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Blocks, block length, lumber and the IRC minimum depth for rows of blocking between deck joists.
With these inputs:
| Result | Value |
|---|---|
| Blocks to cut | 12 |
| Cut length (between 1-1/2 in joists) | 14.5 in |
| 2x8 × 8 ft boards | 2 (6 blocks each) |
| Minimum block depth (IRC R507.6.2) | 4.4 in (60% of 7.25 in) |
| Joist bays per row | 12 |
| Joists | ceil(16 × 12 ÷ 16) + 1 = 13 |
| Blocks | 12 bays × 1 row = 12 |
| Lumber | 14.5 linear ft of blocking |
2021 IRC R507.6.2: joist ends and bearing points need lateral restraint, and joist hangers or blocking used for it must be at least 60% of the joist depth. Full-depth blocking from the same size lumber as the joists always meets that. IRC R502.7.1 adds blocking or bridging at no more than 8 ft for joists deeper than 2x12. Cut a test block first: real joist spacing drifts.
Blocking is short pieces of joist lumber fitted between the joists. It holds the joists upright, stiffens the floor and gives the deck boards something to fasten to at butt joints and picture-frame borders. Enter the deck length, joist spacing, joist size and how many rows you want; the calculator counts the blocks, gives the cut length and how many stock boards to buy, and checks the IRC rule that restraint must be at least 60 percent of the joist depth.
The ends of the joists need lateral restraint so they can't roll over. The 2021 IRC (R507.6.2) lets that come from joist hangers, a rim joist or blocking, with hangers or blocking at least 60 percent of the joist depth. Mid-span blocking isn't required for 2x12 or smaller joists, but many builders add a row to stiffen the floor.
The code only sets a maximum of 8 ft for joists deeper than 2x12 (IRC R502.7.1). For ordinary deck joists, a row at mid-span is common; picture-frame borders, butt joints in the decking and diagonal patterns need blocking wherever a board end lands. Enter the rows you plan in the calculator.
14-1/2 in: 16 in on center minus one 1-1/2 in joist. At 12 in spacing blocks are 10-1/2 in, and at 24 in they are 22-1/2 in. Measure a few bays first — joists are rarely exactly on layout.
Enough rows to support every border board end and the inside board of the frame. A border running along the side of the deck, parallel to the joists, needs a row of blocks under it, and the boards that butt into the border need support at that line too. Add those rows to the calculator.
Only if the IRC 60 percent rule is met where the blocking provides end restraint. For a 2x8 joist (7-1/4 in deep) that means at least 4.35 in, which a 2x4 laid flat or on edge (3-1/2 in) doesn't reach. Using the same size as the joists is simplest.
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