Structural openings and steel for a roof lantern

What a lantern actually weighs

Start with the number, because people consistently underestimate it. A sealed glass unit at 28mm overall weighs roughly 35kg per square metre. Add the frame and a 2500mm by 1500mm lantern is around 160 to 190kg.

A 4000mm by 2000mm lantern is approaching 400kg. That is a grand piano, permanently, in the middle of a roof designed to carry a felt covering and a person kneeling on it.

Then add the kerb the lantern stands on, which is a timber box with insulation and a membrane, and add the imposed load: snow, and the weight of somebody cleaning it. The design case is not the lantern on a still day.

The load arrives as a line, not a point

This is the part that decides the design. A lantern does not press down at four corners. It bears continuously along the top of the kerb, and the kerb bears continuously onto the timbers around the opening.

So the trimmer joists on all four sides of the opening carry a uniformly distributed load along their length, not a point load at midspan. That is the more forgiving case, and it is why a well trimmed opening in a modestly sized extension often needs nothing more exotic than a doubled joist.

Where it stops being forgiving is at the corners. The two trimmers running parallel to the joists collect everything from the trimmed joists on either side and dump it as concentrated point loads onto the trimmers running across. Those cross members are the ones that get deep.

How an opening in a flat roof deck is trimmed

The standard arrangement is four members forming a box inside the joist grid. The joists that would have crossed the opening are cut short and are called trimmed joists. The two members running perpendicular to them, taking their cut ends, are trimmers. The two joists running alongside the opening, taking the trimmers, are trimming joists.

The trimming joists carry roughly one and a half times what an ordinary joist carries, so they are almost always doubled, bolted together rather than nailed, and often tripled on a wide opening.

The cut ends of the trimmed joists hang off the trimmers on galvanised joist hangers sized for the section. Skew nailing is not adequate here and is the single most common shortcut we find when we open up an older installation.

When doubled timber runs out

Doubled C24 timber will handle a surprising amount. As a rough guide, on a domestic flat roof with joists at 400mm centres, an opening up to about 1800mm across the joist direction can usually be trimmed in doubled 47 x 220mm without difficulty.

Past that, the depth needed starts to exceed the depth of the roof zone. You cannot fit a 300mm deep trimmer into a roof built with 220mm joists without either dropping it below the ceiling line, which you can see, or raising it above the deck, which fouls the falls.

That constraint, depth rather than strength, is what usually pushes a project to steel. It is not that the timber cannot be made strong enough. It is that the strong enough timber does not fit.

Flitch beams, and the middle option people forget

A flitch beam is a steel plate sandwiched between two timbers and bolted through at regular centres. It gives you most of the strength of a rolled steel section within a timber depth, and it can be carried up a ladder in pieces by two people.

On a terraced house in Jericho with no rear access, that matters enormously. A 3 metre steel beam weighing 150kg has to get to the back of the property somehow, and the answer is sometimes a crane over the roof, which is a day’s hire and a road closure.

Flitch beams need the bolt pattern, plate thickness and timber grade all specified by the engineer. They are not a site improvisation. But where they work they save a great deal of disruption.

Steel, and where it goes

Where a rolled steel section is needed it is normally a parallel flange channel or a small universal beam laid within the roof zone, with the timbers hung off it on a bolted angle or notched over the bottom flange.

A channel with the open side facing into the opening lets the trimmed joists sit inside the web, which recovers depth. A universal beam is stronger for the same depth but takes more of the roof zone.

Whichever it is, the steel wants wrapping. A steel member in a warm roof build-up must sit within the insulation line or be insulated around, or it becomes a cold bridge running the width of the extension and shows up as a dark line of condensation on the ceiling below.

Padstones, bearings and where the load lands

A beam is only as good as what it sits on. Each end needs a bearing of adequate length, usually 100mm minimum onto masonry, and a padstone to spread the load into the wall.

A padstone is a dense concrete block, sized by the engineer, bedded into the wall to spread a concentrated end reaction over enough brickwork that the brick below does not crush. On a single skin extension wall the padstone often needs to be longer than people expect.

The load does not stop at the padstone. It travels down through the wall to the foundation, and where a lantern beam lands near the end of a wall, on a pier between openings, or over an existing opening, the engineer has to check the load path all the way down. This is where a lantern occasionally turns into a foundation question.

Where the lantern beam meets the beam over the old back wall

Almost every rear extension has a large opening knocked through the original rear wall, carried on a steel. The lantern trimming sits a couple of metres away from it, in the same roof, and the two interact.

If the lantern trimmers bear onto the extension side walls, the interaction is minor. If one of them bears onto the beam over the knock-through, that beam is now carrying a point load it was not designed for, and the section size goes up.

We see this go wrong when the lantern is chosen after the structural drawings are issued. Getting a beam changed once the calculations are with building control means new calculations, a new drawing, and sometimes a larger padstone in a wall that is already built. Choose the lantern first.

Depth: the fight between beam, insulation and headroom

A flat roof has to accommodate the structure, the insulation, the falls and the ceiling within a fixed height, because the top is set by the neighbour’s eaves or by a first floor window sill and the bottom is set by the ceiling height you want inside.

