Guide

What size RSJ do I need for a 3m span? Sizes span by span

Updated 2026-08-14

Everyone wants the table: span in, section out. It does not exist, for good reasons. Here is what engineers typically specify, and how to get the real answer.

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Question 1

What is the clear span of the opening, in metres?

Measure the structural opening. The beam itself will be longer, because it has to bear on masonry at each end; how much bearing, and on what padstone, comes out of the calcs rather than a rule of thumb. Between sizes? Round up.

Why there is no RSJ span table

Two identical 4 m openings can need different beams. One carries a bedroom floor; the other carries the floor, a purlin load from the roof, and the end of another steel.

Timber gets deemed-to-satisfy span tables in guidance linked from Approved Document A. Structural steel in domestic alterations does not. It is sized by calculation to BS EN 1993 (Eurocode 3), weighing strength, deflection, bearing and lateral restraint together. So every serious answer to 'what size RSJ' ends in an engineer.

Steel beam spans and sizes engineers typically specify

Our data table shows section ranges typically specified per span and load band on comparable UK domestic jobs. They run from 152x152 UC23 over small floor-only openings to 356 series UBs at the top of domestic work. Read it as orientation, never a specification: real jobs land outside these bands for reasons only visible on site.

Padstones: the part everyone forgets

A beam is only as good as what it bears on. Padstones spread the beam's end reaction into the masonry below. Their size is calculated from that bearing stress in the engineer's calcs, not by rule of thumb. Domestic padstones are commonly engineering brick or precast concrete, 215–440 mm long. The calcs specify yours, and building control checks them.

The process, start to sign-off

1. Engineer surveys and calculates: site visit, loads measured, section and padstones specified, typically £250–£500 in 2026.
2. Building control is notified via full plans or a building notice, with the calcs.
3. The builder installs to the spec: temporary props, beam in, designed padstones, correct bearing.
4. Inspection and completion certificate: the piece of paper the next buyer's conveyancer will ask for.

Two questions in the checker tell you which range you are probably in, then the engineer's calcs make it real.

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Tell us about the opening and we'll introduce you to up to three firms, each a structural engineer who handles domestic beam calcs for jobs like yours.

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Two questions, then the right next step

See the typical section range, then get the calcs that count.

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Questions, answered directly

What size RSJ do I need for a 3m span?

On comparable UK jobs with only a domestic floor above, engineers typically specify between 152x152 UC23 and 203x203 UC46 for spans up to about 3 m. That is observed practice, not a design: your beam must be calculated by a structural engineer, and building control will require the calcs.

What size RSJ do I need for a 4m span?

With only a domestic floor above, a 4 m span is in the 3–4.5 m band: comparable UK jobs typically see 203x203 UC46 to 254x146 UB37. With roof or masonry above too, it moves to 254x146 UB to 305x165 UB. That is observed practice, not a design. A structural engineer must calculate yours, for building control.

What size RSJ do I need for a 5m span?

A 5 m span is in the 4.5 to 6 m band. With only a domestic floor above, comparable UK jobs typically see 254x146 UB37 to 305x165 UB46. With roof or masonry above, 305 to 356 series universal beams, sometimes paired. Deflection usually governs, and only the engineer's calculation settles the section.

What size RSJ do I need for a 6m span?

6 m is the top of our range. A floor-only beam typically lands at 254x146 UB37 to 305x165 UB46, and floor plus roof or masonry in the 305 to 356 series, sometimes paired or as a frame. Beyond 6 m we give no indicative range: splices, crane access and connection design dominate.

What size padstone does an RSJ need?

Whatever the calcs say: padstone size is computed from the beam's end reaction and the strength of the masonry it bears on. Engineering brick or precast concrete padstones around 215–440 mm long are common in domestic work, but the engineer specifies the exact size for each end.

What steel beam sizes are used in UK houses?

Most domestic openings use universal columns from 152x152 UC23 to 203x203 UC46 over short spans. Universal beams run from 254x146 UB37 up to the 305 and 356 series as spans and loads grow. The name gives nominal depth by width in millimetres and weight in kilograms per metre.

Sources

This site's own dataset, every figure quoted from its source and dated:

The UK Steel Beam Cost Index carries 7 published figures read from 7 sources' own pages; the 4 published figures given without a breakdown run from 75 to 1,200 GBP, read 11 November 2026.

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