Concrete deck blocks: a complete buyer’s guide

What are concrete deck blocks used for?

A concrete deck block is a precast pier that supports the posts and beams of a light structure without dug and poured footings. You level the ground, set the blocks, and build on top. That makes them the go-to base for free-standing, ground-level decks, and they are just as common under sheds, gazebos, walkways, ramps, stairs, hot tub platforms, and garden structures.

The top of the block is notched. Our deck block has a 4-way notched top, which means it accepts a 4x4 post standing vertically or standard 2x dimensional lumber laid on edge in either direction, so beams and joists drop in and sit securely without brackets or hardware.

Deck blocks shine where the structure is free-standing and close to the ground. Once a deck attaches to a house, gains height, or carries serious load, local building codes usually take over and may require footings, which is covered below.

How many deck blocks do I need?

The honest answer is that it depends on your framing plan, not your deck area alone. The working rule: a block belongs under every post location, which in practice means under each end of every beam and every 4 to 6 feet along it. Spacing depends on the size of your beams and joists, the load, and the ground. Closer spacing lets you use smaller lumber; wider spacing demands bigger beams.

A worked example: a 10 by 10 foot free-standing deck framed with three beams typically sits on a 3 by 3 grid, so 9 blocks. A 12 by 16 deck framed with three beams spanning the 16 foot direction, supported every 5 to 6 feet, needs 4 blocks per beam, so 12 blocks. When in doubt, add a block rather than stretch a span: blocks are the cheapest part of the deck.

Soft or disturbed ground needs more support than firm, undisturbed ground. Set every block on soil that has been tamped firm, or on a bed of compacted gravel, never on topsoil, and check that the tops are level with each other before framing.

Deck block sizes and dimensions

Our deck block is a 14 inch, 4-way notched block, cast in the same dense, freeze-thaw rated mix as the rest of our precast line. The wide 14 inch base matters more than it looks: the bigger the footprint, the more the block spreads the load, and the less it settles into the ground over the seasons.

For heavier point loads, or where you want the support buried flush with grade, a solid pier block is the stronger sibling. Ours come in 12 by 12 inch footprints at 8 or 12 inches tall. Many builds mix the two: pier blocks under the heaviest bearing points, deck blocks under the rest.

Deck blocks vs. poured footings

Poured footings win on one thing, and it is an important thing: depth. A poured footing extends below the frost line, so frost heave cannot move it. Building codes generally require frost-depth footings for any deck attached to a house, because the house does not move with the frost and an attached deck must not either.

Deck blocks win on everything else for the builds they suit: no digging, no forms, no concrete truck or mixing, no waiting for cure, and no inspection of a hole. A free-standing deck on blocks can ride the seasons as a unit; if a corner moves slightly with frost, it settles back and nothing tears.

The decision is simpler than it looks. Attached to the house, or elevated: poured footings, and check your local code. Free-standing and ground-level: deck blocks are usually all you need, and they leave the option of moving or extending the deck later. Your local building office has the final word, and a five minute call beats a redo.

Precast blocks vs. lightweight big-box blocks

Big-box stores sell lightweight deck blocks, often foam-filled or cast in a lighter mix so they are easier to carry to the checkout. Light is convenient in the parking lot, and a drawback under the deck: a lighter, smaller block bears less, settles more readily, and can degrade faster through freeze-thaw cycles.

A dense precast block is heavier to place once, and then that mass works for you for decades. Our blocks are cast in the same freeze-thaw rated concrete we pour for architectural facades, dense enough to shrug off the wet-freeze cycling that eats porous concrete, with a footprint that spreads load properly. Under a structure you plan to keep, the few extra pounds per block are the cheapest durability you can buy.