Over time, the joists on above-ground wood decks have a tendency to twist or bend.This is due to the constant exposure to the elements and changes in humidity that can cause increases and decreases in the swelling of the boards. This problem is most common on decks that feature long-spanning joists of 12 feet or more.
One way to prevent this problem is to install blocking between the joists. Also called “solid bridging,” blocking consists of installing short pieces of the same type of wood used for the joists in between the joists. By installing the blocking between the joists in a staggered row, you will shore up the deck’s structure and provide it with the extra stability it needs to last a long, long time.
Installing blocking can prove to be a difficult job, especially if the joists are already badly bent. However, with someone to assist you, this job can be accomplished by the do-it-yourselfer with a general knowledge of carpentry.
Tools and Materials: What you'll need
  • Measuring tape
  • Circular saw or table saw
  • Power drill and bits
  • Hammer
  • Lumber (same dimensions as the existing joists)
  • Galvanized deck screws, 3 and ½ inches
  • Ladders (if necessary)
  • Chalk line

When working with wood, especially when cutting it, it is very important to wear safety gear. For this job, you should wear safety goggles, hearing protection, work gloves, heavy duty work boots and a NIOSH-approved respirator or dust mask.

Adding Blocking between Joists to a Wood Deck
Step 1: Preparation
Before you start cutting and installing the blocking, run a chalk line across the tops of the joists at the mid-point of the deck. This will ensure you keep a straight installation, even though the blocking will be staggered.
Step 2: Cutting the Lumber
Since the distance between the joists can change throughout this installation, you don’t want to cut all of your wood blocks at once. Instead, cut only four at a time and then re-measure the distance between the next joists.
When sizing your cuts, use the measuring tape to learn the distance between the joists. Then, subtract an inch and a half from that number and make your cuts according to the answer. For instance, if the joists are spaced 16-inches apart, you'll want to cut the blocks 14 and ½-inches wide.
Step 3: Installing the Blocking
  1. Start at the first open space between the joists and line up the edge of one piece of block along the chalk line and set it in place. More than likely, you'll have to use the hammer to wedge the block in place.
  2. Once the block is between the joists and the edge is even against the chalk marks, secure it to the joists by driving galvanized deck screws through the face of the joists and into the ends of the block.
  3. When you install the next block, line this one up so the bottom edge of the block is even with the top edge of the previous one. In other words, the blocking between one joist will be below the chalk and the next one will be above it.
  4. Continue installing the blocks in this staggered format.
  5. With the blocking installed, simply paint or stain the wood to match the deck’s existing color and this job is complete.

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