Purlin Size for Metal Roofs: How to Choose the Right Steel Purlins

Selecting the correct purlin size for metal roof is critical for structural safety, long-term performance, and cost efficiency. This guide explains types, sizing factors, load calculations, common dimensions, materials, installation tips, and buying considerations to help make informed decisions.

Common Purlin Type	Typical Section (in)	Common Span (ft)	Typical Use
Z Purlin	3×1.5×0.12 – 8x2x0.25	6–24	Roof and wall supports, cantilevering
C Purlin	3×1.5×0.12 – 10x3x0.30	8–30	Wall girts, simple roof spans
Box/Tube Purlin	2″–6″ square/rectangular	10–40	High load or architectural exposed applications

Types Of Purlins And When To Use Them

Purlins are horizontal structural members supporting roof decking or sheeting. The most common types are Z purlins, C purlins, and built-up box or tube sections. Z purlins are efficient for overlapped splicing and continuous spans, while C purlins are simpler and often used as girts on walls. Box or tube purlins offer superior torsional resistance and are chosen where aesthetic or higher load capacity is required.

Selection depends on connection details: Z sections allow nesting and continuous runs, reducing required steel weight for the same span compared to simple-span C sections. Box sections reduce deflection and are preferred for long unbraced spans.

Factors That Determine Purlin Size For Metal Roof

Several variables influence necessary purlin size: roof span, purlin spacing, roof live and dead loads, wind and snow loads, support conditions, bracing, and serviceability limits such as deflection. Design must balance strength (moment capacity) and stiffness (deflection limits) according to applicable codes and project requirements.

Environmental loads vary regionally: coastal or high-wind areas require higher design loads, and snow zones increase roof live loads. Local building codes and ASCE 7 wind/snow standards inform load values used in calculations.

Common Purlin Sizes, Sections, And Dimensions

Standard cold-formed purlin gauges and section depths vary by manufacturer. Typical C/Z purlin depths range from 3″ to 10″, with flange widths from 1.5″ to 3″ and thicknesses from 0.12″ (approx. 14 ga) to 0.30″ (approx. 11 ga). Heavier gauges and deeper webs increase moment capacity and reduce deflection.

Section	Depth (in)	Thickness (in)	Approx Weight (lb/ft)
C100-12	4.0	0.12	1.2
Z150-16	6.0	0.16	2.1
C200-20	8.0	0.20	3.5

Manufacturers publish span tables that map purlin section to allowable spans at specific spacings and loadings. Always consult the producer’s load-span tables or a structural engineer for final selection rather than relying solely on general tables.

Load Calculations And Span Tables

Design involves calculating tributary loads from roofing, insulation, mechanical attachments, snow, and wind pressures. Loads per foot on a purlin equal roof load (psf) times purlin spacing (ft). Use bending moment and deflection formulas for simply supported or continuous spans to check adequacy of a candidate section.

Key checks include: allowable bending stress (Fy or allowable stress for cold-formed), shear capacity, local buckling, and L/Δ deflection limits (common limit L/240 for live loads or L/120 for total load depending on code). Deflection often governs purlin selection more than pure moment capacity.

Material, Coating, And Durability Considerations

Cold-formed steel purlins are typically produced from ASTM A446, A570, or equivalent with yield strengths from 33 ksi to 50 ksi. Galvanized coatings (G60, G90) or prepainted galvanized (PVDF) provide corrosion resistance. Higher galvanization weight and paint systems extend service life, especially in coastal or industrial environments.

For aggressive environments, consider hot-dipped galvanized or stainless steel purlins and additional protective measures like sacrificial washers or isolation at dissimilar metal connections. Thermal bridging and condensation should be addressed at the roofing assembly level through insulation, vapor barriers, and underlayments.

Bracing, Fastening, And Connection Details

Proper lateral and torsional bracing prevents premature buckling of slender cold-formed purlins. Continuous bridging, diagonal braces, or roof decking with adequate diaphragm action provide stability. Connections to rafters or primary frames must transfer shear and minimize eccentricity to avoid induced moments.

Fastener selection and spacing are crucial for attaching metal roofing to purlins: self-drilling screws with neoprene washers are common, and pull-out capacities for screws into thin purlin flanges should be checked. Use recommended edge distances, staggered patterns, and avoid overdriving fasteners.

Installation Best Practices For Metal Roof Purlins

During erection, ensure purlins are placed at specified spacing and camber, and that alignment and level tolerances are maintained. Pre-marking lines and using temporary bracing during sheeting reduces installation errors. Check for field cuts that remove protective coatings and perform touch-up galvanizing or paint as needed.

Coordination with roofers and other trades avoids conflicts with rooftop equipment and ensures penetrations are properly flashed. Avoid walking on unsupported spans and follow manufacturer guidance for deck handling to prevent damage or excessive deflection during construction.

Cost Considerations And Sourcing Purlins

Cost drivers include section size, steel grade, coating, and quantity. Cold-formed purlins are economical for moderate spans; box section or heavier rolled shapes increase material costs but may reduce installation labor and need for bracing. Factor life-cycle cost—initial price versus maintenance and replacement—when selecting coatings and materials.

Buy directly from reputable roll-formers or distributors and confirm delivery lengths, tolerances, and mill certifications. Ask for load/span tables, certification of steel grade, and finish documentation before ordering to ensure compliance with design assumptions.

Frequently Asked Questions About Purlin Size For Metal Roof

How Is Purlin Spacing Chosen?

Purlin spacing is chosen based on roof sheet manufacturer requirements, load calculation, and target deflection. Tighter spacing reduces required section size but increases material cost. Typical spacings range from 2.0 to 6.0 feet depending on deck type and loads.

Can Purlins Be Used For Long Spans?

Yes, with heavier sections, box sections, or by using closer spacing and intermediate supports. For very long spans, designers often use built-up beams or primary rafters rather than cold-formed purlins alone.

Is A Structural Engineer Required?

Local codes often require engineered design for buildings above certain size or complexity. For most commercial metal roofs and any project with significant wind, snow, or long spans, a licensed structural engineer should verify purlin sizing and connections.

References And Resources

Reliable resources include manufacturer span tables, ASTM and AISI standards for cold-formed steel, and ASCE 7 for environmental loads. Reference these sources during design and consult local building codes for jurisdiction-specific requirements.

If additional help is needed, contacting a structural engineer or purlin manufacturer with project-specific plans, loads, and desired material/coating will yield the most accurate purlin selection and cost estimate.