Which shimming material is approved behind studs on uneven masonry walls?

While InSoFast documentation does not list specific brand-name shims, non-compressible plastic horseshoe shims, high-density polyethylene composite shims, and structural polyurethane adhesive foam are industry-approved materials to plumb studs against uneven masonry walls.

Uneven, bowed, or out-of-plumb concrete and brick foundations across the United States and Canada routinely deviate by 1/2 inch or more across an eight-foot vertical span under ACI 117 concrete tolerances. Standard timber shims fail behind basement framing because capillary moisture from damp masonry causes wood to rot, mold, and compress over time.

If you only do one thing: Never install organic wood shims against raw masonry; use rot-proof plastic horseshoe shims or expanding polyurethane foam adhesive to bridge substrate gaps up to 1/2 inch behind stud attachment points.

• High-density plastic horseshoe shims: Slotted polypropylene or polyethylene shims slide directly around masonry fasteners, providing non-compressible solid backing that resists moisture absorption and prevents wallboard deflection.
• Polyurethane foam adhesive bridging: Expanding construction-grade polyurethane foam adhesive injects directly behind stud flanges to fill irregular voids between 1/8 inch and 3/8 inch, curing within 24 hours to create continuous structural backing.
• Fastener embedment depth: Concrete screw fasteners, such as 3/16-inch or 1/4-inch masonry anchors, must maintain a minimum 1-1/4 inch embedment depth into solid concrete, requiring longer screws whenever shim thickness exceeds 1/4 inch.
• Plumb reference plane alignment: Snapping chalk lines on the concrete slab floor and overhead framing establishes a true plumb line, allowing builders to gauge precise shim thickness across 16-inch or 24-inch stud centers.
• Stud flange support: For continuous insulation panels with built-in composite studs, placing plastic shims or low-expansion adhesive directly behind the internal stud eliminates hollow flex without introducing thermal bridging (heat loss through structural framing).

• Watch out for: Using cedar or pine wood shims against basement foundation walls, which absorb concrete moisture and decay within two to five years.
• Watch out for: Over-tightening masonry screws across unsupported hollows, which pulls the stud flange out of alignment and causes wavy drywall seams.
• Watch out for: Leaving gaps wider than 3/8 inch un-shimmed behind studs, which allows wallboard to flex and crack under standard furniture loads.

Map the wall surface using a 6-foot carpenter level or laser to locate high spots, then stock 1/16-inch to 1/4-inch plastic horseshoe shims before driving masonry fasteners.

General information only, not medical advice. Decisions about health, treatment or wellbeing should be made with a doctor or other qualified healthcare professional.