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.

Flat roof purlin spacing.

The figures are based on quality materials. End purlins and periphery of roof to be fixed every 2nd crest. 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. The tables below give maximum spans for rafters and beams.

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. Note compliance with e2 as1 is dependent on purlin spacing and gauge of roofing. Of course how many trusses a purlin spans over must be considered as it alters their moment and shear diagrams. With approved fasteners alone.

Joints between purlins next to one another should be staggered. C3 fix side laps and miss 2 hit 1 miss 3 hit 1 miss 2 etc with approved fasteners alone. Wider panels allow for greater spacing between rafters. 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.

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. 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. 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. Or less x 1 1 cfu the flat use factor for applying the load to the wide face of a member.

The two deciding factors are spacing of trusses span of purlins and the max load purlins must handle. When constructing a roof framework the rule of thumb is that any purlin should have a minimum nominal depth and width of 76 mm or 50 mm and max centre to centre spacing between the purlins ought to be 1 2 m. You can solve them as simpl. A metal roof must also allow for some overlap on each side of the panels which will reduce the effective width of the panel.

But the tables that follow are a lot more specific. 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. Dead load will be the actual weight of the roof steel and the roof purlins. Roof live load roof dead load x spacing x span of purlin 2.

In areas with mild climates patio roofs are generally designed for loads of 30 psf pounds per square foot.