The steel that carries a roof lantern opening

Who designs it

A structural engineer, not the installer and not the builder. The engineer establishes what the roof carries, what the opening removes, and what section is needed to carry the residual load across the span.

The output is a drawing with a specified section, bearing details at each end, and any padstone requirements. That drawing is what Building Control will want to see.

Bearings

A steel is only as good as what it sits on. Each end bears onto masonry, usually through a padstone that spreads the point load into the wall. Under-sizing the bearing is a way of making a correctly sized beam fail, and it is the detail most often skimped.

Sequence

This is the part that catches people out.

1. Engineer designs the steel from the intended opening.
2. Steel is fabricated and installed, and its final position is measured.
3. The kerb is built to that measured position.
4. The kerb is measured, and only then is the lantern ordered.

Ordering the lantern earlier to save time is a false economy. A made to size unit cannot be trimmed, and a steel that ends up 30mm from the drawing position, which is entirely normal, will make it not fit.

Cold bridging

A steel is a highly conductive element passing through the insulation line. Unless it is insulated, it becomes a cold bridge, and condensation forms on the ceiling below it in winter, usually in a neat straight line that people find alarming.

Insulating the steel is cheap while it is exposed and expensive afterwards.

Cost

On a large lantern, design, supply and installation of the steel frequently exceeds the cost of the lantern. That is the main argument for two smaller lanterns over one large one: two modest openings may need no steel at all.

Why steel enters the conversation at all

A roof lantern is heavy, and everything it weighs has to be carried by the structure around the opening it sits in.

On a new extension that is designed in from the start and frequently costs very little extra. On a retrofit, the joists were sized for the roof as built with no allowance for a large hole in it, so cutting them means the load has to be picked up and carried somewhere else.

Beyond a certain span, timber trimmers get impractically deep and steel becomes the sensible answer. That is where the largest line on a retrofit lantern quotation comes from, and it has nothing to do with the glazing.

What the engineer is working out

Dead load. The permanent weight: the lantern itself, the kerb, and the roof construction that remains.

Imposed load. Snow, and maintenance access. On a lantern that is modest compared with a walk on unit, and it is not nothing.

Span. The distance the new member has to bridge. Depth and cost rise steeply rather than proportionally, because a longer span needs a stronger section to carry the same load.

Deflection. As important as strength here, and the reason lantern openings are designed more tightly than ordinary roof openings.

A structure that sags slightly under load is entirely acceptable for a roof. It is not acceptable under a rigid glazed frame, because a lantern is manufactured to your dimensions and it needs to sit on something that stays where it was when it was measured.

Which way the joists run

Changes the answer more than the size of the lantern does, and it is the first thing a survey establishes.

An opening running parallel to the joists, set between two of them, needs comparatively little: the joists either side are doubled up and the load is carried without cutting anything.

An opening cutting across the joists means the cut members have to bear onto trimmers, and those trimmers have to span the opening and deliver that load into the joists either side.

On a rectangular lantern, rotating the design so it runs with the joists rather than across them sometimes removes the steel entirely. That costs nothing at the drawing stage and it is worth asking about before the layout is fixed.

Timber trimmers or steel

Timber is fine up to a point, and the point arrives sooner than people expect.

A doubled or tripled timber trimmer handles a modest opening comfortably. As the span grows, the required depth grows with it, and a trimmer deep enough to do the job starts to interfere with the ceiling below, the insulation zone, or the head height in the room.

Steel achieves the same strength in far less depth. That is usually why it appears: not because timber could not carry the load in principle, but because a timber member deep enough to do so would not fit.

Steel is also stiffer for a given depth, which matters for the deflection limit.

What steel involves in practice

Design. A structural engineer produces calculations and a drawing specifying the section, the length, the bearing details and the connections.

Fabrication. The section is cut to length, and any plates or cleats are welded on, before it comes to site. That is a lead time in its own right.

