How do I set a fence post in concrete?

To set a fence post in concrete, get your utility lines marked first, then dig a hole about three times the post's width and a third of its length deep, plus 6 inches. Put gravel in the bottom, brace the post plumb, pour fast-setting concrete in dry, add water, and finish the top few inches with soil. Most of the work is in the layout and the digging; the concrete itself takes minutes.

Call Virginia 811 before you dig

Virginia law says no person shall begin any excavation without first submitting a locate request to the notification center,^[1] which is Virginia 811. A post hole counts. After you submit the request, you may start only after you have confirmed, on the center's positive response system, that each utility has marked its lines, said it has none in the area, or otherwise posted a response that digging may begin.^[1] Virginia 811 asks you to contact it three days before any digging project, fences included, and the service is free.^[2] A request stays valid for 15 working days,^[1] so plan to dig within that window.

The marks show the approximate location of each line. Keep post holes well away from them, and if a hole has to land near a mark, move the post or change the spacing rather than dig on top of a line.

Choose posts that can live in the ground

Posts should be long enough to cover the fence height, the setting depth and another 6 inches.^[5] Buy lumber treated for ground contact, not the above-ground grade sold for deck boards: in the treating industry's system, UC4 is the use category for wood used in ground contact in exterior applications.^[6] The tag stapled to the end of each post usually names the use. QUIKRETE's own list calls for a post that is rot-resistant wood, rust-resistant metal or an appropriate synthetic.^[4]

If you cut a treated post, the fresh end is less protected than the factory-treated surface. The Forest Products Laboratory says the damage can be partly made up with a thorough coat of copper naphthenate (1% to 2% copper) on the cut surface.^[6] Put the factory end in the ground and the cut end on top, under a post cap.

Dig the right hole

QUIKRETE's project sheet says to make the hole about three times the post's diameter, and as deep as one-third of the post's overall length plus 6 inches for a gravel base.^[3] QUIKRETE's web guide gives a slightly different rule, one-third to one-half of the above-ground length plus 6 inches.^[4] Corner, end and gate posts take the most strain, so err toward the deeper figure for those.

Soil changes the hole too. For loose or sandy soil it recommends a tube form,^[4] and the project sheet says to dig a larger, dish-shaped hole for posts set in loose or sandy soil.^[3]

A clamshell post-hole digger works for a few holes in soft ground. A long digging bar breaks up clay and lifts out rocks. Keep the sides straight so the concrete collar is even all the way down.

Set, pour and cure

Tamp the sides and bottom of the hole, then add about 6 inches of all-purpose gravel or crushed stone to help drainage.^[3] Stand the post on the gravel.

Bracing is what keeps the post straight while the concrete sets. Attach angled 2x4 braces to two adjacent sides of the post with one screw each, drive a stake near the lower end of each brace, check plumb on both of those sides with a level, and fasten the braces to the stakes.^[4] On a long run, set the end posts first, stretch a string between them, and line up every post in between to the string.

With fast-setting mix, pour the dry mix straight from the bag until it is 3 to 4 inches below the top of the hole,^[3] check plumb again, and pour water over it: about 1 gallon of water for each 50-pound bag, depending on the soil.^[3] For holes deeper than 2-1/2 feet, fill to 2-1/2 feet or less, add water and let it soak all the way in before adding the rest of the mix and water.^[3]

QUIKRETE's fast-setting mix sets in 20 to 40 minutes, and you should wait 4 hours before putting a heavy load on the post.^[3] Below 72 degrees, it needs extra curing time.^[3] On a cold autumn day in the Blue Ridge, leave the braces on longer than the bag suggests.

Finish by filling the last few inches with soil or sod. QUIKRETE suggests the gravel base and sloping the finished top so water runs away from the post^[4] as ways to protect a wood post from rot. If you are building a slab or footing rather than a fence, how to pour a small concrete slab covers that work.

Pulling out an old fence first

Old pressure-treated posts may be the older arsenic-based kind. In December 2003, makers of chromated arsenical (CCA) treated wood stopped making it for homeowner uses.^[7] If your fence is older than that, EPA advises goggles and a dust mask when sawing CCA-treated wood, washing your hands after handling it, and never burning treated wood at home.^[7] Old concrete collars usually come out with the post; a digging bar and a lever under the post help.

Stop and call a pro if

• A utility mark, a buried cable or an unmarked pipe turns up where a post has to go.
• A post hole hits a large rock, a tree root bigger than your wrist, or a slab you cannot dig through by hand.
• The fence will be a swimming pool barrier. The state building code exempts fences from a permit except fences required for pedestrian safety or used as a swimming pool barrier,^[8] so that fence is permitted work.
• The posts will hold up a gate wider than a walk gate, a deck, a roof or anything else that carries weight beyond the fence itself.
• The fence line is on a steep slope, near a retaining wall, or you are unsure where the property line is.

For costs and rules, read what a fence costs in Virginia, whether you need a permit for a fence and why new fence posts lean. Browse fence installers, all exterior answers, or exterior businesses around Blacksburg. If you would rather have it built, describe the fence job once and local businesses send written quotes.