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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.
Half the span of the floor/roof bearing on each side of the beam.
Length of the padstone along the wall (bearing direction).
Width through the wall thickness. With the length, this forms the bearing face on the masonry.
Vertical height of the padstone (e.g. one or two brick courses). Does not affect the bearing area.
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How We Calculate This
This calculator estimates structural steel beam requirements using simplified loading analysis per current UK structural standards (BS EN 1990, BS EN 1991-1-1 and BS EN 1993-1-1, each with its UK National Annex — the Eurocodes that replaced the withdrawn BS 6399 / BS 5950).
Loading calculation
Characteristic dead loads (structure self-weight and finishes) and imposed loads (people, furniture, roof maintenance) are combined at the ultimate limit state per BS EN 1990 equation 6.10: 1.35 × dead + 1.5 × imposed. Load per metre on the beam = the area load × the tributary width it collects.
Beam selection
The ULS bending moment of a simply supported beam under a uniform load is M = wL²/8. The calculator picks the smallest Universal Beam (S275) whose design bending resistance Mc,Rd = 0.275 × Wpl,y (kNm, with Wpl,y in cm³) is at least M. It then checks a simple uniform-load deflection δ = 5wL⁴/384EI (E = 210 kN/mm²) against span/360 for imposed load (plastered/brittle finishes) and span/250 for total load, using the unfactored service loads, and steps up to a stiffer section if either limit is exceeded.
What this does NOT check
The indicative size assumes the beam is fully laterally restrained. It does not assess lateral-torsional buckling, web bearing/buckling, shear, connection design, notching, fire protection or any bespoke point loads — any of which can govern and require a larger section. The padstone figure is an indicative masonry bearing-stress check, not a verified padstone design. A structural engineer MUST carry out the full design and Building Control must approve it.
Frequently Asked Questions
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.
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.
A padstone is a precast concrete block that sits on the wall at each end of the steel beam. It spreads the concentrated beam reaction over a larger area of masonry so the localised bearing stress stays within the masonry's capacity (BS EN 1996-1-1 / BS 5628). Common domestic padstones are 225×225×100mm, precast in C40/50 concrete to BS 8500-2 — far stronger than the blockwork beneath. The required size is set by the engineer from the beam end reaction and the strength of the actual masonry units; this calculator gives an indicative bearing-stress check only, not a verified padstone design.
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.
Legally you can, but it is not recommended. Steel beams are heavy: a 4m Universal Beam ranges from about 64kg (a light 152×89×16) up to roughly 230kg (a 356×171×57), so a typical domestic beam is well over a one-person lift. They also need 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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Last updated: July 2026
Verified against UK standards · estimates only, confirm with your supplier.