Build a Privacy Fence With Posts That Stay Plumb for Decades
Fences fail at the posts, and posts fail because of what happened in the hole. Set them right and a $3,000 fence lasts 25 years instead of leaning by year five.
Walk any neighborhood and you can date the fences by their lean. The straight ones and the leaning ones were built from the same lumber, usually the same way above ground. The difference is underground: how deep the posts went, what they were set in, and whether the builder let the concrete cure before hanging a hundred pounds of pickets on each one. This is a project where 80 percent of the work is digging and setting, and the pickets at the end are the reward.
Before you dig
Find your property pins with a metal detector and the plat, and build a few inches inside the line. Call 811 several business days ahead to have utilities marked; hitting gas or fiber turns a weekend into a very bad month. Check zoning — most towns cap backyard fences at 6 feet and front yards at 4, many require a permit above a set height, and some dictate that the finished side faces the neighbor. HOAs add rules on material and color.
Lay out with string, not a tape on the ground
Drive stakes past both ends of each run and stretch mason’s line at post-top height, 2 inches off the fence face. Check corners with the 3-4-5 method. Mark post centers every 8 feet on center — pickets, rails, and panels are all built around that number, and saving one post by going to 7 feet 10 inches costs an afternoon of recutting. Drop a plumb bob and paint a dot for each hole.
Dig deep and bell the bottom
A post needs a third of its length below grade: for a 6-foot fence on 8-foot posts, 30 to 36 inches, and always below frost line. Hole diameter should be three times the post width, so 10 to 12 inches for a 4×4. Clean the bottom with a clamshell digger and make the hole slightly wider at the bottom than the top; a footing that widens downward can’t be gripped and lifted by frost.
Set on gravel, crown the concrete
Tamp 4 to 6 inches of crushed gravel in the bottom so water drains away from the post end — a post standing in a concrete cup rots from the bottom up. Stand the post, brace it plumb both ways with scrap 2x4s to stakes, and check it against the string. Pour and mix the concrete, fill to just above grade, and trowel the top into a dome that sheds water away from the wood.
Wait, then cut the tops in a line
Fast-setting mix holds a post in 40 minutes but isn’t at working strength for a day, and hanging panels on a green footing is how posts end up leaning at exactly the angle of the first bay you pushed against. Leave the braces on overnight. Next day, snap a line at finished height and cut every post top with a circular saw. Cutting after setting is the only way to get tops that line up, because holes never dig to identical depth.
Rack or step across the slope
Where the ground drops less than a foot over an 8-foot bay, rack the fence: rails parallel to grade, each picket hung plumb, so it follows the ground like a ribbon. Prefab panels won’t rack more than a few inches, so build racked runs stick by stick. On steeper ground, step it: every rail level, each bay dropped a set amount, with a kickboard filling the triangle at the bottom. Pick one method per side; mixing them looks like a mistake.
Hang rails, then pickets, off a string
Three 2×4 rails per bay — 6 inches off grade, 6 inches below the post top, and one centered — with two 3-inch structural screws at each post, set between the posts rather than face-mounted. Run a string along the top for picket height and hang pickets with two screws per rail, a nickel’s width apart for cedar to move. Level every fifth picket; drift compounds fast. Cap the posts and stain after the wood has dried a few weeks.
Never dig before the utility locate is complete, and hand-dig within 2 feet of any paint marks — marked depth is never guaranteed. A gas auger in rocky or rooted soil can throw you; keep both hands on it and stop the engine before clearing the bit. And a gate is a lever: set its posts deeper and in more concrete than their neighbors, or tie the two gate posts together below grade.
A fence is only as straight as the concrete it was leaning on when you hung the first panel.
Making it last
Stain or seal within a season and again every three or four years; a fence that stays dry stays straight. Keep sprinklers off the bottom rail and trim grass so pickets don’t sit in wet mulch. Once a year, push on each post — a wobble at the rail is a loose screw, while movement at the ground wants a concrete collar before it becomes a lean. Posts set on gravel, crowned, and cured before loading routinely outlive two sets of pickets.
Keep the momentum going
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