6 Best Steel Shims for Gap Adjustment

Precision gap adjustment is the difference between a roof frame that holds steady for decades and one that develops an annoying, structural creak within months. Whether leveling a steel base plate or shimming a structural header, the right metal shim prevents load distribution failures that lead to long-term sagging. Avoiding the wrong material here is vital, as compressed debris or flimsy alternatives will eventually fail under the weight of heavy-duty roofing or framing loads.

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Precision Brand Slotted Shims: Best Overall

Precision Brand slotted shims remain the industry standard because they strike the perfect balance between rigidity and ease of use. These shims allow for quick lateral insertion around a bolt without needing to fully back out the hardware, which saves immense time when adjusting structural columns on a commercial build.

The material consistency is key, as these shims maintain their specified thickness under heavy compression. If a steel beam isn’t sitting flush on a concrete pier, slipping one of these into the gap ensures a solid metal-on-metal contact point. Relying on makeshift materials like wood or plastic in these scenarios invites future settling that ruins your work.

Bottom line: Keep a variety of these in the tool shed for any situation requiring precise, load-bearing leveling.

C.S. Unitec Magnetic Shims: For Steel Framing

Working with vertical steel columns or overhead assemblies often requires a third hand just to keep the shim in place while the fastener is tightened. C.S. Unitec magnetic shims solve this by adhering directly to the structural steel, allowing for hands-free adjustment. This is particularly valuable when working on a high-slope roof structure or any environment where you are tethered and need to minimize extraneous movement.

These shims are purpose-built for scenarios where precise vertical alignment is required before full torque is applied to the bolts. Because they don’t shift during the tightening process, the risk of a misaligned joint is significantly reduced. They are specialized tools that pay for themselves by eliminating the frustration of dropped shims in hard-to-reach framing corners.

Bottom line: If the project involves structural steel framing, the magnetic feature justifies the premium price.

Fastenal Assorted Shim Kit: Best for The Truck

Job-site logistics demand versatility, and an assorted kit from Fastenal covers the unexpected gaps that appear during complex framing jobs. Having a range of thicknesses—from paper-thin to heavy-gauge—allows for micro-adjustments that prevent uneven load distribution. A well-stocked kit prevents the temptation to “make it work” with inferior substitutes when a specific gap size isn’t met.

These kits are best kept in the truck for immediate access when a structural member isn’t sitting true. Using the correct thickness immediately eliminates the need for rework, keeping the site productivity high. Always prioritize a kit that includes stainless steel options if the project site is located in a coastal zone where salt air accelerates oxidation.

Bottom line: The ultimate “ready for anything” insurance policy for site-based repairs and adjustments.

McMaster-Carr Shim Stock: For Custom-Cut Jobs

Sometimes the gap isn’t standard, and the bolt spacing requires a shim shape that simply doesn’t exist on the shelf. McMaster-Carr allows for the purchase of high-quality shim stock in sheets, which can be custom-cut to fit non-standard base plates or irregular structural joints. This is the professional choice for custom fabrication where precision dictates the structural integrity of the entire assembly.

Cutting your own shims requires snips or a cold saw, but the outcome is a perfectly tailored fit that no off-the-shelf slotted shim can match. Be mindful of the edges after cutting, as steel burrs can act as stress risers. A quick pass with a file ensures a smooth finish that won’t compromise the bearing surface of the joint.

Bottom line: When a custom job demands a custom fit, buying bulk stock is the only professional approach.

Grainger U-Shaped Shims: For Bolt Alignment

Grainger’s U-shaped shims are the workhorse for leveling equipment pads or machinery anchors. The design provides a stable, wrap-around fit that ensures the shim stays centered under the nut and washer assembly. In scenarios where heavy vibration is expected, this stable profile prevents the shim from “walking” out of the joint over time.

These shims are frequently overlooked in basic framing but are indispensable when installing mechanical roof units or heavy HVAC curbs. Proper seating under a bolt reduces the point-load stresses that cause bolts to snap or anchors to loosen under vibration. Investing in a set of these ensures that your mounted equipment remains level throughout its lifecycle.

Bottom line: A must-have for any installer dealing with mechanical attachments on a flat or low-slope roof.

All-Spec Solid Shims: For Full-Bearing Support

All-Spec solid shims provide maximum surface area contact, which is essential when the entire base plate must be supported to prevent buckling. Unlike slotted shims, these require the bolt to be fully removed, which makes them more labor-intensive. However, the trade-off is superior load transfer, especially in heavy-duty structural applications where gaps exceed the norm.

Use solid shims when you are dealing with significant structural spans where precise, uniform bearing support is required to avoid localized deformation. Because these lack the “weak point” of a slot, they are generally stronger and provide more consistent structural stability. They are the premium choice for permanent, high-load installations.

Bottom line: Use solid shims when the structural integrity of the joint is the absolute priority over installation speed.

How to Properly Measure Gaps for Steel Shims

Measuring a gap for a shim is not as simple as using a tape measure; you need accuracy down to the thousandth of an inch. Use a feeler gauge to determine the exact clearance beneath a beam or plate. If a gap varies across the length of a base plate, measure at multiple points to ensure the shim thickness accounts for the highest point of interference.

Avoid guessing the thickness, as even a 1/16th-inch error can lead to significant bolt stress. If the gap is wide, measure the deflection of the structural member while the load is partially applied. This “loaded gap” measurement ensures the shim will be snug once the full weight of the assembly is locked down.

The Do’s and Don’ts of Stacking Steel Shims

Never stack more than three shims in a single joint, as the cumulative surface error becomes a failure point. The more pieces you add to a stack, the more “spring” or potential movement is introduced into the connection. Always aim to use the fewest number of shims possible, ideally a single shim that matches the required gap.

If you must stack, arrange them by thickness with the thickest shim on the bottom and the thinnest on top. This maintains a more stable bearing profile and reduces the likelihood of the stack splaying under pressure. Never mix materials—if you start with stainless steel, finish with stainless steel to avoid galvanic corrosion.

Slotted vs. Solid Shims: What’s The Difference?

Slotted shims are engineered for speed and convenience, allowing for installation without dismantling the joint. They are excellent for fine-tuning leveling where structural loads are well-distributed across multiple points. The slot itself is a structural trade-off, as it reduces the overall surface area for load-bearing compared to a solid counterpart.

Solid shims provide complete contact, which is ideal for high-pressure points or where vibration resistance is critical. They lack the convenience of the slot but offer superior rigidity and longevity in high-stress environments. Choose slotted for initial leveling and adjustment, and reserve solid shims for permanent, high-load foundation or support anchoring.

Carbon vs. Stainless Steel: Which Shim Is Best?

Carbon steel is a durable, cost-effective choice for internal, dry environments where the shim won’t be exposed to moisture. It handles high-compression loads exceptionally well and is standard for most industrial framing applications. The primary trade-off is its susceptibility to rust, which means it should always be painted or coated if used in an environment with high humidity.

Stainless steel is the mandatory choice for exterior work, roof-mounted installations, or any area prone to condensation and water accumulation. While it is more expensive, it eliminates the risk of hidden corrosion expanding and loosening the joint over time. If you are working on a roof in a coastal zone, never spec carbon steel; the long-term maintenance costs and potential for structural failure simply aren’t worth the upfront savings.

Properly shimming a steel connection is a foundational skill that separates a amateur fix from a professional structural repair. By choosing the right shim material and thickness for the environment, you ensure that the load-bearing integrity of the assembly remains uncompromised for years to come. Regardless of whether the job requires the speed of a slotted shim or the unyielding support of a solid one, consistency in selection and precision in measurement will always be the best insurance against future site callbacks.