Guide · Roof Lantern Installation
Structural openings and steel for a roof lantern
A lantern is a hole in a structure that was carrying load, and the load does not disappear because you have decided you want daylight. It goes around the opening instead, into the members either side, and those members have to be capable of taking it. Below about 1500mm across the span, timber trimming handles that. Above it, or where the opening runs the wrong way relative to the joists, the answer is usually a steel and a calculation to go with it.
This page sets out where the threshold sits, what the engineer produces, what a steel does to the programme and the price, and what Building Control wants to see at each stage. It is written for the flat roofs that lanterns sit on, which across Chelmsford and the surrounding towns means timber joisted decks far more often than anything else.
How a flat roof carries load before you cut it
A domestic flat roof is a set of parallel timber joists, typically 47mm by 195mm or 47mm by 220mm at 400mm centres, spanning the short way between two supports. Each joist carries a strip of the roof: its own weight, the deck, the insulation, the covering, and an imposed load for snow and for someone walking on it. It delivers all of that to a wall or a beam at each end.
Cutting one joist out of that array removes a support from the middle of a continuous surface. The deck above it now has nothing under it for 400mm on either side, and the load that joist was carrying has to be picked up by whatever you put in its place. That is the whole of structural roof lantern work in one sentence.
Trimming an opening in joists, and how it is actually done
The carpentry is well established. The joists that are cut become trimmed joists. The two full length joists immediately either side of the opening become trimming joists and are doubled, meaning a second joist of the same section is bolted alongside each of them, because they are now carrying their own strip plus half the load of everything cut. Across the ends of the cut joists, a trimmer runs perpendicular, spanning between the two trimming joists and collecting the cut ends.
The connections matter as much as the members. Cut ends are hung off the trimmer on proprietary joist hangers, nailed with the full pattern of square twist nails the manufacturer specifies, not with two nails in the corners. The doubling is bolted or nailed to a schedule rather than left to judgement. Done properly this is a load path with no weak link in it, and it is the reason a modest opening needs no steel at all.
The threshold, and where a steel becomes the answer
Three conditions push a job from trimming to steel, and any one of them is enough.
- Width across the joists beyond about 1500mm. The trimmer is now spanning far enough that a timber section deep enough to do it will not fit inside the roof build-up without dropping below the ceiling.
- An opening running with the joists rather than across them. A long lantern parallel to the joist direction takes out several joists over most of their span, which is a much larger structural intervention than a short opening across them.
- An opening near mid span. A trimmer at the middle of a joist run attracts far more load than one close to a bearing wall, because bending moment peaks in the middle.
Where the roof is already spanning a long way, or where the extension has been designed with a beam under the flat roof already, the lantern opening may need to work with that beam rather than around it. That is settled on a drawing, not on site.
What a structural engineer actually produces
The engineer’s output is not a sketch. It is a set of calculations that name the loads, the load path, the section chosen and the deflection at design load, together with a plan and a section drawing showing the member sizes, the bearing lengths, the padstone sizes and the connection details.
Building Control will ask for exactly that package before work starts, and it becomes part of the record for the property. It is a modest cost against the job as a whole and it removes the entire category of argument on site about whether a member is big enough. Sunspire works from the engineer’s drawing rather than instead of it, and where a job needs one, that is established at survey rather than discovered on the first morning.
A steel that has been sized by eye is a steel that has been guessed, whatever the person doing the guessing has been doing for how long.
The sections that get used, and what each is for
| Section | What it looks like | Where it suits a lantern opening |
|---|---|---|
| Universal beam, UB | An I shape, deep and efficient | The general case for a trimmer picking up cut joists |
| Parallel flange channel, PFC | A C shape, flat on one face | Where joists hang off one side only, or against a wall |
| Rectangular hollow section, RHS | A closed box | Where the beam is exposed and wants a clean face, or where torsion is present |
| Flitch beam | Steel plate bolted between two timbers | Where depth is tight and the beam must be handled by two people |
| Universal column, UC | Squarer than a UB | Where the depth available is limited and the width is not |
The flitch is worth knowing about. A 10mm steel plate bolted between two 220mm timbers gives most of a small steel’s capacity in a component that arrives as three manageable pieces and is assembled in place. On a rear extension reached only through a house, that difference decides whether a crane is needed.
