Purlin and girt spacing calculator

Purlin spacing for a metal roof: the table

Purlin size and spacing in inches on center, on edge, for each truss spacing and ground snow load the designer accepts. Each cell is the first size, at the widest spacing, that stays at 95% of its bending or deflection limit or less:

How each cell is worked out

The purlin is a beam spanning from truss to truss. The designer loads it with the ground snow you enter plus 4 psf for the steel and purlins (the roof dead load in the Cass County, Missouri standard), checks it in bending and in deflection at span over 180, and reads the span each size can carry at each spacing. If the purlin would use more than 95% of the limit that governs, it moves closer, from 24 in to 19.2 in and then 16 in, before it goes up a size. On 10 ft trusses at 30 psf that gives 2x6 at 24 in; on 8 ft trusses at 70 psf, 2x8 at 24 in.

What the county guides say

The prescriptive guides agree on the starting point. Clermont County, Ohio: 2x4 purlins laid flat for spans up to 4 ft, on edge up to 8 ft, and never more than 24 in apart. Larimer County, Colorado draws continuous 2x4 purlins on edge at 24 in over trusses 8 ft apart. The Colorado ICC chapter guide asks for purlins on edge with roll blocking when the trusses are more than 4 ft apart, and shows 2x6 at 24 in. Cass County, Missouri tabulates spans at 24 in for spruce-pine-fir: 2x4 to 8 ft, 2x6 to 12 ft, 2x8 to 15 ft, for its 20 psf snow load. Those tables suit light snow; the designer's check is what moves the purlins at heavier loads.

Girt spacing for the walls

Girts are the horizontal boards on the outside of the posts that carry the wall steel. The designer lays them flat at 24 in on center: 2x4 on bays up to 8 ft, 2x6 on bays up to 10 ft. Clermont County asks for girts no smaller than 2x4 and no more than 24 in apart; the Colorado guides show 2x6 at 24 in; the Cass County table allows 2x6 at 36 in on 10 and 12 ft spans. At the bottom, a 2x6 pressure-treated skirt board on edge at grade, the splash board the Colorado ICC guide asks for.

Girts and the steel panel

The steel sets an upper limit too: Union Corrugating rates its MasterRib panel for roof purlins 2 ft apart at most and sidewall girts 3 ft apart at most. Girts at 24 in sit inside that limit and inside the Clermont County rule.

Purlins and trusses together

The purlin span is the truss spacing, and the truss spacing is the post spacing: post-frame trusses are commonly 4 or 8 ft on center, the Post-Frame Building Handbook (NRAES-1) notes, against 2 ft in house framing. Read how to build a pole barn for the order the frame goes up in, and the post spacing and depth calculator for the posts under them. The cost calculator prices the purlins and girts with everything else.