Stand under the truss and follow the weight down
In a stick built garage the roof load runs into a top plate, down through the studs, and lands as an even line along a wall. In a post frame building there is no wall doing that. The trusses land on posts, and those posts are the only thing between the roof and the ground.
So everything the building weighs, plus whatever snow is sitting on it that week, gathers at a handful of points instead of spreading along a line. That is why the pad in the bottom of a post hole matters more than people expect. It takes a concentrated load and hands it to the dirt over a much wider area than the end of a post could. Too small a pad on soft ground and the post walks down into the soil, slowly, and it does not go down evenly.
It is the same idea as a pier under a deck or a porch. One point of load, one pad of concrete carrying it to ground that will hold it.
What actually goes in the bottom of the hole
The hole gets augered to the depth on the plan, which has to reach below the line where the ground freezes for your address and elevation, and it has to land on firm undisturbed dirt rather than on backfill somebody pushed in last week. Then the pad goes in the bottom and the post sets on it.
The ground around here decides a lot of this. Red clay over rock is common, clay holds water and swells, and rock shows up shallower than anybody planned for. Both are far cheaper to deal with while the auger and the excavator are still on site than after a row of posts is standing.
Uplift is the other half, and it is easy to forget on a barn. A post frame building is large, light and mostly roof, and wind pulls up on it about as hard as gravity pushes down. Whatever your plan calls for to hold the post down against that gets placed while the hole is still open.
- Hole augered to the depth the plan gives, below the line where the ground freezes
- Bottom cleaned out and bearing on firm undisturbed soil, not on loose spoil or fill
- A concrete pad placed in the bottom, sized off the load that post carries
- Whatever hold down detail the plan schedules, set before anything gets backfilled
- Backfill around the post brought up in lifts with real compaction, not dumped in and stomped
The floor slab inside is a separate thing entirely
People assume the concrete floor in a pole barn is part of the foundation. It is not. The posts and their footers are already carrying the building before a floor goes anywhere, and the slab on grade poured inside is a floor. Something to park on, work on and sweep.
That is also why the floor wants to be free to move on its own rather than tied hard to the posts. Concrete shrinks a little as it cures, and the ground under a floor moves for its own reasons on its own schedule. An isolation joint around each post keeps the two from fighting each other. A slab poured tight against a post cracks at the post, and that is the first place to look when somebody says their barn floor cracked.
The useful consequence is that the floor can wait. Plenty of pole barns go up as a shell first and get the floor poured later, under a roof and out of the weather, once the building is done being a construction site.
What we pour on a post frame job, and what your builder keeps
On a pole building we are the concrete side of it. The footers in the bottom of the post holes, and the slab when the time comes for it. Your builder sets the posts, frames the building, and owns the plan and the inspections.
Post holes are also a good example of why we bring the pump. A row of holes across a field, a barn sited past a gate at the back of the property, or a pad built up on a slope are all places a loaded ready mix truck has no business being. We run our own trailer mounted line pump on 2.5 inch and 3 inch line, with our operator at the machine and a hose guy at the hole, so the truck stays out on ground that will hold it. Most crews rent a pump when a job calls for one. Ours is our own, so on a job we are pouring the placement is not a second company's calendar sitting between you and your date, and when another contractor is putting up the barn we do the pumping for them.
One scheduling item belongs on the calendar rather than in a surprise. Post footers under a barn are concrete that supports a structure, so the work needs a permit and a footing inspection before concrete goes in the holes. That is standard code administration, and our Virginia permit guide covers what your county will want to know.