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Steel Beams for Rear and Side Extensions
An extension usually needs steel in two places at once — across the opening into the existing house, and holding up whatever the new roof lands on.
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Send us your engineer’s drawings and we’ll work out quantities and price the steel and the processing together — cut, drilled and finished to your specification, delivered direct to site.
Two different jobs, often confused
The beam across the back of the existing house is doing the same job as any structural opening: carrying the wall, and often a floor and roof above it, across a span that used to be solid masonry. That is the beam most people mean when they talk about the steel for an extension.
But the extension itself usually needs steel too. A flat roof spanning onto a wide glazed opening, a side return where the new roof has nothing to land on, a corner that has been opened up — all of those need their own members. It is common for a single-storey rear extension to involve three or four separate steels, and for them to connect to each other rather than all bearing into masonry.
Full-width openings and what they cost structurally
The appeal of an extension is usually the open span, and spans are expensive. Bending moment increases with the square of the span, so going from four metres to six is not a fifty per cent increase in demand — it is more than double. Combine that with deflection limits, which govern most domestic beams beyond about five metres, and the section size climbs quickly.
That is where the weight starts to matter practically as well as structurally. A six metre beam in a 305 section is around a quarter of a tonne. It is not going through a house, it is not being lifted by hand, and the delivery and lift need planning well before the day.
Where there is nothing to bear onto
On a full-width rear opening there is frequently no return wall at one or both ends to carry the beam down. When that happens the answer is usually a goal post frame — a beam with a column at each end, fabricated as a set and bolted together on site.
It is worth establishing early whether your job needs one, because a goal post is a fabricated assembly with connection details rather than a loose beam, and it changes both what has to be drawn and what has to be ordered.
Typical sizes for this job
Sections commonly specified for extension openings, from a modest rear opening to a full-width span.
| Size | Weight | Indicative Span | Typically Used For |
|---|---|---|---|
| 203 x 133 x 25 | 25 kg/m | 3.8–4.5m | Modest rear openings and door heads |
| 203 x 133 x 30 | 30 kg/m | 4–4.8m | The step up where the lighter section fails on deflection |
| 254 x 146 x 37 | 37 kg/m | 4.8–5.5m | Mid-width openings carrying floor and roof |
| 254 x 146 x 43 | 43 kg/m | 5–5.8m | Heavily loaded openings where a deeper section will not fit |
| 305 x 165 x 40 | 40 kg/m | 5.5–6.5m | Full-width rear openings and bi-fold heads |
| 305 x 165 x 46 | 46 kg/m | 5.8–6.8m | Wide spans where deflection governs the design |
| 356 x 171 x 45 | 45 kg/m | 6.5–7.5m | Very wide openings on larger properties |
What else the steel will need
Most beams need more than cutting to length before they reach site. All of it is arranged on the same order:
- Cut to size
- Welded cleats, end plates and connections
- Painting, priming and intumescent coating
- All processing and fabrication services
Related jobs
Common questions
How many steels does a rear extension need?
Commonly three or four: the beam across the existing rear wall, plus members carrying the new roof and any wide glazed opening. They often connect to one another rather than all bearing into masonry.
What size beam for a 5 metre bi-fold opening?
It depends on what sits above the opening as much as the width. A 5 metre span carrying only a flat roof is a different calculation from one carrying a storey above. Send the drawings and we will price what the engineer specifies.
Do I need a goal post frame?
If there is no return wall at one or both ends to carry the beam down, usually yes. It is worth establishing this early because a goal post is fabricated as a set with connection details.
How is a beam that size delivered?
Direct to site, with HIAB lorry crane offload available on request. Tell us at quote stage if access is restricted so the delivery is planned around it.
Ready for a price?
Send your drawings or a cutting list and we’ll come back with quantities and a price. Large and small orders both welcome.
We supply structural steel across London, the South and the Midlands, delivered direct to site. Information on this page is general guidance; the specification for your project must come from your structural engineer.
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