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How many concrete deck blocks a floating deck needs, the load on each, and the gravel pads under them.
With these inputs:
| Result | Value |
|---|---|
| Deck blocks needed | 15 blocks |
| Layout | 3 rows of 5, rows 6 ft apart, blocks 4 ft apart |
| Load on an interior block | 1,200 lb |
| What the soil under one block carries | 1,260 lb — OK |
| Blocks cost | $180 |
| Gravel pads (24 in square, 4 in deep) | 20 cu ft · 0.74 yd³ |
| Gravel by weight (#57 stone) | about 1.04 tons |
| Or in 0.5 cu ft bags | 40 bags |
Loads at 40 psf live + 10 psf dead (IRC R507 / AWC DCA 6); soil values are the IRC Table R401.4.1 presumptive bearing figures. Deck blocks sit above the frost line, so use them only for a low, freestanding deck that is not attached to the house — anything attached, elevated or permitted needs footings below frost depth.
A floating deck sits on precast concrete deck blocks instead of dug footings. The count comes from two spacings: how far apart the rows of blocks are (the joist span between supports) and how far apart the blocks are along each row. This lays the blocks out, checks the load on each against what the soil under it can carry, and works out the compacted gravel pads the blocks should sit on.
Fifteen, laid out as three rows of five. With rows no more than 6 ft apart across the 12 ft width you need three rows, and with blocks no more than 4 ft apart along the 16 ft length each row needs five. Each interior block then carries about 1,200 lb, just under the 1,260 lb an 11 in block can put on 1,500 psf soil.
About 4 ft apart along each row and 6 ft or less between rows is the common layout for a floating deck. The real limit is load: each interior block carries the deck area around it at about 50 lb per square foot, and a small block on average soil can only spread roughly 1,200 lb. Closer spacing or bigger blocks fix an overloaded layout.
They should sit on a compacted gravel pad rather than bare soil. Strip the topsoil, dig down about 4 in, fill with crushed stone such as #57 and tamp it level so the block cannot rock or settle into mud. A 24 in square pad 4 in deep is about 1.3 cubic feet of gravel, or roughly three 0.5 cubic foot bags.
Usually only for a low, freestanding deck that the code exempts from a permit, often under 30 in above grade and under 200 sq ft. Deck blocks sit above the frost line, so the ground can lift them in winter. Anything attached to the house, elevated or permitted needs footings dug below frost depth, which your building department sets.
Yes. Most blocks have a slot for a 2x joist or beam on edge and a pocket for a 4x4 post. Setting beams in the slots and running joists across them keeps the deck lowest. Use short posts only where the ground falls away, and keep them short, because a block does not hold a post against sideways movement the way a buried footing does.
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