It's too small, it might squeek in at a 6圆. I also changed the span in the calc to 140" then entered this info assuming the same species. The fail in deflection shows it would make for a bouncy floor but a passable roof board.Ĥ0square feet of roof bearing on a purlin X 40lbs per square foot= 1600lbs load bearing on each 4圆 purlin. I looked at it as a simple beam in flatways bending I'd call the adjusted Fb-1125psiĪssume a 6" wide board spanning the 40"=1.66 square feet X40psf=67lbs load Ive drawn many pole barns for around here (N MI) w/ trusses at 4oc & 2x4 purlins 2oc up the slope for sheet metal roofing. I would bump up the Fb by 15% for short term loading(snow), 15% for repetitive member use, and you could take 15% more for flat use but I don't think I would. Add 10psf for dead load so 40psf total load. Spanning Capability: Purlins contribute to the spanning capability of the roof by allowing you to double the maximum allowable single span length of a rafter. Your building department can give you the snow load for your area, I'm guessing it will be 30psf. Roughly then, 4' centers on an 8/12 with 4" wide purlins is going to give a clear horizontal span of about 40". The horizontal clear span from plumb to plumb would be the way the NDS measures the span. (basically take a hole saw, saw in about 1/2 inch, and put about a 7/8 inch long section of pipe/like in that hole saw cut).The codebook doesn't alter sheathing, rafters or purlins based on pitch. When positioned, roof purlins should be spaced no further than 1.2 metres apart when using sheeting with a thickness of 0.7mm, and 1 metre when using 0.5mm. Truss tails not properly trimmed around prior to wall steel installation. Inside closures omitted from on top of beveled edge purlin. Improper roof steel overhang past beveled fascia purlin. Beveled fascia purlins not installed behind fly rafters. High Pressure Zones KL 1.5 and 2.0 included. Failure to install fly rafters and beveled fascia purlins with crown up. By choosing the right purlin spacing with a right roof profile, stability in roof is achievable. What Are the Common Purlin Spacing for Corrugated Roofing The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft. The safe spans below are suitable for sheds and class 1 structures with the following assumptions: Non-cyclonic sites only. truss that supports the roof (purlins + roof). The C sections can also form part of the columns and rafters in these smaller applications. A single deck beam has a maximum span of 5 11 and a double span of 8 9 for a standard header. A metal roof’s furring strips are typically 20-24 inches apart. There is a maximum span of 16-4 for 2×6 spacing 12 and so you can use it as a guideline. Not saying your not pushing the modern limits of construction, but if you have enough 2圆's one could double up a few one on each side of the existing rafter, or as far as shear, if your concerned, could consider a split ring type of attachment, Figure 32: Purlin-to-principal rafter joint using metal corner braces. When lean-to rafter spacing is common, 16 and 24 are commonly used. If you space out your purlins by too much, the support they offer to the roof won’t be as sufficient and the structural integrity will be weakened overall. Both the support and the placement of purlins are crucial. I have buildings with 2x4 on 24 spanning 12 foot, slight sag after 90 years,Īnother building 2x4 24 on center spanning 10', no seeable sag, 70 years,īarn lean to sheds, 2圆 24 oc, 15 foot, span slight sag it is 105 years old, (all lumber is Douglas fir)Īll have approximately the same slope as your drawing, all of my buildings are 1x sheeted and have been shingled, with asphalt shingles, the barn is sheeted but tinned now, For example, tiles may come loose, or roof sheets may let water in if the purlin spacing is irregular.
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