Guide
What size RSJ do I need? The honest answer, span by span
Updated
Everyone wants the table: span in, section out. It does not exist for good reasons, but here is what engineers typically specify, and the process that gets you the real answer.
Why a span table cannot answer it
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. That is why every serious answer to 'what size RSJ' ends in an engineer.
What engineers typically specify
Our data table shows the section ranges typically specified per span and load band on comparable UK domestic jobs, from 152x152 UC23 over small floor-only openings to 356 series UBs at the top of domestic work. Read it as orientation, the range you are probably in, never as 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, and their size is calculated from that bearing stress, it is part of the engineer's calcs, not a rule of thumb. In practice domestic padstones are commonly engineering brick or precast concrete in the 215–440 mm length range, but treat that as description, not guidance: 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.