Roof live load roof dead load x spacing x span of purlin 2.
Flat roof purlin spacing.
The spacing between rafters in a given building is part of the engineering calculations that determine the required structure of the roof.
Wider panels allow for greater spacing between rafters.
The tables below give maximum spans for rafters and beams.
End purlins and periphery of roof to be fixed every 2nd crest.
For heavy roofs or in areas with substantial snowfall or winds call your local building department for code requirements.
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The two deciding factors are spacing of trusses span of purlins and the max load purlins must handle.
For sake of discussion we will assume a minimal roof snow load of 20 psf pounds per square foot and a 4 12 roof slope.
With approved fasteners alone.
Of course how many trusses a purlin spans over must be considered as it alters their moment and shear diagrams.
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.
C3 fix side laps and miss 2 hit 1 miss 3 hit 1 miss 2 etc with approved fasteners alone.
End purlins and periphery of roof to be fixed every 2nd crest.
A metal roof must also allow for some overlap on each side of the panels which will reduce the effective width of the panel.
The figures are based on quality materials.
Joints between purlins next to one another should be staggered.
Note compliance with e2 as1 is dependent on purlin spacing and gauge of roofing.
To determine how far apart to set rafters measure the width of the metal panel and then the width of the first hill and valley of the panel.
Dead load will be the actual weight of the roof steel and the roof purlins.
In the valleys due to the angle typically produced at the valley and the use of transition flashing it is essential that the purlins are doubled in the valleys that is two purlins.
In the order shown above purlins laying flat use 2 20d ring shank nails per connection for trusses with minimal spacing hung purlins for wide span trusses lu26 hangers for 2 6 lu28 hangers for 2 8 etc and purlins on edge overlapped not butted for mid range to wide span trusses 8 or 9 spacings installed with 1 60d.
Doubling up in the valleys refer to the installation manual for the proper on center spacing and be sure that the purlin is attached to the roofing truss.
Other relevant factors include the type of wood being used for the rafters the size of the rafters and the load that the rafters need to support.
8 x section modulus of a 4 2 of 1 3125 x fb fiberstress in bending x 1 15 duration of load x 1 15 cr repetitive member factor as the purlins are spaced 24 o c.
Or less x 1 1 cfu the flat use factor for applying the load to the wide face of a member.
