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Purlins & Rails Calculator — Indicative Section Pre-Sizing
Estimate purlin and side rail requirements for steel-framed buildings from rafter spacing, wind/snow loading and cladding weight. Gives an indicative cold-formed Z or C section — confirm against the manufacturer’s load/span tables before ordering.
Distance between portal frame rafters
1.5-2.0m typical for metal cladding
Roof value (ground snow × 0.8 for a 0–15° duo-pitch roof per BS EN 1991-1-3 UK NA). 0.4–0.6 kN/m² typical for England — this tool does NOT apply the shape coefficient for you
Net wind pressure on roof zone
0.1 kN/m² typical for profiled steel
12mm rods for spans over 4.5m
Leave 0 to auto-calc from roof slope length
Length of ONE rafter (eaves to ridge); duo-pitch roofs need 2× the purlins (leave 0 for 5m default)
Purlin material cost per metre
Safety notice
Steel members must be designed or checked by a structural engineer before fabrication or installation. Treat these figures as initial guidance for planning and budgeting, not a structural design.
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How We Calculate This
This calculator gives an indicativepurlin or side rail section for a steel-framed building from the loading due to cladding, snow and wind. It is a pre-sizing aid: it does not replace a full design to the Eurocodes, and the section it suggests must be confirmed against the manufacturer’s published load/span tables before ordering.
Load calculation
- Dead load: cladding weight plus an allowance for the purlin’s own weight (kN/m²)
- Snow load: the roof snow load you enter, s = µ1·Ce·Ct·sk (BS EN 1991-1-3 + UK NA). The tool does not apply the shape coefficient µ1 — enter the roof value yourself (ground snow × 0.8 for a 0–15° duo-pitch roof)
- Wind load: the net wind pressure on the roof zone (kN/m²), from a BS EN 1991-1-4 assessment
- Factored load: EN 1990 ULS — gravity 1.35 × dead + 1.5 × snow, compared against the 1.5 × wind case; the larger governs (kN/m on the purlin)
Section selection
The factored load is multiplied by the span to give the load carried over each bay, then matched against approximate span/load bands to suggest a cold-formed Z or C section. The section self-weights are verified Metsec component weights (sections designed to BS EN 1993-1-3). The bands are indicative single-span estimates, nota reproduction of a published load table — always confirm the final section, restraint and connection details with a structural engineer and the manufacturer’s load/span tables (valid to a 25° roof slope).
Frequently Asked Questions
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.
Z (Zed) sections have a point-symmetric Z shape, so adjacent spans can overlap or be sleeved at the rafter — this makes them the usual choice for continuous multi-span roofs and the most efficient option over several bays. C (Cee) sections are simpler channels used for single-span purlins, eaves beams, side rails and shorter runs where overlapping is not needed. Both are cold-formed from galvanised steel to BS EN 1993-1-3; this tool uses verified Metsec component weights for each. (Sigma/Σ profiles are a third family — e.g. Albion or Hadley — with stiffening ribs for heavier loads; size those directly from that manufacturer's tables.)
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.
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.
UK snow loads come from BS EN 1991-1-3 and its UK National Annex. The roof snow load is s = µ1 × Ce × Ct × sk, where sk is the ground (characteristic) snow load from the NA map (rising with altitude), Ce = Ct = 1.0 for normal UK conditions, and µ1 is the roof shape coefficient — 0.8 for a roof pitched 0–15° (so roof snow ≈ 0.8 × ground snow for a standard duo-pitch). Ground snow ranges from about 0.4 kN/m² in south-west England to 0.7+ kN/m² in the Scottish Highlands; for most of England a roof snow load of 0.4–0.6 kN/m² is typical. Important: enter the ROOF snow load (after the shape coefficient) into this calculator — it does not apply µ1 for you.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.