How to Install Deck Support Posts

Deck posts are the skeleton of your entire structure. They carry every ounce of weight—the boards, the railing, the people standing on it—straight down into the earth. A post installed wrong won't just fail quietly; it'll leave your deck sagging, cracked, and unsafe. The work itself is straightforward: dig below frost line, pour concrete footings, set the posts plumb, and bolt them down. But getting the depth, the spacing, and the leveling right separates a deck that lasts twenty years from one that's a liability in five.

1. Mark Posts Perfectly Straight. Use your deck plans to determine exact post locations. Measure from the house rim or ledger board and mark the ground with chalk or spray paint. For most decks, posts should be no more than 8 feet apart along the beam line. Double-check measurements twice—moving a post after digging wastes time and concrete.
2. Know Your Frost Line. Call your local building department or check online frost line maps for your region. Most northern zones require 36 to 48 inches; southern areas may only need 12 to 24 inches. Frost heave—the upward pressure from freezing soil—will push shallow posts up and crack your deck every winter. This isn't optional.
3. Dig Deep Enough. Rent a power auger or dig by hand with a posthole digger. Dig to at least 4 inches below the frost line—so if frost line is 48 inches, dig 52 inches deep. Make the hole 10 to 12 inches in diameter, wider than the post base. Clean out loose soil and debris from the bottom.
4. Set Level Concrete Footings. Drop 4 inches of gravel into the bottom of each hole for drainage. Mix concrete according to bag instructions (usually 1 part mix to water) and pour into holes until you reach ground level or slightly above. Use a level to ensure the top of the footing is flat. If your deck is on sloped ground, aim for the tops to be at similar heights. Let concrete cure for at least 48 hours before setting posts.
5. Install Hardware First. Once concrete is cured, bolt an adjustable post base (also called a post bracket or pier bracket) to the center of each footing using anchor bolts and washers. These bases allow fine-tuning of post height and keep wood off the wet concrete, stopping rot before it starts. Tighten bolts snug but don't strip the hardware.
6. Check Plumb Twice. Cut pressure-treated posts to length, accounting for the rim joist connection at the top. Drop the post into the adjustable base and use shims or the base's adjustment bolts to bring it perfectly plumb—check with a level on two sides. Secure the post to the base with lag bolts or structural screws through the pre-drilled holes.
7. Align All Posts Level. Once all posts are set and bolted, run a level or string line along the tops to verify they're all at the same height. Temporary diagonal braces (2x4s nailed at 45 degrees) will keep posts plumb while you work on the rest of the frame. Do not remove braces until the beam is bolted and the deck structure is fully tied together.
8. Seal Every Wood Surface. Apply wood sealant or a post cap to the top of each post where it will connect to the rim joist. Seal the base where it meets the post base to prevent water from wicking up the grain. This small step pays dividends by keeping rot at bay for years.