What is the typical purlin spacing for a steel building?

Standard purlin spacing is 1.5-2.0m for profiled metal cladding, depending on the cladding profile's spanning capability. Common spacings are 1.8m for standard profiles (0.7mm gauge) and 1.5m for lighter profiles. Standing seam roofing may allow wider spacing up to 2.5m. Always check the cladding manufacturer's load/span tables.

What is the difference between Zed and Sigma purlins?

Zed (Z) purlins have a Z-shaped cross-section and are the most common type, used for single and multi-span conditions. Sigma purlins have a flattened S-shape and offer better torsional resistance, making them more suitable for heavier loads and asymmetric loading. Sigma sections are more expensive but provide better performance per kg of steel.

When do I need sag rods for purlins?

Sag rods are needed when the purlin span exceeds about 4.5m and the roof pitch is steep enough for the purlin to sag under its own weight. Sag rods are typically 12mm diameter round bars connecting each purlin to the ridge or apex purlin. For shallow-pitch roofs (under 10°), sag rods are less critical but still good practice for spans over 5m.

How do I calculate the wind load on purlins?

Wind load on purlins depends on the site wind speed (from BS EN 1991-1-4 wind map), building height, roof pitch, and zone location on the roof. Typical wind pressures on UK industrial roofs range from 0.3-0.8 kN/m². Internal pressure coefficients also apply. A wind load assessment should be carried out for each specific building.

What snow load should I use in the UK?

UK snow loads are determined from BS EN 1991-1-3 and the National Annex. Ground snow loads range from 0.3 kN/m² in south-west England to 0.7+ kN/m² in the Scottish Highlands. The roof snow load is the ground snow load × a shape coefficient (0.8 for a standard duo-pitch roof). For most of England, 0.5-0.6 kN/m² is typical.

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Purlins & Rails Calculator — Section Size from Load Tables

Calculate purlin and side rail requirements for steel-framed buildings. Determines section size from rafter spacing, wind/snow loading, and cladding weight.

Rafter Spacing (Purlin Span)

Distance between portal frame rafters

Purlin Spacing

1.5-2.0m typical for metal cladding

Snow Load

0.5-0.6 kN/m² typical for England

Wind Pressure

Net wind pressure on roof zone

How We Calculate This

This calculator determines the required purlin or side rail section for a steel-framed building based on the loading from cladding, snow, and wind.

Load calculation

Dead load: Cladding weight + purlin self-weight (kN/m²)
Snow load: Ground snow load × shape coefficient (kN/m²)
Wind load: External and internal wind pressure (kN/m²)
Factored load: 1.35 × dead + 1.5 × imposed (kN/m on purlin)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.