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Calculate optimal roof purlin spacing, size requirements, deflection, and load distribution. Support for steel C-sections and timber purlins with comprehensive load analysis.
Rafter length between walls
Distance between purlin centerlines
Roof weight
Snow/construction
Wind pressure
| Purlin Type | Size | Light Load (psf) | Medium Load (psf) | Heavy Load (psf) | Max Spacing (ft) |
|---|---|---|---|---|---|
| Steel C-Section | C6x8.2 | 20-30 | 30-50 | 50+ | 4-6 |
| Steel C-Section | C8x11.5 | 20-30 | 30-50 | 50+ | 6-10 |
| Steel C-Section | C10x15 | 20-30 | 30-50 | 50+ | 8-12 |
| Timber | 2x4 | 15-25 | 25-35 | 35+ | 4-5 |
| Timber | 2x6 | 15-25 | 25-35 | 35+ | 6-8 |
| Timber | 2x8 | 15-25 | 25-35 | 35+ | 8-12 |
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Purlins run horizontally across the slope of the roof and rest on the main rafters or trusses. Rafters run from the ridge to the eaves and form the primary roof structure. In modern construction, purlins are secondary members that provide intermediate support for roofing material, while in industrial buildings, purlins carry the primary roof loads with fewer main members.
Standard spacing for metal roofing is typically 4-6 feet for residential buildings and 6-12 feet for industrial applications. The exact spacing depends on purlin size, material, roof pitch, and load requirements. Metal roofing is lighter than tiles or heavy materials, allowing wider spacing. Always check local building codes and engineer specifications.
Sag rods are critical for steel purlins, especially with long spans or high winds. They prevent the purlin web from buckling and lateral movement. Timber purlins on short spans (under 6 feet) may not need sag rods, but it's recommended for spans over 6 feet. Check with structural engineer and local codes.
Deflection is calculated using the formula: δ = (5 × w × L⁴) / (384 × E × I), where w is load per unit length, L is span in inches, E is modulus of elasticity, and I is moment of inertia. Maximum deflection typically should not exceed L/240 to L/180 depending on the roofing material and building codes.
Purlins are structural members supporting the roof. Pugging is the process of filling gaps between purlins and rafters or joists. Pugging materials include concrete, foam, or insulation to prevent drafts and provide thermal continuity. They work together in many building designs.
Mixing materials is not recommended unless engineered specifically. Steel and timber have different expansion properties, strength characteristics, and fastening requirements. If you must mix materials, ensure proper transition details and seek professional engineering guidance.
Fastening methods depend on purlin and support material. Steel purlins are typically bolted with ½" or ¾" bolts through welded or rolled cleats, or directly welded to steel beams. Timber purlins are usually nailed with large galvanized spikes or bolted through saddle brackets. All connections should be designed to resist shear and moment forces.
Steel C-section purlins typically cost $0.50-$2.00 per pound, resulting in $200-800 per member for common sizes. Timber purlins cost $2-10 per linear foot. Final cost depends on material market prices, shipping, fabrication, and installation complexity. Request quotes from structural suppliers for accurate pricing.