Floating Deck Calculator

A floating deck is a low, ground-level deck that rests on precast deck blocks instead of footings dug below the frost line, and it is not attached to the house. That makes it simpler and faster to build, which is why it is a popular DIY project for patios and backyard seating areas.

Deck blocks support the beams and joists, typically spaced about every 4 feet in both directions. For a 12×12 floating deck that is roughly 16 blocks. The calculator works out the block grid from your deck size, and you can add more for heavier loads or longer spans.

Most floating decks sit on a compacted gravel base under each block, which keeps the blocks from settling and improves drainage. The calculator estimates the gravel volume for a base pad. Landscape fabric under the gravel helps suppress weeds.

In materials, roughly $520 to $660. A 10 x 10 floating deck works out to 9 deck blocks in a 3 x 3 grid, 8 joists and about 24 deck boards, plus a gravel pad under each block - the calculator above shows the full list. Built by a contractor instead, expect somewhere around two to three times that, because on a deck this small the labour is mostly setup and levelling rather than square footage. It is also the size where DIY makes the most sense: low, unattached and small enough to square up by hand.

About $1,980 to $2,530 in materials: 36 deck blocks in a 6 x 6 grid, 16 joists and roughly 93 deck boards. That is four times the area of a 10 x 10 and almost exactly four times the material, because a floating deck scales cleanly - there is no ledger, no footings below frost line and no railing on a deck this low. At 400 sq ft you are also at the size where many jurisdictions start asking for a permit, so check before you buy the lumber.

It is how you get a square corner without a framing square. Measure 3 feet along one side, 4 feet along the other, and adjust until the diagonal between those two marks is exactly 5 feet - the corner is then a true 90 degrees. Any multiple works and bigger is more accurate, so on a deck frame use 6-8-10 or 9-12-15. This matters more on a floating deck than on most builds: the blocks sit on a grid, and a frame that is out of square puts the blocks where the joists are not.

Leave a gap rather than pushing it tight against the wall - commonly 1 to 2 inches. Three reasons, all of which cost money to fix later: the deck and the house settle independently, so contact transmits movement into the siding; the gap lets water drain and air move instead of trapping moisture against the wall; and a deck touching the structure invites the argument that it is attached, which changes both the permit question and the code requirements. If you want it flush, you are building an attached deck, and that needs a proper ledger and footings.

No - blocks carry the beams, and the beams carry the joists. A typical floating deck runs beams on a roughly 4 ft grid of blocks, with joists spanning across them at 16 inches on centre. Putting a block under every joist would mean four times the blocks and no more capacity, because the limit is the beam span, not the number of contact points. What does call for more blocks is a hot tub, a heavy planter run or soft ground, and the calculator lets you add them.

Rules vary by location. Many areas exempt small, low, unattached decks below a certain size and height, but others require a permit regardless. Check with your local building department before you start — this calculator sizes the materials but does not cover code or permits.