A warm roof puts the insulation above the deck, typically 120 to 150mm of PIR, plus a tapered layer to form the falls. That eats into the height before the structure gets any.

So when a lantern trimmer needs to grow from 220 to 300mm, that 80mm has to come from somewhere. It comes off the ceiling height, or the insulation thickness, or the roof goes up and the planning drawing changes. This is the conversation to have with your engineer and your builder at design stage, in one meeting, with the lantern size on the table.

Warm roof, cold roof and where the structure sits

In a warm roof the deck sits on the joists, the insulation sits on the deck and the membrane on top. The structure is entirely within the warm side, which is good: the timbers stay dry and warm and there is no ventilation to arrange.

In a cold roof the insulation sits between the joists and there must be a ventilated void above it. Cold roofs on flat extensions are a poor arrangement generally and a worse one around a lantern, because the kerb interrupts the cross ventilation path and you end up with a stagnant zone of moist air directly against cold timber.

Where we are asked to fit a lantern into an existing cold roof, we will do it, and we will be explicit about the ventilation route around the new kerb before we start.

Snow, cleaning and imposed load

The imposed load on a domestic flat roof in this part of the country is typically taken as 0.6 kN/m² for snow, with an additional allowance for maintenance access. On a lantern that seems academic until you consider drifting.

A lantern is an obstruction. Snow blowing across a flat roof piles against the upwind face of a kerb and can reach two or three times the uniform depth in that band. The engineer accounts for this as a drift load against the obstruction, and it is one reason the trimmers on the exposed side sometimes come out larger than symmetry would suggest.

Snow slides off a pitched lantern readily, which is one of the practical arguments for a steeper pitch, discussed on roof lantern pitch and how steep it should be. It slides off onto the flat roof beside it, where it then sits.

Wind uplift on a raised glazed object

Downward load is intuitive. Uplift is the one people miss.

A lantern standing 400mm proud of a flat roof sits in the accelerated airflow at the roof edge, and wind passing over a pitched surface generates suction on the leeward facets. On an exposed site on the ridge above Oxford, that suction is a real design case.

The consequence is that the lantern must be mechanically fixed down through its base into the kerb, at the manufacturer’s specified centres, and the kerb must be mechanically fixed to the deck and the trimmers. Bedding a lantern on sealant alone and relying on its weight is not adequate on an exposed roof, and it is the reason we fix down through the frame even when the instructions treat it as optional.

Deflection, and why glass is fussier than plasterboard

A beam can be strong enough and still be unsuitable. Timber design normally limits deflection to span over 250 or 14mm, whichever is less, which is fine for a ceiling.

A lantern kerb sitting on a member that sags 12mm in the middle is a kerb that is no longer flat, and a lantern bedded onto a kerb that is not flat is a lantern with uneven compression on its perimeter seal. Over a few seasons of thermal movement that is where water finds a way in.

So the deflection limit under a lantern should be tightened, typically to span over 360 or better. Say this to the engineer explicitly. It is a one line instruction and it changes the section size occasionally, and it prevents the slow problem that is very difficult to correct later.

What the engineer needs from you, and what you get back

Give the engineer the lantern manufacturer’s technical sheet with the external kerb dimensions, the unit weight in kilograms, and the position of the opening on a roof plan with dimensions to the walls. Add the existing joist size, spacing and span if the roof is already built.

What comes back is a set of calculations, a structural drawing showing member sizes, hanger types, bolt patterns and padstone sizes, and a specification note. That package goes to building control.

The fee for this is modest against the cost of a lantern and it is not a place to economise. We do not produce structural calculations and we will not fit a lantern onto an opening that has been sized by eye.

Building control, and the stages they want to see

Forming a new opening in a roof and installing a rooflight are both notifiable work. The calculations are checked at plans stage, and the inspector will want to see the opening at two points: once with the structure exposed, before the deck and insulation go on, and again at completion.

Photographs are accepted for some elements by some authorities, but the structural inspection is normally a visit. Book it rather than assume it, because a roof closed up before an inspection sometimes has to be opened again.

The wider regulatory position for rooflights is set out on building regulations for rooflights. Whether a lantern needs steel at all, in the simplest cases, is answered on do roof lanterns need a steel beam.

Cutting into a roof that is already built

Retrofitting into an existing extension is entirely feasible and it is a different sequence. The membrane and insulation come off over a working area, the deck is lifted, the joists are exposed and their size and condition confirmed, and only then can the trimming be built.

The uncertainty is what the survey cannot see. Joists at 600mm centres instead of 400mm, a deck that has been overlaid twice, or a beam that turns out to be a timber lintel change the design. We open up, confirm, and if the structure is not what the calculations assumed, work stops and the engineer is called before anything else is cut.

Two figures apply here as everywhere: a range by email once we know the lantern size and the roof, then a fixed price after the survey. The roof is never left open overnight, so the opening is decked and made watertight at the end of each day regardless of where the work has got to.