How to Stiffen Deck Floor Joists

Weathered Wood on a Deck
You can stiffen a bouncy deck by blocking the joists below the deck boards. hipshot/E+/Getty Images

A safe, well-built and attractive outdoor deck adds beauty and value to your home, increases your living space, and provides the perfect spot for entertaining guests. However, all of this use and exposure to the elements eventually takes its toll, and some older decks begin to feel bouncy, spongey, and unstable.

There may be a few causes behind this. Wood planking, a common decking material, expands and contracts in response to direct moisture and relative humidity. These unsightly gaps may partially contribute to the slightly unsteady feeling you experience when you walk across it. More likely though, the problem stems from joists that are no longer as firm as they should be.

By using a technique called blocking, you can easily stiffen a bouncy deck by firming up those joists. Blocking means affixing pieces of wood in a staggered fashion between the joists located underneath your deck.

Tools and Materials

  • Long-sleeved shirt and long pants (optional)
  • Work gloves
  • Work light
  • Drill with screwdriver attachment to remove side panels
  • Chalk line
  • Tape measure
  • Pressure-treated lumber the same size as the joists
  • Circular saw
  • Hammer or rubber mallet
  • Ladder (optional)
  • 16d galvanized box nails

Prepare the Work Area

Make sure no one is standing on the deck as you do these repairs. Remove all items from the deck, including furniture and flower pots. If you have a raised deck with living space underneath, remove all of the items under the deck.

Suit up before starting work. If you have a low-lying deck, wear a long-sleeved shirt and pants to protect your body while crawling underneath. Wear gloves to protect your hands. You will also need a light attached to a headband to wear under the deck or a work light.

Before you get under the deck, you will want to make sure that the area is clean and safe. The sides of some decks are open for easy access underneath; others are covered by latticework or decorative panels. These panels are usually secured with a few screws in the corners. Keep in mind that animals such as raccoons, spiders, and snakes like to hide in small, dark spaces, so be sure to look around carefully before crawling under the deck. Make loud noises or bang on the deck supports to frighten off animals that may be lurking.

If your area is prone to poisonous spiders such as the brown recluse, you may want to take the added precaution of spraying the bottom side of the deck with a garden hose or pressure washer. This should dislodge spiders and other insects. Let the area thoroughly dry before performing repairs.

Inspect the Joists and Planks

Once you are underneath the deck, check to see whether you have any loose or decaying planks, which will need to be reattached or replaced, depending on their condition. There is no point in trying to firm up joists beneath rotting boards.

Create Chalk Lines

Starting near the end of the deck, snap a chalk line perpendicular to the joists. Joists are the wood beams arranged in a parallel pattern to support the deck's overlying structure. The chalk line should run from one end of the deck to other. Repeat this step every 3 to 4 feet across the deck. You will place the blocking along the chalk lines.

Starting with the first chalk line, measure the space between the first two joists. Make sure that the measurements are precise, as you will be cutting your blocking to fit snugly between the joists. Blocking will effectively unify the joists so that any weight borne by one joist is partially distributed to those adjacent to it. Just be sure not to install blocking directly beneath the spaces between the deck boards, as this will form pockets that will catch and hold water, which will only exacerbate your problem.

Cut the Blocking and Place It

Measure and cut a piece of treated lumber to fit between the joists. This will serve as your blocking. Using a hammer or rubber mallet, tap the blocking into place, with the narrow side facing up. Use the chalk line as a guide.

Attach the Blocking

Once the blocking is in place, drive three galvanized nails through the joists on each side of the blocking to hold the wood into place. One nail should be at the top of the blocking, one in the middle, and one at the bottom.

Continue and Stagger the Blocking

Repeat this process along the chalk line, staggering the blocking. The idea behind staggering is that you need to provide space so that you can nail onto the ends of the blocks. If the blocks were to continue in a strictly linear fashion, you would have no room to nail the blocks. When you finish, every second piece of blocking should be aligned.

Perform Regular Maintenance

Remember, the planking that makes up the surface of your deck is exposed to all sorts of wear and tear from the humans who use it, plus whatever else the elements dishes out. Keeping your deck safe as well as beautiful means doing regular maintenance checks at least once a year. Look for damaged or loose boards, inspect for structural damage to the supports underneath the surface, and shore up any weak spots to ensure that all users can continue to enjoy the deck without fear of injury or discomfort.