Pole barn post spacing and depth calculator

Pole barn post spacing: why 8 ft

Eight feet on center is the spacing the county post-frame guides are written around. The Clermont County, Ohio guide (RCO 328) says it outright: the maximum spacing for posts is 8 ft. Larimer County, Colorado draws 6x6 posts 8 ft apart for walls up to 14 ft, and the Colorado ICC chapter guide leaves a blank for the post spacing with "(8' max.)" printed beside it. The National Frame Building Association calls 8 ft posts a pretty typical post-frame design standard.

The spacing of the posts is the spacing of the trusses, because each truss sits on a pair of posts: Clark County, Ohio requires truss spacing to line up with the posts or a girder to carry it. So a 40 ft barn on 8 ft bays has 5 bays, 6 post lines and 6 trusses. Wider bays mean fewer posts and holes, but heavier purlins and girts; narrower bays the reverse.

Endwall posts

Across the endwalls the designer keeps posts no more than 8 ft apart too, and adds a jamb post on each side of every overhead or sliding door. Those door posts carry the header, so the endwall with the big door usually has more posts than the back wall.

When 6 or 10 ft bays make sense

Some lengths do not divide into 8 ft: 30, 50 and 60 ft barns frame on 6 ft or 10 ft bays. Six feet passes every check with lighter purlins (2x4 on edge at 30 psf of snow). Ten feet needs trusses designed for that spacing, 2x6 girts and heavier purlins, and the designer flags it for review, because the prescriptive guides stop at 8 ft.

How many posts for common sizes

Every free design, with its posts and footings for a 36 in frost line:

How deep pole barn posts go

The designer sets the depth to the deeper of 48 in and your frost line, rounded up to the next 6 in, then adds 24 in for posts on an open side. The 48 in floor appears in three separate county documents: the Colorado ICC guide ("piers must be at least 48 inches in depth, or the plan must be engineered"), Alexander County, North Carolina, and Cass County, Missouri, which also never allows less than 30 in. Cass County is the source of the extra 2 ft on an open side, where wind pushes the posts with no wall to brace them.

Hole depth is post depth plus the pad

The post stands on a concrete pad poured at the bottom of the hole, so the hole goes deeper than the post by the pad's thickness. For the 30x40 that is 4' of post in the ground on a pad 24 in across, 12 in thick: dig about 5'.

Frost line and wider buildings

Ask your building department for the frost depth; it is the number the inspector checks. Tall walls and high wind can need deeper posts than the frost line: the Post-Frame Building Handbook (NRAES-1) tabulates embedment by wind speed, bay, eave and soil, and some of its clay rows have no prescriptive answer at all. The designer asks for a review past a 12 ft eave or 115 mph for the same reason: the prescriptive guides stop there.

Footings, collars and concrete

Each post sits on a round pad sized by the building width: 18 in across up to 24 ft wide, 24 in across up to 36 ft wide, 28 in across up to 40 ft wide. The pads are at least 8 in thick and at least half their diameter. Around the bottom of the post goes a 12 in concrete collar with two #5 bars 9 in long through the post, the uplift anchor in the Clermont County guide; two treated 2x6 cleats 12 in long are the alternative it allows. The concrete is 2,500 psi or stronger, the strength the NFBA guide specification asks for in post foundations.

Treated posts

Every post below grade is pressure-treated to AWPA Use Category UC4B, ground contact heavy duty, the category AWPA lists for sawn building poles and the NFBA specification requires within 8 in of grade.

Post size: 6x6 or laminated

The post follows the wall height and the bay. The designer uses 6x6 post up to a 10 ft eave on bays up to 8 ft; 3-ply 2x6 post, nail-laminated up to a 12 ft eave on bays up to 10 ft; 3-ply 2x8 post, nail-laminated up to a 16 ft eave on bays up to 10 ft. Larimer County accepts unspliced 3-ply 2x6 laminated posts; where a ply has to be spliced, the plan staggers the splices and says so, and Larimer asks for spliced laminated posts to be engineered.

Next steps: the purlin and girt spacing calculator for the roof and walls, how to build a pole barn, and the cost calculator for the whole list. Or start from one of the free pole barn plans.