RSJ stands for Rolled Steel Joist - a length of structural steel with an I-shaped cross-section used to span large openings or carry heavy structural loads. In modern engineering the correct term is Universal Beam (UB), but RSJ remains the common trade name used on building sites.

What is the difference between an RSJ and a universal beam?

Historically, RSJs had tapered flanges while Universal Beams (UB sections) have parallel flanges - making UBs more efficient structurally. Today virtually all new steel beams supplied are UB sections, but the term RSJ is still widely used in the trade to refer to any structural steel beam of this type.

How is an RSJ specified?

Universal Beams are specified by depth x flange width x weight per metre - for example a 203x133x30 UB is 203mm deep, 133mm wide, and weighs 30kg per metre. A structural engineer selects the section based on span, the loads to be carried, and allowable deflection limits.

Do I need Building Regulations approval to fit an RSJ?

Yes. Installing an RSJ to replace a load-bearing wall or carry structural load is notifiable building work under Approved Document A. Building Control approval is required and a structural engineer's calculations must be submitted demonstrating the beam's adequacy.

RSJ (Rolled Steel Joist) - Construction Glossary

Definition

An RSJ (Rolled Steel Joist) is a length of structural steel with an I-shaped or H-shaped cross-section, used to span large openings or carry heavy loads where timber or masonry would be insufficient. RSJ is the traditional trade name - the correct engineering term is Universal Beam (UB). The two horizontal plates are called flanges; the vertical plate connecting them is the web.

In practice

RSJs are the go-to solution for spanning removed load-bearing walls, creating open-plan layouts, carrying upper floors over large garage openings, or supporting structures where timber lintels are inadequate. A structural engineer selects the section size based on span, the loads to be carried, and the maximum allowable deflection.

In UK construction, Universal Beam sections are specified by depth x flange width x weight per metre - for example a 203x133x30 UB is 203mm deep, 133mm wide, and weighs 30kg per metre. The beam must be supported at each end on padstones bedded into the masonry to spread the concentrated point load. For longer spans or heavier loads, the beam may need to be encased in concrete or protected with intumescent paint for fire resistance.

Building Regulations

Relevant PartPart A - Structure

Approved DocumentAD A (2010, amended 2013)

Fire protectionPart B - may require intumescent coating or encasing

Engineer's calcs requiredYes - for all RSJ installations

Installing an RSJ to carry structural load is notifiable work under Building Regulations. Approved Document A covers structural adequacy; Approved Document B may require fire protection depending on the beam's location and the fire resistance period required for the building. A structural engineer's calculations are mandatory.

Full Building Regulations guidance

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RSJ (Rolled Steel Joist)

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