Have you ever walked across a deck and felt the boards bounce a little? That bounce is called deflection. It happens when a board flexes under your weight instead of staying still.
A bouncy deck usually isn’t dangerous right away. But it’s one of the clearest signs that separates an amateur build from a professional one.
The fix isn’t a thicker board or a fancier material. It’s a framing step that happens before any decking goes down. That step is called blocking.
What Is Blocking?
Blocking is short pieces of lumber. You install them between your deck’s floor joists, running side to side.
Once the deck is finished, you won’t even see the blocking. It’s hidden underneath the deck boards. But it changes how the whole deck feels and performs.
Why Blocking Matters
Blocking does two important jobs.
First, it reduces bounce. When you nail and screw blocking between joists, it ties the frame together. This makes the deck resist flexing under weight. At the spots where blocking is installed, the frame becomes noticeably stronger against bounce. That matters most when a lot of weight is on the deck at once, like during a party with dozens of guests.
Second, it helps keep your frame square. “Square” means everything lines up at proper right angles. A square frame is easier to work with as you build. It also matters later, when you install your decking boards in a clean, even pattern.
That clean pattern is called a picture-frame finish. You’ve probably seen it on nicer decks: a neat border of decking boards that frames the field boards in the middle. To install that border well, you need a solid, level surface to nail into at the edge of the deck. Blocking provides that surface.
How to Size and Space Your Blocks
Before you cut blocking, you need to measure the gap between your joists. Don’t just use the on-center spacing number (the distance between the centers of two joists). Measure the actual open space between them, since the joists themselves take up some width.
In one example project, the joists sat 12 inches on center. That worked out to a block length of about 10½ inches. Your own measurement may be different, depending on your joist spacing. The process stays the same: measure your actual gap, then cut your blocks to fit.
Here are a few more guidelines to follow:
- Keep blocking runs under 8 feet. If the space you’re blocking is longer than 8 feet, don’t run one long line of blocks. Split it into two rows instead.
- Skip bad lumber. Look at the ends of your blocks before you use them. If you see a large knot or a split starting, don’t use that piece. It’s a weak point that could fail later.
- Don’t stress over a perfect fit. Your blocks don’t need to sit exactly centered or flush every time. What matters is that each block fits snugly enough to do its job.
Why You Need Both Nails and Screws
Once your blocks are cut, you still need to fasten them correctly. This part matters just as much as the cutting.
First, avoid surface-nailing or surface-screwing. That means driving a fastener straight down through the top of the block into the joist below. Instead, drive your fasteners in at an angle, through the ends of the block. This is called toe-nailing or toe-screwing. Stagger your blocks along the run so you always have room to drive a fastener through solid wood.
Next, use nails for shear strength. Shear strength is a connection’s ability to resist sliding forces, like the force of you stepping on the deck. In one example, a single nailed connection provided roughly 320 to 350 pounds of shear strength.
Then, add screws for pull-out resistance. Pull-out resistance is a connection’s ability to resist being yanked straight out. A nail has a smooth shank, so it can pull out fairly easily. A screw has threads, so it grips the wood much better. Adding screws to the same connection pushed the estimated strength up to about 500 pounds.
Finally, don’t forget the underside. After your main blocking run is finished, go back underneath the deck. Add fasteners to the bottom of each block. This gives you extra holding power beyond what you added from the top.
One more note on fasteners: pressure-treated lumber and LVL beams (a type of engineered wood beam) are both treated with chemicals. Those chemicals can corrode regular metal over time. To avoid that, use galvanized nails and screws rated for contact with treated lumber, often labeled as ACQ-compatible.
What to Do With Twisted or Bowed Joists
Not every joist comes out of the lumber pile straight. Some are twisted or bowed. You’ll likely run into at least one of these while installing blocking. Luckily, there’s a simple field method for straightening them.
Step one: soak the joist with water. This isn’t a quick spray. You want to fully saturate the wood so it becomes flexible. Keep coming back and re-wetting it every so often while you work on the blocks around it. Wet wood bends more easily, and it’s less likely to crack or split while you’re straightening it.
Step two: use a joist-straightening tool. This is a strap or ratchet tool with a hook on each end. One hook grips the top of the joist, and the other grips the bottom. You place the hooks a short distance in from the edge of the board, not right on the edge, since the edge can get damaged under pressure. Then you ratchet the tool tight. This pulls the top and bottom of the joist together at the same time, straightening the twist.
Step three: add screws before you remove the tool. While the joist is still held straight by the tool, drive in a few screws to lock it in place. Once the screws are in, you can safely release the tension and remove the tool. Add your nails after that.
Step four: watch for hardware in the way. Some joists sit near metal hardware, like a hurricane tie (a metal bracket that connects a joist to a beam for extra strength in high winds). If a joist near one of these is badly twisted, disconnect the hardware first. This gives the wood room to move as you straighten it. Once it’s straight, reattach the hardware. You may need to move it slightly, such as placing a screw lower on the board than before.
Work through twisted joists patiently. If you notice your joists starting to bow out of line as you install blocking, stop and adjust. You can cut a slightly shorter block, maybe a quarter inch under your normal length, instead of forcing a full-length block into a tightening gap.
Planning Blocking Around Railings and Features
Blocking placement isn’t only about strength. It also depends on what’s happening on top of the deck.
If part of your deck overhangs past where a railing post will go, you may not need blocking behind that overhang. A common spacing guideline is to place a block roughly every 6 feet, adjusting the spacing around your actual railing post locations.
If you’re planning a feature like a hot tub, think ahead. Lay out your blocking sections so you can still access or adjust that area later, without tearing into blocking you’ve already finished elsewhere.
Around 4x4 railing posts, be careful where you drive your fasteners. Avoid the corner edge of the post. The wood there is thin, and a fastener driven into it can split the post. A split post often forces the fastener in at an angle, leaving the screw head sitting up instead of flush with the surface.
Why the Picture-Frame Nailing Surface Matters
Remember the picture-frame finish mentioned earlier? It needs more than just structural strength underneath it. It needs a continuous, level surface to nail into.
A nail roughly every 12 inches along that surface is enough to support real weight. It also helps the whole assembly act as one solid unit, instead of a row of separate boards moving independently.
This matters even more at the top of a staircase. If a decking board’s end isn’t fully supported there, it can flex under a person’s foot. Picture someone stepping near the tip of that unsupported board. Their toe touches down first, and the board holds. Then they shift their weight forward, and the board flexes or rolls under them. On a short unsupported span, this usually isn’t dramatic. But repeated stress at the top of stairs, over months and years, raises the chance of the board eventually breaking. Solid blocking at these transition points is what prevents that outcome.
The Payoff
None of these steps are difficult on their own. You cut blocks, fasten them, and occasionally straighten a stubborn joist. But together, they add up to something bigger: a deck frame that’s stronger, squarer, and ready for a clean finished look.
It’s also the difference between a deck that only looks solid and a deck that actually is solid.
Disclaimer
This article is for informational and educational purposes only. Techniques, measurements, spacing guidelines, and fastener recommendations described here should be verified independently against your specific project conditions, lumber species, local building codes, and manufacturer documentation before use. Sketchronix Insights makes no guarantees as to the accuracy or completeness of this information and is not liable for any damages, injury, or losses resulting from its use. For structural or safety-related decisions, consult a licensed professional or your local building department.