· Michael Blair
Fences fail at the posts. Middle Tennessee clay expands when wet and shrinks when dry, and a post that is not deep enough or not drained gets levered a little further out of plumb every season.
Depth and the frost line
A post should be set below the frost line and deep enough that the buried length resists overturning — commonly a third of the above-ground height as a rule of thumb, and deeper for gate and corner posts.
Posts set shallow because the digging was hard are the most common cause of leaning fences here, and it shows up within a few seasons.
The concrete collar problem
Concrete poured in a bell shape wider at the top than the bottom gives clay something to grip and lift. The collar should be widest at the base and sloped away at the top so water sheds.
A post set in concrete with no drainage beneath sits in a bucket. A few inches of gravel at the bottom of the hole lets water leave rather than pool against end grain.
- Below frost line, roughly a third of height buried
- Gravel at the base for drainage
- Concrete widest at the bottom, sloped at the top
- Corner, end and gate posts set deeper and larger
- Bracing held until the concrete has cured
Gates are their own problem
A gate post carries a swinging cantilevered load that constantly tries to pull it out of plumb, and gates sag as a result more often than any other part of a fence.
The fix is a larger, deeper gate post, a diagonal brace in the gate itself running from the bottom hinge corner upward, and hinges bolted through rather than screwed.
Resetting versus replacing
A sound post that has simply leaned can be reset. A post that has rotted at grade cannot, and if one has rotted its neighbours of the same age are usually close behind.
Check by probing at ground level with a screwdriver. Soft wood there means the post is finished regardless of how good it looks above.