Do roof lanterns need a steel beam?

When timber is enough

On a modest opening in a new flat roof, the lantern sits inside an opening formed by timber trimmers, and those trimmers carry the load to the surrounding joists. That is standard construction and needs no steel.

When a steel is needed

Widen the opening and the trimmer span increases. Past a point, timber of a sensible depth stops working, and the answer is a steel.

It also arises where the lantern takes up most of the roof, where the opening interrupts a structural wall line, or where the roof carries something else such as a terrace above.

Why the order matters

A steel has to be designed by a structural engineer, because the size depends on the span and the load. That design sets exactly where the steel sits, and the steel position sets the kerb position, and the kerb sets the lantern size.

Ordering a made to size lantern before the steel is designed is how a unit ends up not fitting. It cannot be trimmed.

What it costs

On a large lantern the steel and its installation frequently cost more than the lantern. That surprises people, and it is the main reason two smaller lanterns are sometimes both better and cheaper than one large one: two modest openings may need no steel at all.

We flag whether a steel is likely at survey and put it in the quotation as its own line rather than burying it.

Sometimes, and it depends on the opening rather than the lantern

A lantern is heavy, and everything it weighs is carried by the structure around the opening it sits in. Whether that needs steel depends on the span, the joist sizes, what else the roof is carrying, and whether the opening runs with or across the joists.

The honest answer is that it is a calculation rather than a rule of thumb, and it is done by a structural engineer rather than judged on site.

When steel is likely

A wide opening across the joists. Cutting several joists means the load they carried has to be picked up by trimmers, and those trimmers have to span the opening. Beyond a certain span, timber trimmers get impractically deep and steel is the sensible answer.

A retrofit into an existing roof. The joists were sized for the roof as built, with no allowance for a large hole in it. New build extensions are designed with the lantern in them from the start, which frequently avoids steel entirely.

A large or unusual lantern. Weight goes up faster than area, because larger panes need thicker glass.

Where the roof also carries something else. A flat roof used as a terrace, or carrying plant, is a different loading case.

When it is not needed

A modest lantern in a new extension, where the opening is designed in and the joists either side are simply doubled up, needs no steel at all. That is the commonest situation and it is why lanterns on new extensions are so much cheaper to install than retrofits.

A lantern set within the span of existing joists rather than cutting across them can also avoid it, which is why the orientation of the opening relative to the joists is worth establishing early.

Why it matters to the quotation

On a retrofit lantern, the structural work is frequently the largest single line and it has nothing to do with the glazing. A quotation that does not mention structure at all, on a retrofit into an existing roof, has not been thought through.

Ask whether structural design is included, whether an engineer’s fee is in the price, and whether the steel and its installation are in the figure or excluded.

The sequence

Survey, then structural design, then order, then fit. The engineer’s drawing has to come before the order because the opening dimensions depend on the trimmer design, and a lantern is manufactured to those dimensions with several weeks of lead time.

Ordering first and designing afterwards is how these jobs go wrong.

Usually, and the engineer decides

Forming an opening in a flat roof means interrupting joists, and their load has to be carried into trimmers around the opening. Beyond a modest span, timber trimmers are not adequate and steel is the answer.

Most domestic lanterns are large enough for that to apply.

What the engineer is designing for

The dead load of the lantern, which on a glazed structure is substantial. The imposed loads of wind and snow. And the loads from the roof area now being carried around the opening rather than through it.

Deflection matters as well as strength. A structure that sags slightly will put the kerb out of level, and a lantern on an out of level kerb does not sit down evenly.

Why this sets the sequence

The steel is designed and installed first, the kerb built to it, then the lantern surveyed to the finished kerb and ordered.

Ordering from a drawing before the structure exists is the common shortcut, and it produces a unit that is a few millimetres out, discovered on the day, with weeks of lead time to correct.

There is usually a span at which the steel steps up in cost, and it is worth asking where that falls before fixing the lantern size.