6 Best Leveling Shims For Subfloor Board Installation

A flat, stable subfloor is the foundation of every successful flooring project, yet many installers skip the critical step of addressing minor dips and unevenness. Ignoring these imperfections leads to squeaky floorboards, cracked tile grout, and premature wear on finish surfaces. Shimming is not merely a suggestion; it is the industry-standard method for ensuring a level plane for hardwood, tile, or luxury vinyl planks. Investing time in the proper materials now prevents the nightmare of pulling up finished floors to fix a bounce three years down the line.

Disclosure: As an Amazon Associate, this site earns from qualifying purchases. Thank you!

Nelson Wood Shims: The All-Around Workhorse

Nelson wood shims remain the classic choice for a reason. These kiln-dried pine wedges offer excellent compression strength, which is essential when the goal is to transfer the load of heavy furniture or foot traffic directly to the joists.

They are incredibly easy to trim on-site with a utility knife or a chisel. Because they are porous, they bond exceptionally well with wood glue or construction adhesive, effectively becoming one with the joist structure.

The tradeoff is susceptibility to moisture. In basements or bathrooms where humidity levels fluctuate, plain wood shims can eventually shrink or swell, potentially leading to slight movement in the subfloor.

Wobble Wedges: Best for Precision Leveling

Wobble Wedges are not your typical tapered shims. These are high-impact, interlocking plastic shims designed for micro-adjustments where precision is the absolute priority.

They feature patented ridges that “lock” into place, preventing them from sliding out of position while the subfloor panels are being installed. This is a massive advantage when working alone or dealing with awkward spans.

Because they are rigid plastic, they do not compress like wood. This makes them perfect for leveling under concentrated loads where even a fraction of an inch of “give” would cause a squeak.

EZ-Shim Composite Shims: Won’t Split or Rot

Composite shims represent the evolution of the material, combining the workability of wood with the environmental stability of plastic. EZ-Shim composite options are specifically designed to resist splitting, which is a common failure point when driving nails or screws through a traditional cedar wedge.

They maintain a consistent thickness under load, unlike wood which may compress over time. This makes them a reliable choice for long-term installations where you want to “set it and forget it.”

While slightly more expensive than traditional wood, the durability pays off in labor savings. You won’t be wasting time discarding shims that split the moment a fastener hits them.

Builder’s Best Cedar Shims: Top Rot-Resistance

Cedar is the go-to wood species for any area prone to moisture. If the subfloor installation is happening in a crawlspace or a room with exterior exposure, natural cedar shims are the professional’s choice due to their inherent tannins.

These tannins act as a natural deterrent to mold, fungus, and wood-boring insects. They are slightly more expensive than pine, but the protection against rot is invaluable in high-humidity zones.

Be aware that cedar is softer than dense pine or composite materials. If you are shimming a high-traffic area, ensure the shim is fully supported by the joist beneath to prevent it from crushing over time.

Go-Ez Color-Coded Shims: For Fast Easy Sizing

Speed is the primary currency on any job site, and Go-Ez color-coded shims eliminate the guesswork. When you are crawling through a tight floor cavity, identifying the correct shim thickness by sight rather than feel saves precious time.

The color-coding system ensures that different areas of the room stay consistent. If the low spot requires a 1/4-inch lift, grabbing the corresponding color ensures the entire subfloor stays level across the span.

These are typically manufactured from high-density materials that offer a balance between ease of installation and structural integrity. They are ideal for large-scale projects where consistency across multiple rooms is a logistical priority.

Pound-Ez Plastic Shims: Most Durable Shim Option

Pound-Ez plastic shims are engineered for brute force. These are the shims to choose when installing heavy subfloor sheathing or working on subfloors that will support heavy stone or ceramic tile.

They have an extremely high compressive strength, meaning they won’t flatten out under the weight of the floor and the people walking on it. They are also completely impervious to water, making them the ultimate solution for laundry rooms or mudrooms.

The primary disadvantage is that they can be slippery. When using plastic shims, apply a dab of construction adhesive to keep them from migrating before the subfloor panels are secured.

Finding High and Low Spots in Your Subfloor First

Before reaching for a box of shims, the entire subfloor must be mapped. Use a long, straight edge—preferably a six-foot level—to scan across the joists and identify dips.

A laser level is the most accurate tool for finding high spots. If you see a hump in the middle of a room, shimming the rest of the floor won’t help; you must plane down or sand the high spot first to establish a true baseline.

Mark the low spots with a pencil and indicate the approximate depth needed. A standard tape measure or a shim gauge can confirm the gap height, ensuring you don’t over-shim and create a new high spot.

Wood vs. Plastic Shims: Which Is Right for You?

The decision between wood and plastic should be dictated by the location and the finish flooring. Wood shims are generally preferred for interior living spaces where they can be glued to the framing, creating a quiet, monolithic structure.

Plastic shims are superior in high-moisture environments or when dealing with concrete slab applications. They provide a permanent, rot-proof barrier that won’t move regardless of the humidity level.

If you are using screws to fasten the subfloor, plastic shims are often better because they won’t split when the screw thread engages. Wood shims perform better when held in place with pneumatic finish nails or construction adhesive.

How to Properly Install and Secure Subfloor Shims

Shims must be placed directly over the joists, not in the gaps between them. If a shim is placed over a void in the subfloor sheathing, it will inevitably lead to a soft spot and eventual cracking of the finish floor.

Always ensure the shim is long enough to cover the width of the joist. A thin shim hanging off the edge of a joist will crush under the weight of the floor, defeating the purpose of the leveling effort.

Once the subfloor panel is positioned, ensure it is fastened through the shim into the joist. Using an adhesive between the shim and the subfloor is recommended to prevent squeaks caused by the wood panels rubbing against the shim surface.

Understanding Shim Load Ratings for a Stable Floor

While shims are small, they bear the full weight of the floor system and the occupants. For residential projects, standard-grade wood or plastic shims are usually sufficient.

However, if you are installing heavy tile, stone, or high-density furniture, verify the manufacturer’s load ratings on the packaging. Overloaded shims can undergo “creep,” a process where the material slowly deforms under constant pressure.

If the floor feels bouncy, it is rarely the fault of the shim material itself, but rather the spacing or the support. Ensure the joist span is adequate for the floor load before attempting to fix it with shims alone.

Successful subfloor leveling is the difference between a floor that lasts a decade and one that lasts a lifetime. By selecting the right shim for the job—whether for moisture resistance, precision, or structural load—the finished surface will remain flat and silent. Take the time to map the floor, choose quality materials, and secure them firmly to guarantee a professional result that stands the test of time.