6 Best wood shims for leveling structural beams

Leveling a structural beam is a high-stakes task where precision determines the long-term integrity of the entire building frame. When a beam sits unevenly, it translates stress directly into the joists, flooring, and eventually the finish materials of the home. Choosing the right shim isn’t just about closing a gap; it is about selecting a material that will maintain its thickness and structural density under constant compression. Ignoring these nuances leads to cracked drywall, uneven floors, and failing door frames months or years down the line.

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Nelson Contractor Cedar Shims: Best for Damp Areas

Cedar possesses natural oils that resist rot, decay, and insect infestation far better than white pine or other softwoods. In areas where moisture migration is a concern—such as crawl spaces, near foundation walls, or rim joists—these shims provide a critical layer of defense.

The primary advantage here is durability in environments where standard wood would quickly soften and compress. While they lack the brute strength of hardwood, they excel in applications where environmental exposure is the primary threat to the shim’s lifespan.

Use these when leveling sill plates on concrete or in areas with high humidity. Avoid using them under high-point-load areas where a denser material like oak or steel is required to prevent “crushing” over time.

Real Wood Oak Shims: Top Choice for Heavy Loads

Oak is a hardwood with a high Janka hardness rating, making it nearly immune to the crushing forces exerted by heavy structural beams. When a project involves supporting a load-bearing post or a main carry beam, these shims provide a solid, non-compressible interface.

The density of oak ensures that the shim maintains its exact thickness under the weight of the floor system. Unlike softwoods, which can deform slightly as the building settles, oak provides a predictable, rigid point of contact that stops movement in its tracks.

Always verify the thickness of the shim against the required gap to ensure a proper fit. Do not stack too many thin shims to fill a large void, as this creates a “sandwich” effect that can slide or shift under heavy vibration.

Baldwin Pro Hardwood Shims: Best Value Bulk Pack

Baldwin Pro hardwood shims are the industry standard for general-purpose framing and light-duty leveling. These bundles offer a consistent thickness and a taper that makes them easy to drive into tight gaps without excessive force.

When tackling a large-scale project—like leveling a long run of floor joists—the cost-efficiency of bulk packs becomes apparent. They provide enough material to shim every joist across a room without running out mid-task or worrying about inconsistent sizing.

For heavy structural beams, these should be used in moderation. If a gap exceeds a quarter-inch, transition to solid blocking or LVL cut-offs to ensure the structural load path remains robust and continuous.

Job-Cut LVL Blocks: The Ultimate Structural Shim

Laminated Veneer Lumber (LVL) is an engineered product designed for extreme structural loads, making it the superior choice for leveling primary beams. Because LVL is manufactured under high heat and pressure, it has virtually no moisture content and zero tendency to shrink or warp.

Using scraps of LVL as shims is the most professional approach for rectifying significant variances in beam height. Because the material matches the composition of the beam itself, it creates a monolithic-like connection that does not compress or deteriorate over time.

When the gap under a beam is substantial—anything over a half-inch—rely on these engineered blocks. They offer the highest load-bearing capacity of any wood-based product, ensuring the structure stays level for the life of the building.

Job-Cut PT Lumber: For Masonry or Sill Contact

Pressure-Treated (PT) lumber is the only logical choice when shimming a beam that comes into direct contact with masonry or concrete. The chemical treatment prevents the wood from wicking moisture from the stone, which would otherwise lead to rapid rot in untreated lumber.

Working with PT lumber requires awareness of fastener compatibility, as the treatment chemicals can corrode standard steel nails and screws. Use hot-dipped galvanized or stainless steel fasteners if the shim needs to be secured through the beam.

Treat these as a bridge between the foundation and the structural members. They are intended for moisture protection rather than precision structural leveling, so use them in conjunction with steel shims if the required tolerances are extremely tight.

Ez-Shim Composite Shims: A Rot-Proof Alternative

Composite shims are manufactured from high-density polymers that are entirely impervious to water, insects, and humidity. They are an excellent solution for exterior projects, such as deck framing or porch beam leveling, where moisture cycles are aggressive.

The advantage of composites lies in their structural consistency. They do not compress, they never rot, and they provide a clean, uniform surface that won’t splinter during the driving process.

Be cautious, however, with the sheer weight capacity. While they work well for minor adjustments, they are not designed to handle the massive concentrated loads of a multi-story home’s central beam. Use these for leveling rim joists or non-load-bearing partitions.

Wood vs. Composite vs. Steel: Choosing Your Shim

Choosing the right material depends entirely on the location and the force being applied to the beam. Wood shims offer a natural friction hold, composite shims offer moisture immunity, and steel shims offer the highest compressive strength for high-load scenarios.

• Wood: Best for interior load-bearing beams where moisture is controlled.
• Composite: Best for exterior framing, decks, or high-humidity zones.
• Steel: Required for heavy-duty structural adjustments where precision and absolute zero-compression are non-negotiable.

Always assess the load path before selecting the material. A beam carrying the weight of a two-story house requires a different shim approach than one supporting a standard partition wall.

How to Properly Bed and Set Structural Shims

A shim is only as effective as its placement and the surface area it covers. When setting shims, always place them directly under the load point—typically where a beam crosses a post or pier.

Never allow a shim to hang in the air or support a load at its thin, tapered edge. If the gap is wide, use a shim pair, driving them from opposite sides so they overlap and provide a level, uniform bearing surface for the beam.

Check for “shim creep” during installation. Use a light adhesive if necessary to hold the shims in place until the weight of the structure pins them, ensuring they do not vibrate loose before the frame is locked down.

Understanding Load Points and Shim Placement

Structural loads travel vertically from the roof, through the beams, down to the foundation. Every shim introduced into this path creates a potential point of failure if it is not placed directly beneath the load-bearing member.

If you shim the center of a beam span instead of the support point, you are effectively creating a fulcrum that could cause the beam to crack under stress. Always maintain the integrity of the load path by shimming at the bearing points only.

Observe the grain direction when using wood shims. Always position the grain parallel to the load, as wood is significantly stronger in compression when loaded along the grain than across it.

Are Shims a Permanent Fix for a Sagging Beam?

Shims are an excellent tool for correcting initial level issues during construction, but they are a symptomatic fix for a sagging beam that has already begun to deflect. If a beam is sagging, shimming it only addresses the gap, not the underlying cause of the structural failure.

Before shimming, consult a structural engineer or a licensed contractor to determine why the beam sagged in the first place. Overloaded joists, failing posts, or foundation settlement are common culprits that require a permanent, structural solution like reinforcing the beam or adding support columns.

Treat shims as the final alignment step, not as the primary structural support. If a beam is sagging, jack it back to its original position using proper temporary shoring before installing permanent, high-strength shims to lock it in place.

Effective leveling is as much about material selection as it is about understanding the physics of the load path. By using the right shim for the specific environment and pressure requirements, you ensure that the home’s skeleton remains rigid and true for decades. When in doubt, prioritize density and moisture resistance, and always verify that your shims are fully supporting the beam’s intended bearing surface.