Getting it in. A steel beam is heavy and rigid and it has to reach the position through a finished house or from outside. On a rear extension with side access that is manageable. On a terraced property it may need to come through the house, and on a large section it may need mechanical assistance.

Bearings. The steel has to sit on something that can carry it, which frequently means padstones built into the supporting walls. That is part of the design rather than an afterthought.

Fire protection and corrosion protection, depending on where the steel sits and what it is exposed to.

Where it goes in the build up

Worth thinking about early, because it interacts with the insulation.

A steel within the insulated envelope is warm and dry, which is what you want. A steel sitting outside the insulation line, or bridging it, is a cold bridge and a potential condensation point, and on a warm roof that means the insulation detail has to accommodate it.

Where a steel is left partially exposed within a ceiling, it also has to be finished, and boxing it in is a joinery and plastering job that belongs in the quotation.

The sequence that avoids waste

Survey. Then structural design. Then order the lantern. Then book the fitting to the confirmed delivery date.

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

Ordering a lantern from a tape measure held before the structure is designed is how units arrive the wrong size, and a bespoke lantern cannot be returned.

Building Control

Forming a structural opening is notifiable under Part A, and where steel is involved the engineer’s calculations form part of the submission.

Treat the inspection as a benefit. An independent check of the structural work while it is open is exactly what you want, and it produces a certificate a buyer’s solicitor will ask for.

Keep the calculations with the house paperwork alongside it. In fifteen years, somebody asking whether the roof can take a different use will need them.

The question worth asking first

Whether a lantern is the right product for the room at all.

On a long narrow extension, two or three flat roof rooflights spaced along the length light the space more evenly than a single lantern, cost considerably less, are light enough to handle without a lift, and frequently need no structural work beyond doubling a joist.

A lantern earns its cost where the room is broad, the ceiling is flat at normal head height, and the volume it adds is the point. Where that is not the case, the structural line on the quotation is buying you something the room did not need.

What to ask on a quotation

Whether structural design is included and whether an engineer’s fee is in the figure.

Whether any steel, its fabrication, its installation and the padstones are included or excluded.

And on a retrofit, a quotation that does not mention structure at all has not been thought through.

What the steel is doing

Forming an opening in a flat roof interrupts joists. Each cut joist was carrying a strip of roof, and that load now has to travel sideways into trimmers around the opening and then into the structure either side.

Beyond a modest span, timber trimmers cannot do it, and steel takes over. Most domestic lanterns are large enough for that to apply.

The steel also carries the lantern itself, which on a glazed structure is a substantial dead load, plus wind and snow.

Deflection, which decides more than strength

The part that gets overlooked.

A beam can be strong enough and still deflect more than the installation can tolerate. If the steel sags under load, the kerb goes out of level, and a lantern on a kerb that is not level does not sit down evenly. The gap gets taken up in sealant, and sealant carrying a structural gap is a temporary arrangement.

So the engineer designs to a deflection limit as well as a strength requirement, and that limit is tighter than it would be for a plain ceiling.

Where a lantern has started leaking a few years after installation with no obvious cause, structural movement is worth considering before the glazing is blamed.

Padstones and bearings

A steel beam concentrates load at its ends, and that load has to be spread into whatever it bears on.

On masonry that usually means a padstone: a concrete or engineering brick block sized to distribute the point load into the wall below. Sitting a beam directly on ordinary blockwork crushes it over time.

The padstone size comes from the engineer, and it is not a detail to improvise.

Fire protection

Exposed structural steel in a domestic setting may require fire protection depending on its position and what it supports.

Usually that means boarding or an intumescent treatment, and it is decided with building control rather than assumed. It affects the finished dimensions, so it belongs in the design rather than being added afterwards.

The sequence it forces

Structural design first. Steel installed and the opening formed. Kerb built to the steel, square, level and insulated. Waterproofing dressed up the kerb. Survey to the finished kerb. Then order the lantern.

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