Rack Shim: What It Is And When Warehouses Should Use One

Rack Shim A leveling plate placed under rack uprights to compensate for uneven floors. Rack shims are thin pieces of material—commonly steel or polymer—inserted between a rack upright's baseplate and the concrete floor to correct small variations in elevation, align column lines, and stabilize pallet racking on imperfect slab surfaces.

What A Rack Shim Does

Rack shims level individual uprights so rows of pallet racks run true along aisles. They remove gaps under baseplates that would otherwise concentrate load on a small contact area and can reduce rocking, migration of loads, and cumulative misalignment across a rack bay. When installed correctly, shims restore positive bearing between the rack baseplate and the floor while maintaining the designed vertical load path.

Why Leveling Matters

An uneven rack undermines the structural behavior manufacturers design for. Small vertical offsets at multiple uprights add bending and eccentric loading to columns, increase bolt shear, and change beam-seat reactions. Consequences include premature component wear, beam disengagement, and a higher risk of collapse during impact or seismic events. Leveling with shims is a low-cost step that preserves capacity and reduces repair costs.

Typical Materials And Profiles

• Steel Shims: Cold-rolled or stainless steel plates, thicknesses from 1/32" to 1/4" per shim layer; used where heavy loads and chemical/temperature exposure are concerns.
• Galvanized Shims: Zinc-coated steel for corrosion resistance in damp environments.
• Polymer Shims: High-density plastics used for minor adjustments and where galvanic corrosion between dissimilar metals must be avoided.
• Stacked/Stepped Shims: Pre-cut stepped plates that allow incremental height changes without trimming on site.

How Leveling Methods Vary

Shimming is one of three common strategies for addressing uneven floors: grouting, surgeoning (machining of the baseplate), and shimming. Grouting creates a permanent bearing surface beneath the baseplate and is the recommended solution for offsets larger than about 1/8"–3/16" or where a structural connection must be continuous. Shims are faster and cheaper but are considered appropriate for small, localized corrections and temporary adjustments prior to grouting.

When To Use Rack Shims

Use shims when floor variation is minor, when immediate operational continuity is required, or as a temporary measure while planning a grout repair. Typical triggers include new racking installations on older slabs, small repairs after dock modifications, or correcting isolated uprights that sit on saw cuts or surface irregularities.

Installation Considerations

Correct shim installation preserves capacity and safety:

• Bearing Area: Position the shim so the baseplate bears fully on the shim; never allow the shim to sit outside the plate edge.
• Stacking Limit: Limit the number of stacked thin shims; combine thicknesses to produce a single solid piece when possible to avoid slip between layers.
• Fastening: Tighten anchor bolts to the manufacturer’s torque specification after shimming to compress the shim and create full-bearing contact.
• Compression and Fill: If the gap exceeds manufacturer guidance, use non-shrink grout instead of relying on shims to transfer load.

Inspection And Compliance

Document shim use during installation and include shim condition in routine racking inspections. Inspectors should look for bent or corroded shims, shim migration, and gaps between shim and baseplate. Where building codes, seismic requirements, or the rack supplier’s recommendations disallow certain shim materials or thicknesses, follow the stricter specification.

Practical Example: Small Distribution Center

A 3PL installs a new 3‑bay rack line on an existing concrete slab. Several uprights exhibited a 1/16"–3/32" depression due to finish troweling. Installers used stainless steel shims in matched thicknesses under the affected baseplates and torqued anchors per rack supplier specs. The line was later permanently grouted during planned downtime, but the shims allowed immediate, safe operation without misalignment.

Common Mistakes To Avoid

• Underestimating Gap Size: Relying on shims when the gap exceeds recommended thickness risks bearing failure.
• Poor Placement: Allowing shims to hang off the baseplate edge reduces effective bearing area.
• Unrecorded Temporary Fixes: Leaving temporary shims indefinitely without scheduling grout or repair invites long-term deterioration.

In short, the Rack Shim is a practical, cost-effective tool for correcting minor floor irregularities beneath rack uprights. When selected and installed according to load, material, and manufacturer guidance, shims restore bearing, align rows, and postpone or eliminate more invasive repairs.