6 Best Structural Grade Shims for Uneven Concrete Substrates

Uneven concrete substrates act as the silent enemy of a level, structural installation. Whether setting base plates for steel columns or mounting heavy-duty roof equipment supports, a gap between the substrate and the hardware creates a point of failure. Proper shimming bridges this gap, ensuring uniform load transfer across the entire footprint. Choosing the correct structural shim is the difference between a rock-solid build and a long-term liability.

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Grove Shims: Best Overall High-Strength Shim

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Grove shims are recognized for their exceptional compressive strength, making them a go-to for standard structural steel applications. The material density prevents the “creep” often seen in cheaper plastic alternatives.

When bolting down heavy equipment or structural steel, maintaining a consistent gap is critical to preventing bending moments in the hardware. These shims provide a rigid, predictable base that stands up to the torque applied during installation.

Reliability in the field comes down to consistency. Because these are manufactured to precise tolerances, stacking them doesn’t lead to the unpredictable “mushiness” that compromises structural integrity.

Deslauriers E-Z Shims: Best for Heavy Loads

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When dealing with massive dead loads—such as those found in industrial mechanical units or multi-story structural steel—the E-Z Shim series provides a massive surface area to distribute force. These are engineered specifically for high-capacity projects where failure is not an option.

The structural design includes a notched profile that allows for easier placement around anchor bolts. This saves significant time on the job site when dozens of base plates require leveling in a single shift.

The primary advantage here is the sheer density of the composite material. It refuses to compress under extreme pressures, ensuring the gap remains perfectly closed for the life of the installation.

BigFoot Shims: Most Versatile Stacking System

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BigFoot shims feature an interlocking design that prevents slippage when multiple layers are required. In scenarios where the concrete slab is severely out of level, the ability to stack without sliding is a major safety and quality advantage.

The design distributes weight across the entire footprint of the shim, reducing the likelihood of point-loading the concrete below. This is essential when working on older, cured slabs where surface spalling could become an issue.

These are an ideal choice for contractors who need a modular solution. Carrying a single, versatile stackable system keeps the truck lean and ensures the right thickness is always available.

US-Shim Packs: Best Value for High Volume Jobs

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For large-scale projects where thousands of shims are required for wall plates or perimeter flashings, US-Shim packs offer the most logical economic solution. They provide consistent performance at a price point that doesn’t eat into the bottom line of a competitive bid.

While they may lack the high-end additives of specialized composite brands, they meet industry requirements for structural compression. They are a workhorse product meant for standard duty.

Effective site management relies on using materials that match the project scope. Do not overspend on specialty high-pressure shims if the application is simple base-plate leveling for light-gauge steel.

Precision Steel Shims: For Extreme Point Loads

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Steel shims are non-negotiable when dealing with extreme point loads or fire-rated assemblies. Unlike plastics or composites, steel will not melt, degrade, or deform under the intense heat or crushing weight that can occur in industrial settings.

When shimming steel-to-steel connections, the thermal expansion rates remain consistent, preventing internal stress buildup. This is vital in environments with significant temperature fluctuations, such as rooftop equipment support frames.

Be aware that steel shims require proper corrosion protection. Always verify that they are galvanized or coated if used in an exterior, moisture-prone environment to prevent rust staining or structural degradation.

G-Shim Composite Shims: Best for Corrosion Risk

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G-Shim composite shims excel in wet environments or high-moisture climate zones. If the project involves mounting systems on a flat roof where standing water or ice dams could be a factor, these prevent the wicking and corrosion common with metal shims.

The composite material is essentially inert. It won’t react with the concrete or the fastener, meaning no galvanic corrosion will occur at the contact point.

This is the expert’s choice for projects near salt air or high-humidity regions. They provide the strength needed to hold a load while remaining completely indifferent to environmental weathering.

Plastic vs. Steel vs. Composite: Which to Use

• Plastic: Use for non-structural, light-duty, or temporary leveling. Low cost, but prone to creep and UV degradation.
• Steel: Use for heavy-duty, high-heat, or extreme load-bearing applications. Requires corrosion management.
• Composite: The industry standard for structural applications. Resistant to moisture, high compressive strength, and chemically inert.

Selecting the wrong material usually results in “set-down,” where the shim compresses over time, leading to loose bolts and rattling connections. Never compromise on material grade when the shim is supporting a load-bearing column.

Always match the shim material to the substrate and the fastener. A mismatched pair often leads to galvanic corrosion, which degrades the metal over time and eventually compromises the entire anchor system.

How to Properly Stack and Grout Your Shims

Stacking shims should be done with minimal layers whenever possible. The more layers used, the higher the risk of the stack becoming unstable or sliding under vibration.

If a gap exceeds one inch, stop shimming and prepare for a dry-pack or non-shrink grout pour. Shimming beyond this depth creates a cantilever effect that can snap bolts under seismic or wind loads.

Ensure the shim footprint is as wide as the base plate it supports. A narrow shim under a wide plate creates a fulcrum, which can crack the concrete or bend the steel base plate under tension.

Reading Shim Specs: Compressive Strength 101

Compressive strength is the most important number on any shim spec sheet. Measured in PSI (pounds per square inch), it dictates how much weight the shim can bear before it suffers permanent deformation.

Most structural composite shims range from 8,000 to 12,000 PSI. Always ensure the shim’s rating exceeds the actual load transferred by the structure by a safety factor of at least two.

Ignore marketing claims and look directly at the testing standards. Look for ASTM D695 compliance to ensure the manufacturer has verified the load-bearing capacity through standardized, repeatable testing.

When to Use Grout Instead of Shimming a Gap

Shims are for leveling, not for filling large voids. If the gap is substantial, the shim stack acts as a point load, which can actually damage the concrete slab by concentrating force into a small area.

For gaps wider than one inch, or where the load is dynamic and heavy, non-shrink structural grout is the superior choice. Grout provides 100% surface-to-surface contact, eliminating the risk of internal voids.

The rule of thumb is simple: If you can slide a shim into the gap, use a shim. If you need a masonry trowel to fill it, use structural grout.

The integrity of a structure is only as strong as the connection to its substrate. By prioritizing high-strength, compatible shim materials, you eliminate the risk of premature settling and fastener failure. Selecting the right tool for the gap ensures the project stays square, level, and secure for decades to come.