7 Best Floor Leveling Shims for Sagging Joists That Last

Floorboards that dip or bounce when walking across a room signal more than just an aging house; they indicate a structural gap that needs immediate attention. Correcting sagging joists restores the integrity of the home’s skeleton and prevents long-term damage to finished flooring and subflooring. Choosing the right shim ensures the repair remains stable, quiet, and secure for years to come.

Simpson Strong-Tie SH Shims: For Heavy Support

When a sagging floor is caused by heavy structural loads or a joist that has significantly deflected over time, standard plastic wedges often fail. These steel shims are engineered for high-capacity load bearing, making them the industry standard for serious floor leveling projects. Their rigid construction ensures they will not compress or crack under the weight of heavy furniture or appliances.

Because these shims are made of galvanized steel, they resist corrosion, which is vital if the joists are located in a basement or crawl space with fluctuating humidity levels. They are thin, allowing for precise adjustments when only a minor lift is needed to bring a joist back into alignment.

• Best for: Large-scale renovations and heavy-duty load correction.
• Bottom line: If the floor sag is significant, do not gamble with lighter materials; go with the structural integrity of steel.

Nelson Composite EZ-Shims: Best All-Purpose Pick

For the average homeowner dealing with mild squeaks or slight unevenness in a bedroom or hallway, composite shims offer a perfect balance of durability and ease of use. These shims do not split or rot like wood, yet they are easier to cut and adjust than steel options. They are ideal for DIYers who want a reliable, “set-it-and-forget-it” repair without needing professional machinery.

The composite material provides enough flexibility to conform slightly to the joist’s surface, ensuring a tight fit that stops movement. They work well with standard construction adhesives to lock the shim into place permanently.

• Best for: Routine maintenance and fixing common floor squeaks.
• Bottom line: The most versatile choice for standard household floor leveling.

Precision Brand Steel Shims: For Permanent Fixes

Sometimes a floor requires a “shim and forget” approach, especially in older homes where the joists have reached their final settling point. Precision Brand shims are available in various thicknesses, allowing for surgical accuracy when leveling a subfloor before installing tile or hardwood. Their uniform thickness ensures that the entire length of the joist receives even support.

Because steel does not shrink or expand with seasonal temperature changes, these shims eliminate the risk of the floor “re-settling” a few months after the repair. They are an investment in the long-term stability of the home’s structure.

• Best for: Precision work before installing new flooring.
• Bottom line: Choose these when perfection and permanency are the primary goals.

Wobble Wedges: Best for Precise, Stackable Lifts

Wobble Wedges are a staple in many toolboxes because of their unique design that allows for easy, incremental stacking. If a joist has a varying degree of sag across its length, these wedges can be inserted and nested to match the exact curve of the wood. They are also non-conductive and non-marking, which protects the surrounding materials during installation.

The textured surface of these wedges provides excellent grip, ensuring they do not slip out of place once the load is reapplied. They are particularly effective for quick fixes in tight, hard-to-reach areas where a hammer and large tools are difficult to maneuver.

• Best for: Irregular gaps that require varying thickness along a single span.
• Bottom line: A highly practical solution for tricky, uneven spaces.

G-Pro Plastic Shims: Top Value for Large Projects

When leveling an entire room, the cost of shims can add up quickly. G-Pro plastic shims provide an affordable, durable solution for those managing a larger project, such as renovating a dining room or a full basement suite. They are lightweight, easy to transport, and can be snapped or cut to length with simple hand tools.

While they are plastic, they are dense enough to support the floor without deforming under normal foot traffic. They are an excellent way to bridge gaps across multiple joists without exceeding a modest renovation budget.

• Best for: Large-scale subfloor leveling on a budget.
• Bottom line: Unbeatable value for projects that require a high quantity of shims.

Contractor-Grade Cedar Shims: A Tried-and-True Fix

Many traditional homes were built using wood shims, and cedar remains a popular choice for its natural resistance to moisture and decay. These shims offer a degree of friction that plastic and steel cannot match, as the natural grain bites into the joist and the subfloor to prevent movement. They are the traditional choice for tightening up floor joists in historic or older homes.

However, keep in mind that wood can compress over time, especially if the joist movement is dynamic. Always ensure that the wood is dry before installation to prevent it from shrinking and creating a new squeak in the future.

• Best for: Traditional homes where compatibility with existing materials is preferred.
• Bottom line: A classic, reliable option, provided the wood is high-quality and moisture-free.

Lumberock Plastic Shims: For Damp Crawl Spaces

Crawl spaces are notorious for high moisture levels, which can turn standard wood shims into a breeding ground for mold and rot. Lumberock shims are specifically designed for harsh environments; they are essentially impervious to water, insects, and decay. They provide a structural bridge that remains stable regardless of how damp the environment becomes.

These are the go-to for homeowners living in high-humidity climates or those with homes that have historically damp basements. They provide peace of mind knowing that the repair will not degrade, even if the foundation conditions are less than ideal.

• Best for: High-moisture areas and homes in humid climates.
• Bottom line: The ultimate insurance policy against moisture-related structural decay.

Steel vs. Composite vs. Wood: Which Shim is Best?

Choosing the right material depends on the environment and the degree of the sag. Steel shims are the strongest but are the most expensive and least forgiving to work with. Composite materials are the modern “middle ground,” offering a blend of strength and ease of use that suits most household repair needs.

Wood is the most affordable and provides excellent friction, but it is vulnerable to moisture and long-term compression. Evaluate the location of the repair—if it is under a kitchen or bath, prioritize moisture resistance; if it is for structural support under a heavy piano or entertainment center, prioritize the rigidity of steel.

• High load: Choose steel.
• General use: Choose composite.
• Moisture prone: Choose plastic.

How to Measure Joist Sag for the Right Shim Size

Before purchasing shims, you must identify the exact depth of the sag. Use a long, straight board (a “straightedge”) or a laser level to span across the joist. By measuring the distance between the straightedge and the top of the sagging joist at various points, you can determine how many shims are needed and what thickness is required.

Never guess the measurement, as inserting a shim that is too thick will create a “hump” in the floor, which is often more noticeable than the original sag. Always account for the thickness of the adhesive if you plan to glue the shim into place.

• Step 1: Place a long, level straightedge over the joist.
• Step 2: Measure the gap at the deepest point of the sag.
• Step 3: Add the thickness of the adhesive to your total measurement.

When to Shim a Joist and When to Call a Pro

Shimming is a perfectly safe and effective solution for minor to moderate floor sag caused by standard settling. However, if the sag is accompanied by cracks in the foundation, signs of termite damage, or if the joist itself shows signs of rot or splitting, a shim is only a band-aid. These issues point to structural failure that requires a professional assessment.

If the floor feels “spongy” over a wide area, the joists may be undersized or under too much stress, necessitating additional support like a sistered beam or an added lally column. When in doubt, have a qualified contractor inspect the crawl space to ensure the floor system is safe to support the weight of the home.

• Shim it if: The issue is minor, localized, and the wood is structurally sound.
• Call a pro if: You see rot, deep cracks, foundation movement, or widespread structural fatigue.

Addressing floor sag promptly prevents a minor nuisance from becoming a significant structural repair. By selecting the right shim material and confirming the extent of the gap, most homeowners can restore a level floor with minimal effort. Consistent maintenance in this area preserves both the comfort and the long-term value of the home.