How to Add Blocking Between Deck Joists
Blocking is the unsung hero of deck framing. These short pieces of lumber between joists prevent twisting, stiffen the frame, and provide extra support where you need it most. Taking time to install blocking correctly makes your entire deck more stable and long-lasting.
Quick Summary
1-2 hours
Easy
Same as joists
Why Blocking Matters
Without blocking, tall, narrow joists can twist or rotate under load. This twisting causes the joist to lose its vertical orientation, reducing its load-carrying capacity and creating uneven support for decking. Over time, a twisting joist can work fasteners loose and create a bouncy, unstable deck.
Blocking locks joists in their upright position by connecting them to each other. Think of it like the rungs of a ladder - individually, each rung could rotate, but connected together, they're locked in place.
Blocking Provides:
- - Prevention of joist twisting
- - Lateral load transfer between joists
- - Extra stiffness underfoot
- - Additional nailing surface for decking
- - Support for railing posts between joists
When It's Required:
- - Joists deeper than 2x10 (most codes)
- - Joist spans over 8 feet
- - Under heavy load areas (hot tubs)
- - At railing post locations
- - When specified by engineer
Pro Tip: Even when code doesn't require blocking, experienced deck builders often add it anyway. The small investment in time and material pays off in a noticeably more solid deck that stays square and level over time.
Types of Blocking
There are two main approaches to blocking, each with advantages for different situations.
Staggered Blocking (Recommended)
Blocking pieces alternate on either side of a center line, creating a zigzag pattern when viewed from above.
- Advantage: Can face-nail through joists into blocking ends - much stronger
- Advantage: Easier to install - no awkward toe-nailing
- Note: Uses slightly more lumber than inline blocking
Inline Blocking
All blocking pieces sit in a straight line across the joists.
- Advantage: Creates a continuous line of support
- Advantage: Slightly less lumber needed
- Disadvantage: Every other piece must be toe-nailed (weaker connection)
Tools and Materials You'll Need
Tools
- Circular saw - For cutting blocking pieces
- Tape measure - For measuring between joists
- Chalk line - For marking blocking location across joists
- Framing nailer or hammer - For driving nails
- Speed square - For marking square cuts
Materials
- Pressure-treated lumber - Same size as your joists (2x8 blocking for 2x8 joists, etc.)
- 16d galvanized nails - For face-nailing through joists (3-4 per blocking piece)
- 10d galvanized nails - For toe-nailing if using inline blocking
Estimating Material
For a 12x16 foot deck with joists at 16" on center, you'll have about 11 joist bays. One row of mid-span blocking needs 11 pieces at roughly 14.5" each. That's about 14 linear feet of lumber per row of blocking - less than two 8-foot boards.
Step-by-Step Installation
Step 1: Determine Blocking Location
Blocking typically goes at the mid-span of joists - the point halfway between the ledger and the outer rim joist.
- 1.Measure the joist span (ledger face to rim joist face)
- 2.Divide by 2 to find the mid-point
- 3.Mark this distance on both end rim joists
- 4.Snap a chalk line across all joists at this mark
For longer spans: If joists span more than 10-12 feet, install two rows of blocking at third-points (divide span by 3 instead of 2).
Step 2: Measure and Cut Blocking Pieces
Each blocking piece fits between two joists. With joists at 16" on center, blocking pieces are about 14.5" long (16" minus two 3/4" joist thicknesses).
- 1.Measure between the first pair of joists at the chalk line
- 2.If all bays are consistent, cut all blocking pieces at once
- 3.If joist spacing varies, measure each bay individually
- 4.Cut blocking from the same size lumber as your joists
Pro Tip: Use joist offcuts for blocking. Those 14-16" scraps from cutting joists to length are perfect for blocking pieces.
Step 3: Install Staggered Blocking
Staggering the blocking allows you to face-nail through each joist into the blocking end - a much stronger connection than toe-nailing.
- 1.Position first piece - Place on one side of the chalk line, between joists 1 and 2
- 2.Align tops flush - The top of blocking must be flush with joist tops
- 3.Face-nail through joist 1 - Drive 3 nails through joist 1 into the end of the blocking
- 4.Position second piece - On the OPPOSITE side of the chalk line, between joists 2 and 3
- 5.Face-nail through joist 2 - Drive 3 nails through joist 2 into BOTH blocking pieces
- 6.Continue alternating - Each piece on the opposite side of the line from its neighbor
Important: Keep blocking tops perfectly flush with joist tops. Even small variations create humps or dips in your finished decking surface.
Step 4: Nail Pattern for Different Joist Sizes
The number of nails depends on your joist size. More depth means more nails for proper load transfer.
| Joist Size | Nails Per End | Pattern |
|---|---|---|
| 2x6 | 2 nails | Staggered top and bottom |
| 2x8 | 3 nails | Top, middle, bottom |
| 2x10 | 3-4 nails | Staggered pattern |
| 2x12 | 4 nails | Two pairs, staggered |
Step 5: Check Alignment and Adjust
Before moving on, verify all blocking is properly aligned.
- Run a straightedge across the joist and blocking tops - should be continuous
- Check that all joists are still vertical (blocking hasn't pushed them out of plumb)
- Verify nails are driven flush or slightly countersunk
- Add any missing nails
Special Blocking Situations
Railing Post Locations
Railing posts that fall between joists need solid blocking behind them for a strong mounting surface.
Solution: Install double blocking at post locations - two pieces stacked to equal the post width, or blocking on each side of where the post will mount.
Hot Tub or Heavy Load Areas
Areas supporting significant weight benefit from additional blocking and possibly closer joist spacing.
Solution: Install blocking at 2-foot intervals across the entire load area. Consider using 2x12 blocking even with smaller joists for maximum rigidity.
Diagonal Decking
When running decking at 45 degrees, the unsupported span of each board increases, requiring more blocking.
Solution: Add blocking rows at 4-foot intervals rather than just mid-span. This provides adequate support for diagonal decking boards.
FAQ
Is blocking required for deck joists?
Codes typically require blocking for joists over 2x12 or spanning more than 8 feet. However, blocking is beneficial for any deck - it prevents twisting, adds stiffness, and many builders add it regardless of code requirements.
Where should blocking be installed?
Install at mid-span for most decks. For joists over 10-12 feet, use two rows at third-points. Also add blocking at railing post locations and under heavy load areas like hot tubs.
Why stagger blocking instead of inline?
Staggering lets you face-nail through each joist into the blocking end, which is much stronger than toe-nailing. With inline blocking, every other piece must be toe-nailed.
Can I use different lumber for blocking?
Use the same size lumber as your joists - 2x10 blocking for 2x10 joists. This ensures flush tops for flat decking. Shorter offcuts from joist material work perfectly for blocking pieces.