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Loft Steel Beam Calculator — Purlin & Ridge Beam Sizing
Estimate steel beam size for loft conversion purlins and ridge beams based on rafter loads, span and roof loading to BS 5950.
Clear span between supports
Slope length from ridge to eaves
Fabricated and delivered
Snow + maintenance, roof category H (0.6 typical; BS EN 1991-1-1 UK NA)
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How We Calculate This
This calculator provides indicative steel beam sizes based on simplified loading calculations. Always consult a structural engineer for final design.
Loading (BS 5950-1)
Roof loads are factored to the ultimate limit state using the BS 5950-1:2000 partial factors 1.4 × dead + 1.6 × imposed. A ridge or valley beam collects the rafters from both roof slopes, so the tributary load width is taken as the full rafter run; the factored uniformly distributed load is then UDL (kN/m) = factored load (kN/m²) × tributary width (m).
Bending check
Bending moment M = UDL × Span² / 8
Required plastic modulus Wpl,y (cm³) = M (kNm) × 1000 / py, where py = 275 N/mm² is the S275 design strength (BS 5950-1 Table 9, flange ≤ 16 mm).
Deflection check
A serviceability deflection check (δ = 5wL⁴ / 384EI, E = 210 kN/mm²) is also applied with a span/360 limit for brittle plaster finishes, and the tool selects the lightest Blue Book Universal Beam that satisfies the governing of the two checks.
Important note
These are indicative sizes only. A structural engineer must verify beam sizes, confirm bearing and deflection, design connections, allow for any point loads, and specify padstone requirements.
Frequently Asked Questions
When removing collar ties, purlins or struts to create open space in a loft, the roof structure needs alternative support. Steel beams (ridge beams and purlins) take the rafter loads and transfer them to the supporting walls or posts. A structural engineer must design these.
Purlin size depends on the span, rafter loading and spacing. For a typical 3m span carrying concrete-tiled rafters at 400mm centres, a 152×89×16 UB is often sufficient. For longer spans or heavier loads, larger sections like 203×133×25 UB may be needed. Always get a structural engineer's calculation.
A padstone is a concrete or dense block pad placed under each end of a steel beam to spread the concentrated point load over a larger area of the supporting wall. Without a padstone, the beam could crush the brickwork. Typical domestic padstones are 215×215×100mm or 450×215×100mm concrete.
Yes. Before removing any existing structural elements (collar ties, purlins, struts), temporary support must be erected using Acrow props and scaffold boards to carry the roof loads while the new steelwork is installed. This is critical for safety and must be planned by a structural engineer.
No. This calculator provides indicative beam sizes for preliminary budgeting only. All structural steelwork in a loft conversion must be designed by a qualified structural engineer and submitted to building control for approval. The engineer will consider specific loading, bearing conditions and building geometry.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.