Deck Blocking Calculator
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.
What you'll need
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Shopping for someone? See our backyard gift guide for deck builders & DIYers.
Block lengths and minimum depths
| Joist | Actual depth | Min. block depth (60%) | Block at 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|---|---|
| 2x6 | 5.5 in | 3.3 in | 10.5 in | 14.5 in | 22.5 in |
| 2x8 | 7.25 in | 4.35 in | 10.5 in | 14.5 in | 22.5 in |
| 2x10 | 9.25 in | 5.55 in | 10.5 in | 14.5 in | 22.5 in |
| 2x12 | 11.25 in | 6.75 in | 10.5 in | 14.5 in | 22.5 in |
Blocks for one row across common decks
2x8 joists 16 in on center, cut from 8 ft boards.
| Deck length | Joists | Blocks per row | 8 ft boards |
|---|---|---|---|
| 10 ft | 9 | 8 | 2 |
| 12 ft | 10 | 9 | 2 |
| 16 ft | 13 | 12 | 2 |
| 20 ft | 16 | 15 | 3 |
| 24 ft | 19 | 18 | 3 |
What the code asks for
The 2021 IRC (R507.6.2) wants the ends of deck joists and the points where they bear on a beam restrained against rolling. A rim joist, joist hangers or blocking does it, and hangers or blocking must be at least 60 percent of the joist depth. Joists deeper than 2x12 also need blocking or bridging at least every 8 ft (R502.7.1). Rows beyond that are a choice: one at mid-span stiffens the floor, and picture-frame borders, decking butt joints and diagonal patterns need blocking wherever board ends land. Size the joists with the joist spacing calculator and the rest of the frame with the deck framing calculator.
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Frequently asked questions
- Does a deck need blocking between joists?
- 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.
- How far apart should deck blocking be?
- 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.
- How long are blocks for 16 inch joist spacing?
- 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.
- How much blocking does a picture-frame deck need?
- 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.
- Can I use 2x4s for deck blocking?
- 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.
