what these openings look like. The 1930s semi belt, of which south Essex has an enormous amount, was not built with open-plan living in mind. The classic project here is a rear extension over a knocked-through kitchen, and that almost always means the original back wall coming out and a beam going in. So the lantern that ends up over the dining table is riding on a structural opening that was really about the wall, and the two have to be designed together from the start.

The Victorian and Edwardian villas and terraces along the cliff and through the older parts of the city bring a different structure again: deeper roofs, sometimes with older timbers that need respect and careful survey before anything is cut. Post-war and newer estates at Shoeburyness and out towards Basildon tend to have more predictable, regular joist layouts that make openings simpler to trim. Knowing which of these you are standing on changes the survey before it changes the steel.

There is also the aspect. A lantern over a rear extension on a south-facing garden, which describes a large share of the work here, is pointed almost straight up at the estuary sun for most of the day. That has nothing to do with whether the beam is big enough, but it has everything to do with what glass goes into the frame once the structure is sound. A structurally perfect opening filled with the wrong glass gives you a room that overheats every July afternoon, which is why we treat the structure and the glazing specification as two halves of one job and match the solar-control glazing to the direction the roof faces. The estuary aspect across Southend is the whole reason we start every survey with a compass.

The order of work matters as much as the parts, because a roof opening is a controlled operation, not a demolition. It starts with a survey that establishes which way the structure spans, what is bearing on what, and whether a wall below is part of the picture. From that comes a structural design: an engineer sizes the beam or the trimmers, specifies the bearings and padstones, and produces the calculations that Building Control will want to see.

Only then does anything get cut. The temporary support, the propping that holds the roof and anything above it while the permanent beam goes in, is planned before the first joist is touched. The beam or trimmers are installed and made to bear properly. The opening is trimmed square and true, because an out-of-square opening fights the lantern frame for the rest of its life. The insulated kerb is built up to its full height off the trimmed opening. The roofing membrane is dressed up the kerb and terminated. And last of all, the lantern itself is set on top, fixed down, and weathered in.

Done in that order, with the structure designed rather than assumed, a lantern goes in once and stays put. We have been forming these openings and fitting lanterns across south Essex for over fifteen years, and every installation carries a ten-year workmanship guarantee. If you are planning a lantern and are not sure whether your roof needs steel or timber under it, that is exactly the sort of thing the survey settles. Ask us to look at the roof and we will tell you what the opening actually needs before anyone talks about glass.