How to Shim Deck Joists: A Step-by-Step Guide

Sagging or uneven floors can be a sign of worn-out or improperly spaced joists. Shiming deck joists is an effective way to restore your deck's stability and safety. This process involves filling gaps between joists and the beam they rest on, ensuring a solid connection and preventing further damage. Let's dive into the step-by-step process of how to shim deck joists.

Before you start, gather your tools and materials. You'll need a tape measure, a level, a circular saw, a drill with a screwdriver bit, joist hangers, screws, and shims. Ensure your shims are made of pressure-treated lumber to resist rot and decay. Now, let's get started.

Preparation and Assessment

Begin by inspecting your deck's joist system. Look for any signs of damage, rot, or excessive wear. Measure the gaps between the joists and the beam they rest on. Ideally, these gaps should be no more than 1/8 inch. If they're larger, you'll need to install shims.

Before installing shims, ensure your joists are level. If not, you may need to sister (add a new joist alongside an existing one) or replace them entirely. Once your joists are level, you're ready to proceed.

Choosing the Right Shim Size

Shim size depends on the gap you're filling. For gaps up to 1/2 inch, use 1/4-inch shims. For gaps between 1/2 inch and 1 inch, use 1/2-inch shims. For gaps larger than 1 inch, you may need to use multiple shims or consider sistering the joist.

Always use shims that are at least as wide as the joist they're supporting. This provides a stable base and prevents the shim from bending or breaking under pressure.

Installing the Shims

Start by cutting the shim to the correct length using a circular saw. The shim should extend from the top of the joist to the bottom, filling the entire gap. If the gap is too large for a single shim, install multiple shims stacked on top of each other.

Using a drill and screwdriver bit, predrill holes through the shim and into the joist. This prevents the wood from splitting when you drive the screw. Then, secure the shim to the joist using screws. Ensure the screws are long enough to penetrate at least 1 inch into the joist.

Securing the Joists with Joist Hangers

After installing the shims, you'll want to secure the joists to the beam using joist hangers. This provides additional support and prevents the joists from moving over time.

Joist hangers come in various sizes, so choose one that fits your joist and beam sizes. Cut the hanger to length using a hacksaw or reciprocating saw. Then, position the hanger over the joist and beam, ensuring it's level and square.

Installing the Joist Hangers

Using a drill and screwdriver bit, predrill holes through the hanger and into the joist and beam. This prevents the wood from splitting when you drive the screw. Then, secure the hanger to the joist and beam using screws. Ensure the screws are long enough to penetrate at least 1 inch into the wood.

Repeat this process for each joist, ensuring they're all securely fastened to the beam. Once you've finished, your deck should be stable and safe for use.

Finishing Up

After installing the shims and joist hangers, it's a good idea to inspect your work. Check that all the joists are level and securely fastened. If necessary, make any adjustments before moving on.

Finally, consider applying a sealant to the shims and joist hangers to protect them from moisture and further decay. This will help extend the life of your deck and keep it looking its best.

Shiming deck joists is a crucial step in maintaining your deck's safety and longevity. By following these steps and taking the time to do the job right, you can ensure your deck remains a safe and enjoyable space for years to come. Don't forget to regularly inspect your deck for any signs of wear or damage, and address any issues promptly to keep it in top condition.