How do I size a steel beam for an extension opening?

Beam sizing depends on the span (opening width), the load it carries (roof, floor or both) and the tributary area (how much of the structure bears on the beam). A structural engineer calculates the bending moment and shear force, then selects a Universal Beam (UB) from the steel tables. This calculator provides indicative sizes only.

What is tributary area loading?

The tributary width is the distance from the beam to the next point of support (typically half the room depth on each side). The beam carries the weight of everything within this width: floor joists, roof structure, walls above and imposed loads (furniture, people). Multiply the loading per m² by the tributary width to get load per metre of beam.

What is a padstone and what size do I need?

A padstone is a concrete block that sits on the wall at each end of the steel beam. It spreads the concentrated beam load over a larger area of masonry. Typical domestic padstones are 225x225x100mm of C40 concrete. For heavier loads or weaker masonry, larger padstones or concrete pad foundations may be needed.

Do I need temporary support when installing a steel beam?

Yes, always. Before removing any load-bearing wall, Acrow props with timber needles must support the structure above. Props should be placed at maximum 1m centres across the opening on both sides of the wall. The temporary support must stay in place until the beam is properly installed, packed and the mortar has cured.

Can I install a steel beam myself?

Legally you can, but it is not recommended. Steel beams are heavy (a 4m beam can weigh 90–130kg), require precise installation and must be designed by a structural engineer. Building Control will inspect the beam installation and check it matches the engineer's design. Most homeowners hire a builder with experience in structural alterations.

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Extension Steel Calculator — Beam Sizing & Padstones

Estimate structural steel beam sizes for extension openings based on span, tributary area loading and padstone requirements. Engineer design essential.

Beam span (opening width)

Load type

Tributary width

Bearing type

How We Calculate This

This calculator estimates structural steel beam requirements using simplified loading analysis per UK building standards.

Loading calculation

Dead loads (structure weight) and imposed loads (people, furniture) are factored per Eurocode combinations (1.35 x dead + 1.5 x imposed). Total load per metre = factored load x tributary width.

Beam selection

Bending moment = wL²/8 for a simply supported beam. The indicative beam is selected from standard UB sections. Final design MUST be by a structural engineer.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.