· John Roth
How Deep Should Fence Posts Be Set in Middle Tennessee?
People ask me about the "frost line" a lot. Here in Middle Tennessee, frost is not what decides your post depth. Leverage, wind and our clay are.
The frost line around here is shallow
- Nashville (Metro): the building code's frost depth is 12 inches (Metro Code §16.08.014).
- Mt. Juliet: the city code sets it at 6 inches (City Code §8-20).
Those numbers are for house footings. Fences are generally exempt from building permits in Mt. Juliet up to 7 feet, but any post set right is well below the frost line anyway.
The rule we build by
A post holds a fence up like a lever. The deeper it goes, the less the wind can rock it. Around here, we set our posts about 2 feet in the ground, in concrete:
| Fence height | Post we use | In the ground |
|---|---|---|
| 4 ft | 4x4x8 | about 2 ft (trimmed to height) |
| 5 ft | 4x4x8 | about 2 ft (trimmed to height) |
| 6 ft privacy | 4x4x8 | about 2 ft, in concrete |
| Gates | 6x6 or steel (wood fences); heavier on wider gates | about 2 ft; big, heavy gates up to 4 ft, in more concrete |
A privacy fence acts like a sail, so it catches the most wind. That's why our posts go in concrete, and why gate posts get bigger posts and more concrete: they carry the weight of a swinging gate every day.
Gate posts: the one place I won't cut corners
A gate is the hardest-working part of any fence. Every time it swings, it pulls on its post. A post that's too light will lean, and then the gate sags and drags.
- Wood fences: on any gate over 4 feet wide, we always use a heavier post, a 6x6 or a steel post. I recommend it on 3- and 4-foot gates too.
- Aluminum fences: standard 3- and 4-foot gates are fine on the 0.125-wall post the supplier ships. For a single gate 5 feet wide or more, you want a 3- or 4-inch aluminum post with a 0.125- to 0.250-inch wall.
- Chain link: same idea. The bigger the gate, the bigger the post.
- Big, heavy gates: we go deeper than 2 feet, up to 4 feet when the ground allows, depending on the size of the gate. On something like a 1,500-pound gate, we dig as deep as we can possibly get.
What if we hit rock? Then we go as deep as the rock lets us. Depth matters less in solid rock than in soft dirt: once the concrete footing bonds to a solid rock formation, the whole formation is holding your post up. That's far stronger than the same depth in loose soil.
Not every contractor bids an upgraded gate post, so our estimate can look a little higher at first. It's the difference between a gate that still latches in ten years and one you're lifting to close.
Middle Tennessee soil is its own animal
- Clay holds water. A post sitting in a puddle of clay rots faster, so we keep the concrete crowned at the top to shed water.
- Rock is everywhere in Middle Tennessee, from Davidson and Robertson counties to Wilson, Smith and DeKalb. You can hit limestone 3 inches down in a yard where you'd never expect it. I wrote about how we handle that here: Setting fence posts in rock.
- PostSaver sleeves go on at the ground line, where wood posts rot first. It's cheap insurance on a pine fence.
Thinking about a new fence? We'll look at your ground on the estimate and tell you straight what we expect to find.