Pole Barn Truss Spacing: How Far Apart They Go

Pole barn trusses are spaced the same distance as the posts they sit on, usually 8 ft apart, and 10 ft is the widest bay our designs use. A post-frame building puts a truss on every pair of posts, so the post layout and the truss layout are the same decision.

For a 30x40 with a 4/12 roof, 8 ft bays give 6 trusses and 19 posts. The truss manufacturer designs the trusses for your span, pitch, spacing and snow load, often with an engineer’s seal on the drawing, and that drawing wins over anything in a general guide (NFBA¹, Frame Builder, 2022). What follows is how to choose the spacing before you send the order.

Truss Spacing Follows the Post Spacing

The bay is the distance between two posts along the sidewall, and a truss lands on each post pair. Our barn designer offers 6, 8 and 10 ft bays. Eight feet is the standard post-frame bay, and 10 ft is the most we draw, with trusses and girts sized for it. The barn length is the number of bays times the bay: a 40 ft barn is five 8 ft bays or four 10 ft bays.

All three at 30 psf of ground snow, a 10 ft eave and a 4/12 roof. The 6 ft row is a 30x60 because 40 ft does not divide into 6 ft bays, and the designer only draws whole bays. For the truss count of every size, see the table at the end.

What Spacing Does to Purlins, Girts and Posts

Purlins run across the trusses to carry the roof steel, on edge and 24 in on center in our plans. They span from truss to truss, so wider spacing means a longer span and a bigger purlin, and the same goes for the wall girts and the posts that carry each truss. In the table above, the purlin grows from 2x4 to 2x6 between 6 and 8 ft, and at 10 ft the girts and posts grow too. The designer picks each purlin so it uses at most 95% of its bending or deflection limit at your snow load; past that, the purlins move closer or go up a size.

Snow pushes the same way. At 70 psf of ground snow the 30x40 on 8 ft bays goes up to 2x8 on edge at 24 in. The purlin spacing calculator shows the purlin for any truss spacing and snow load.

Pole Barn Trusses: Span, Count and Cost

One truss spans the full width, so a 30 ft wide barn takes 30 ft trusses. The count is one per post pair: the number of bays plus one, with a gable end truss at each closed endwall. Wider spacing buys fewer trusses and fewer posts, and gives most of that back in bigger members, so the saving is small. For the 30x40, materials come to $14,078 on 8 ft bays and $13,910 on 10 ft bays.

The trusses themselves are priced by span, not by spacing, in the published range we use: $120 to $330 each at 30 ft (HomeGuide², Jan 2026). The cost guide breaks the 30x40 down, and the cost calculator does it for your own size. Treat the truss line as an estimate until the truss maker quotes you.

Who Decides the Final Spacing

You pick a spacing to draw the barn. The truss manufacturer confirms it and designs the trusses for it, often with an engineer’s seal on the drawing; ask for the seal if your building department needs one, as many do at inspection (Larimer County³, Colorado asks for the stamped truss design). To order:

1. Draw the barn

   Pick the width, the bays and the eave in the designer, for your snow, wind and frost line.

2. Send the order sheet

   Our plan includes a truss order sheet with the span, pitch, heel, overhang, spacing, loads and counts, so the quote is for the barn you drew.

3. Ask for three quotes

   One at your spacing and one step either side, before you commit.

4. Match the posts

   Lay out the posts to the spacing on the truss drawing, not the other way round.

Truss Count for Each of Our Designs

Any of these opens in the designer with the spacing, the span and the truss order sheet. Change the size or the bay and the count updates. Next: how to build a pole barn for where the trusses go in the build order, or the post spacing and depth calculator for the posts they sit on.