Assess the Project
The ultimate goal here is to have the top 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.
Grab a string longer than the width of your deck and add weights to both ends of your string (this could be a couple of washers or bolts).
Lay the string across the width of your deck. Starting at the back of the deck, move the string forward. Assess where your high and low spots are in conjunction with the other joists. Mark the areas on each joist that are either crowning or sagging.
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.
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 your joists will be level with each other.
Fixing Crowned Joists
There are a few 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
You can always just start over and replace the bad joist with a new straight board.
ncG1vNJzZmivp6x7p63MoqOyoJGjsbq5wKdlnKedZK6zwMico55nlp7FqrrGZq6aqqCasW6wxJyiZqKfnsC1v44%3D