Bearings, padstones and where the load lands
A steel is only as good as what it sits on. The end of the beam delivers a concentrated point load to the wall, and blockwork or brick will crush under it if the load is spread over too small an area.
The answer is a padstone: a precast concrete block, commonly 215mm by 215mm by 100mm on domestic work, bedded on the wall to spread the reaction over enough masonry. The engineer sizes it, and the minimum bearing length of the steel onto it, usually 100mm to 150mm. Where the wall below is a cavity wall, the inner leaf is normally the load bearing leaf and the padstone sits on that. Where the wall is a single skin garden wall or a poor quality Victorian brick, the calculation may call for a pier, a longer padstone or a plate.
Party walls, and the notice that goes with them
On a terrace or a semi, one end of a lantern beam very often lands on the party wall. Cutting into a party structure to insert a beam is notifiable work under the Party Wall etc. Act 1996, and the notice period is two months for that kind of work.
This is a paperwork item rather than an obstacle. The adjoining owner can consent in writing, in which case nothing further is needed, or they can dissent, in which case a surveyor is appointed and a party wall award is drawn up. What matters for programme is that two months is two months, and finding out about it after the glass has been ordered is the expensive way to learn it. The survey establishes early whether a bearing lands on a shared wall.
Openings that run with the joists rather than across them
This is the case people underestimate. An opening running across the joists interrupts each one at a point. An opening running parallel to them removes whole joists along most of their length, and the trimmers at each end then have to carry the full accumulated load of everything taken out.
Practically it means a long lantern aligned with the joist direction almost always needs steel at both ends, or a pair of beams either side of the opening running the full span. The alternative, which is often better, is to rotate the lantern so its long axis runs across the joists. Where the room shape allows it, that single decision can take a steel out of the job entirely, and it is one of the reasons sizing and structure are settled together rather than in sequence.
Deflection, and why glass is unforgiving
Strength is not the governing criterion on a lantern opening. Deflection is. A beam can be perfectly safe and still move enough under snow load to distort the opening it is framing, and a rigid rectangular lantern set into a frame that goes out of square will bind, whine and eventually crack a corner of glass.
The standard limit for a domestic beam is span over 360 for imposed load, or span over 200 overall. Where a beam is supporting a glazed element, engineers routinely tighten that, often to span over 500, and it is worth telling the engineer that a lantern is going on the opening rather than a plasterboard ceiling. That single note can change the section by one size and it removes a whole category of problem that surfaces two winters later.
Insulation around a steel, and the cold bridge it makes
Steel conducts heat about four hundred times better than the timber it replaces, so an unwrapped beam in a roof build-up is a straight line of cold running across a ceiling. In a kitchen, that line shows itself first as a strip of ceiling that stays cooler than the rest and then as a mark where dust settles on it differentially.
The detail is to keep the insulation continuous over, under or around the steel so that the beam sits on the warm side of the insulation line. On a warm deck flat roof, where the insulation sits above the deck, this happens naturally and the steel is inside the envelope. On a cold roof it takes deliberate detailing. It is worth asking the question at design, because correcting a cold bridge after the ceiling is boarded means taking the ceiling down.
Fire protection where a beam is exposed
Structural steel loses strength rapidly above about 550 degrees, so where a beam contributes to the stability of a dwelling it needs protecting to the required period, normally thirty minutes in a domestic setting.
Two layers of 12.5mm fire rated plasterboard boxed around the section is the conventional domestic answer and it satisfies Building Control on most jobs. Where the design intent is an exposed beam left visible in the ceiling, intumescent coating is the route, applied to a specified dry film thickness with a certificate to go with it. That is a decision to make at design, because a beam intended to be seen is ordered with a different finish and delivered with different handling.
