Fix Sagging Deck Boards

Deck boards sag when the structure beneath them fails to do its job. A board spanning more than sixteen inches between joists will eventually bow under foot traffic, furniture weight, and its own mass. Sometimes the problem is worse — rot has compromised a joist, or the original builder spaced supports too far apart. The good news is that most sagging happens gradually, giving you time to intervene before the deck becomes unsafe. A proper fix addresses the cause, not just the symptom. Driving in a few extra screws might tighten a loose board temporarily, but it won't solve inadequate support. You need to get underneath the deck, assess what's failing, and rebuild the support system so it actually holds weight. Done right, this repair adds years to your deck's life and makes it safe to use again.

1. Find Where It's Breaking. Walk the deck and mark every sagging board with chalk. Press down on each one to test how much it flexes. Get underneath and look up at the joists supporting the problem areas. Check for rot by prodding the wood with a screwdriver — if it sinks in easily, that joist is compromised. Measure the joist spacing with a tape measure.
2. Strip Boards to Access Joists. For sections with rotted joists, you need access from above. Use a drill to back out all screws or a pry bar to pull nails from the affected boards. Remove only the boards you need to reach the problem joist. Stack them in order if they're reusable — you'll put them back the same way.
3. Replace Compromised Support. If a joist is rotted, cut away the damaged section with a reciprocating saw. Install a new joist segment alongside the remaining good wood, fastening it to the ledger board and beam with joist hangers. If the entire joist is compromised, remove it completely and install a full-length replacement using the same size lumber as the existing structure.
4. Bridge the Gap with Blocking. For boards sagging due to wide joist spacing, cut blocks from matching joist lumber to fit snugly between joists at the sag points. Install them perpendicular to the joists, toenailing through the block into each joist with three-inch nails or securing with structural screws. Space blocks every four feet along the joist run in problem areas.
5. Level Every Board Precisely. Place a straight edge across multiple boards to find low areas. Cut composite shims or use cedar shingles to bring low boards flush with their neighbors. Slide shims between the board and joist, then drive screws through the board, through the shim, and into the joist. Use two screws per joist at each board.
6. Anchor Boards with Precision. Replace any boards you removed, ensuring each one sits flat against its supports. Use deck screws rated for exterior use, driving two screws at every joist intersection. Pre-drill screw holes on hardwoods to prevent splitting. Countersink screw heads slightly below the surface but avoid over-driving, which weakens the connection.
7. Verify the Deck is Solid. Walk the entire repaired section, bouncing slightly to test for flex. Any remaining movement means a fastener missed its joist or blocking needs adjustment. Add screws where needed and verify that every board feels solid underfoot. Check that gaps between boards remain consistent — warped boards may need replacement rather than repair.
8. Seal Against Future Rot. Apply deck stain or sealant to any new or exposed wood, matching your existing finish. Pay special attention to cut ends of joists and blocking, which absorb moisture readily. Let the sealant cure for the time specified on the label before placing furniture or heavy objects on the deck.