Why do I need steel beams in a loft conversion?

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.

What size steel beam do I need for a loft purlin?

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.

What is a padstone and why do I need one?

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.

Do I need temporary support during installation?

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.

Is this calculator a substitute for 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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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.

Beam Span

Clear span between supports

Rafter Length

Slope length from ridge to eaves

Beam Type

Roof Loading

Steel Price Per kg (optional)

Fabricated and delivered

How We Calculate This

This calculator provides indicative steel beam sizes based on simplified loading calculations. Always consult a structural engineer for final design.

The formula

UDL (kN/m) = Total Load (kN/m²) × Tributary Width (m)

Bending Moment = UDL × Span² / 8

Section Modulus = Moment / Steel Yield Strength (275 N/mm²)

Important note

These are indicative sizes only. A structural engineer must verify beam sizes, check deflection limits, design connections and specify padstone requirements.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.