Trussed roofs, and openings that run into a pitch
Lanterns sit on flat roofs, but the flat roof often abuts a pitched one, and across the estates at Great Notley, South Woodham Ferrers, Beaulieu Park and most of Basildon and Harlow that pitched roof is trussed.
A trussed rafter is an engineered component with its members in calculated tension and compression, and the webs and bottom chord cannot be cut. Where an opening has to run close to a junction with a trussed roof, or where the new flat roof structure wants to bear on something the trusses are using, the truss manufacturer or the engineer has to be consulted. There is nearly always a route through, but it is designed rather than improvised.
What happens on a Victorian rear addition
The older stock brings a different set of problems. Joists in a Victorian rear addition are frequently undersized by current standards, spaced irregularly, notched by a century of plumbers, and bearing into a single skin wall with no padstone anywhere.
The consequence is that an opening in that roof often needs more structural work than its size suggests, because the surrounding structure is not up to picking up the redistributed load. It is one of the situations where an engineer earns their fee immediately. The particular conditions of that house type are worked through on the rear addition page.
Building Control, and the inspection points
Structural work of this kind is inspected, and there are usually three visits. The first is at the point the opening is formed and before anything covers it, so the inspector can see the beam, the bearings, the padstones and the connections. The second is at insulation and covering stage. The third is at completion.
The paperwork trail is the calculation package, the inspection record and finally the completion certificate. Sunspire handles the notification and the booking of those inspections as part of the job. It matters far more than it sounds, because an unrecorded structural alteration to a roof is exactly the item a buyer’s solicitor asks about, and the moment it surfaces is the worst possible moment to be looking for it.
Sequencing, and what a steel does to the programme
A job with timber trimming only is normally two to four days from strip-back to finished reveal. A job with a steel adds two things: lead time before, and days on site during.
The engineer needs a week or so from survey to calculation. Fabrication of a steel to size, drilled and primed, is typically one to two weeks. On site, forming the bearings, propping the roof, lifting the beam in and dry-packing the bearings adds a day or two to the programme before the kerb can even be started. None of it is difficult and all of it is predictable, which is why it is priced and programmed at survey rather than absorbed as it goes. Whatever stage the work is at, the roof is made watertight before the site is left each evening.
Propping, and keeping the roof up while it is open
While joists are cut and before the beam is in, the roof has to be held up by something. That is temporary propping: adjustable steel props on spreader plates, taking the load off the area being worked and delivering it to a floor below that can take it.
The plate underneath matters as much as the prop. A prop standing directly on a suspended timber floor concentrates the load onto one or two joists, and the correct arrangement spreads it across several with scaffold boards or a sole plate. Where the floor below is solid, this is straightforward. Where it is a Victorian suspended timber floor over a shallow void, it wants thinking about, and the survey looks at it.
What the survey looks for, structurally
The structural half of a lantern survey is done from inside, in the roof void or through a lifted section of ceiling. It establishes the joist size, the spacing, the direction of span, where each end bears, whether there is already a beam in the roof, and the condition of the timber and the masonry it sits on.
It also looks for the awkward truths: a joist already notched to half its depth for a waste pipe, a bearing that is one brick of a single skin wall, a previous alteration with no visible support at all. Those things change the answer and they change the price, and finding them before a figure is agreed is the entire reason the survey happens before the quote. Sunspire gives a range by email once the basics are known and a fixed written price once the roof has been seen, and the range for structural work is set out under costs.
Keep reading
More on roof lantern installation
Aluminium and timber roof lanterns compared
Read the guide →Roof lantern glazing and solar control
Read the guide →Roof lantern ridge vents and how they are operated
Read the guide →Roof lantern pitch and how steep it should be
Read the guide →Roof lantern kerbs and who builds them
Read the guide →
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