How to build a ground-level deck

When a ground-level deck makes sense

Low decks are usually built freestanding: they stand on their own supports and are not bolted to the house, so there is no ledger to flash and no house wall to open up. Many building departments exempt a deck from a permit when it is under about 30 inches above grade, under 200 sq ft and not attached to the house. A 12x16 deck is 192 sq ft, just inside that size, but check your own rules in the deck permit guide before you dig.

The trade-off is frost. Deck blocks sit on the surface, above the frost line, so the ground can lift them a little in a hard winter. A freestanding deck rides that movement; an attached one would pull on the house. That is why blocks are only for low, detached decks. If you want the deck tied to the house, use real footings and read freestanding vs attached decks.

Lay out the blocks

For a 16 ft by 12 ft deck, the deck block calculator puts the blocks in 3 rows of 5: rows 6 ft apart across the 12 ft width, blocks 4 ft apart along each 16 ft row. That is 15 blocks.

The floating deck cost calculator prices a more conservative 4 ft grid (20 supports) for the same deck, so the costs below have a little room in them.

Frame it low

Set three beams, one in each row of blocks, in the block slots: doubled 2x8s or a single 4x6 for a 6 ft joist span. Run the joists across the beams at 16 inches on centre: 13 joists over 16 ft, the same count the deck joist calculator gives. With the beams sitting in the blocks, the top of the decking ends up roughly 12 to 14 inches above the ground, depending on the blocks and the boards.

• Use lumber rated for ground contact for everything below the decking. Air under a low deck moves slowly and the wood stays damp.
• Leave the ends of the frame open, or use vented skirting, so the space under it can dry.
• Tape the joist tops before the boards go on; the joist tape calculator counts the rolls.
• Where the yard falls away, put short 4x4 posts in the block pockets to level the frame, and keep them short, because a block does not brace a post the way a buried footing does.

The material list

What it costs

The floating deck cost calculator for a 16x12 deck on deck blocks:

In the DIY pressure-treated build, decking is $768-$1,344 and framing lumber $672-$1,056, so the blocks and gravel are a small share of the total. Switching to composite boards is the biggest single jump. For other sizes and finishes, the DIY deck cost calculator adds tools and the permit.

Build sequence

1. Mark the outline with stakes and string, and square it by checking the diagonals are equal.
2. Strip the turf, lay landscape fabric to stop weeds, and dig the 24 in pads about 4 in deep at each block position.
3. Fill the pads with gravel, tamp them, and set the blocks so their tops line up, using a long level or a string line.
4. Drop the beams into the block slots, shimming with composite shims (not wood) until they are level.
5. Fasten the joists to the beams with hurricane ties or toe screws, add the rims, and check the frame for square again.
6. Tape the joist tops, then lay the decking from the outside edge in, with the gap your boards call for.

Two people can do it in a long weekend once the blocks are level. For a deck that needs footings and a ledger, follow the full how to build a deck guide instead.