Fixing Warped Deck Joists

Warped deck joists can be a problem, whether you’re building a new deck or resurfacing an old one. Joists dry out, changing the moisture content and causingthem to twist and curl. That makes the decking sag or lift (or both) and can make the deck itself unsafe to walk on.

It’s important to check joists for straightness before you install them, but even when you do, they may start warping after a few years of wear. If you spot warped deck joists (and we’ll show you how), you don’t necessarily have to replace them. Here’s how to minimize or remedy this problem without doing that.

Assess the Problem

The ultimate goal here is to have the tops of all of your deck joists level with each other. Get eye level with the joists and take a good look across the top surface of your deck. There’s a good chance some of your joists are crowning or sagging.

Lucia Villegas, the brand manager for an Ohio decking company, offers these tips for identifying sagging joists:

• Do a sightline inspection: Stand at one end of the deck and visually align your sight with the joists—warped joists will bow (crown or cup); sagging joists will dip noticeably.
• Perform a walk-test for bounce and flex: Walk across the deck and feel for areas that flex or bounce. Soft or springy spots underfoot indicate deflection or spring in the joist framing.
• Use a string line or laser level: Stretch a taut string or project a laser line across multiple joists. Inconsistent gaps or deviations from straightness signal sag or warp.
• Look for decking clues: Uneven or bouncy joists often show up in the decking surface as lifted boards, gapping, or gaps that catch your foot.

Once you’ve determined you’ve got a problem with sagging deck joists, here’s what to do.

Ways to Fix Warped Deck Joists

Add blocking

Blocking refers to the boards that are added between each joist every four to six feet to give the structure rigidity and minimize the movement of the joists. Here’s how blocking helps, according to Villegas:

• Stabilizes joists: Joist blocking ties adjacent joists together, reducing the tendency to twist or rotate, which is key in preventing warping under load.
• Distributes load evenly: Properly spaced blocking (typically mid-span every 6 to 8 feet, depending on spacing/joist span) spreads live loads across a broader section of joists, which minimizes sag between supports.
• Increases frame stiffness: Blocking enhances structural rigidity, reducing bounce and deflection for a sturdier, more stable deck frame.
• Keeps decking tight and aligned: A rigid joist grid holds boards in place, preventing gaps and uneven wear in decking. It is especially important for composite or diagonal board layouts that demand tight tolerances.

Important. Before adjusting a joist, remove the blocking on both sides of it.

Pro Tip: Before you add blocking, attach 2x6s (on edge) on the top of the joists spanning the width of the deck, using screws or pipe clamps. This will ensure that all the joists will be level with each other.

Fixing Crowned Joists 

There are two ways to fix a crowning joist. You can move the joist or remove material from the joist.

Move the Joist

Push down on the joist by attaching a pipe clamp or Deck Devil to the top of the joist and to the bottom of the beam. Once the crowned joist is level with the adjacent joists, attach it to the beam below with deck screws or hurricane clips on each side of the joist. Reinstall the blocking on both sides of the joist.

Remove Material From the Joist

Remove the blocking on each side of the joist. Snap a chalk line on the side of the joist from one end to the other, revealing where the high spot is. At the high spot, use a power planer or a circular saw with a straight edge to remove material.

Fixing a Sagging Joist

Likewise, there are a few ways to fix a sagging joist. You can turn it upside down, add a sister joist, pull up on it or just replace it.

Turn the Joist Upside Down

Remove the blocking on each side of the joist. Remove the screws or nails in the joist hangers. As long as you have the board out, you might as well straighten it. Snap a line and straight line cut the board using a circular saw and straight edge. You can also plane it with a power planer or run it through the jointer.

Add a Sister Joist

Attach a straight sister joist next to the sagging joist with screws and or carriage bolts.

Pull Up on the Joist

Remove the blocking on each side of the joist. Lay a four-foot 4×4 across the top of the sagging joist and two adjacent joists. Attach a bar clamp or Deck Devil to the bottom of the joist and to the top of the 4×4. Pull up on the joist until the top of the joist is flush with the bottom of the 4×4. Reinstall the blocking on both sides of the joist.

Replace the joist

As a last resort, you can always just start over and replace the bad joist with a new straight board.

FAQ

How can I prevent future warping?

If you’re building a deck, select dry, pressure-treated wood for the joists. Prevent warping deck joists on an existing deck by painting the joists (yes, you can paint pressure-treated wood) or sealing them with a clear or semi-transparent wood sealer or stain.

Is it okay to install decking over slightly crowned joists?

Generally, yes. As long as the crown is slight, the element of waviness it introduces into the deck surface probably won’t be noticeable. It will still be there, though, so if you want to avoid it and have a perfectly flat deck, use the techniques described above for correcting crowning.

About the Expert

Lucia Villegas is the brand manager for Archadeck, a deck and patio contracting service based in Akron, OH.