Less expensive (1/3rd less board footage of lumber), fewer pieces to handle, fewer joist hangers to have to attach and only one half as many screws to attach the roof steel. Rather than having 2×4 purlins every 12″ it would make far more sense economically to use 2×6 purlins every 24″. Without even running the calculations, I would say there is a good chance the 2×4 purlins will not make the deflection criteria required for a roof framing member supporting steel. In bending, 2×4 #2 purlins would work at 12″ on center, however I personally would not want to walk on top of them. They are typically 2 by 4 feet and installed in the same way as the metal roofing. Roof purlins are generally installed horizontally beneath the metal roofs and over roof rafters with a vapour barrier such as a felt underlayment on top. HemFir has a base Fb of 850 psi multiplied by the size factor of 1.5 = 1275 psi Typically, purlin sections take on the slope of the roof they are supporting. The COS of the purlin spacing is because the load on the purlins is vertical and the purlin spacing is with the run of the roof.įor sake of discussion we will assume a minimal roof snow load of 20 psf (pounds per square foot) and a 4/12 roof slope.ĭead load will be the actual weight of the roof steel and the roof purlins. The COS of the Live Load is due to us only having to check for bending about the strong axis of the member as the purlin is restrained in bending in the weak axis direction by the roof steel. Here is the formula for calculation of a roof purlin: ![]() (You can brush up on bending moments here: ) Spacing them on each side of the column causes the trusses to work independent from each other and takes away the advantages of the true double truss system (load sharing and minimization of truss bracing).Ĭould one use 2×4 roof purlins in this system? In this fashion they truly act as a two ply member. If the new panel cannot meet the required uplift pressures (PSF) for a 5' purlin spacing, then an additional Roof Hugger will be necessary between the existing building's purlins. My question is can I use 2” X 4” on edge spaced 16” or 12” apart? I intend to have a metal roof on top.ĭEAR DAVE: My preferred method of post frame construction actually places the two trusses face-to-face nailed together and notched into one side of the column. If the existing purlin spacing is 5' (typical in older buildings) you would compare the capacity of the new panel on 5' purlin spacing. That is exactly what I have, double trusses on each side of a post on 12’ centers. ![]() ![]() Reader DAVE in MICHIGAN writes: Hi, I saw on your webpage the Pole Barn Guru stated the trusses could be spaced 12’ apart (I called and was told it is a double truss one on each side of a post that is on 12’ centers).
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