6 Best Pressure Treated Shims For Wood Framing Adjustments

Framing a structure requires precision, and even a slight discrepancy in a sill plate or window opening can compromise the entire integrity of a build. Pressure-treated shims serve as the essential, rot-resistant buffer between foundation concrete and wood, or between framing members in moisture-prone zones. Selecting the right shim isn’t just about filling a gap; it is about ensuring that a load-bearing connection remains stable and protected against decay over decades. Utilizing improper materials in these critical areas can lead to structural settlement or premature rot that is nearly impossible to repair once the building is closed in.

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Nelson Treated Shims: The Go-To Contractor Pack

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Nelson treated shims are a staple on residential job sites because they offer a consistent thickness and reliable chemical treatment. When a crew is framing multiple door and window openings, having a stack of shims that don’t split under the pressure of a driven fastener is a massive time-saver.

These shims are typically cedar or pine, treated specifically to resist termites and fungal growth. They excel in general framing where minor adjustments are needed to bring headers or studs into true plumb.

Because they come in large, bundled contractor packs, they minimize waste and reduce the need for constant supply runs. For the contractor who values uniformity, Nelson is the standard for high-volume, standard-tolerance framing work.

Severe Weather PT Shims: Ultimate Rot Resistance

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When a build involves contact with masonry, such as setting sill plates on a basement wall or slab, standard wood won’t cut it. Severe weather treated shims are infused with deeper, high-performance preservatives that provide a superior barrier against direct ground contact and moisture wicking.

These shims are dense and manufactured to withstand the heavy compression of load-bearing walls. In climates with high humidity or frequent freeze-thaw cycles, this added level of protection is non-negotiable to prevent the shim from becoming a conduit for rot.

While they may cost a premium compared to basic shims, the structural insurance they provide is worth the investment. Use these specifically in “wet zones” where standard treated lumber might struggle to hold up over a thirty-year lifespan.

HDX Heavy-Duty Shims: Best Value for Big Projects

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Large-scale framing projects often require shims by the pallet, making cost-per-unit a significant factor in the bottom line. HDX shims offer a balance between sufficient treatment levels and a price point that makes them accessible for DIYers and contractors alike.

These shims are designed for high-stress applications where precise leveling is required across wide spans, like long runs of floor joists. They are rigid enough to hold their shape under the weight of heavy sub-flooring without crushing.

If the goal is to manage a budget while maintaining structural code compliance, HDX serves as an excellent workhorse. They handle general framing adjustments with ease, provided the project is not in a high-submersion or constant-dampness environment.

Blue Hawk Treated Shims: A Solid DIY Option

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Blue Hawk shims are frequently found in local hardware stores, making them the most accessible option for someone mid-project. They provide an adequate level of treatment for light-duty framing, such as exterior trim or secondary interior walls.

These shims are uniform enough to provide a stable shim profile for window installation. However, they are generally softer than professional-grade specialty shims, so careful handling is required to avoid splitting them during installation.

For a homeowner fixing a sagging door frame or leveling a small interior partition, these are perfectly suited. They save time on logistics while providing enough rot resistance for standard above-grade interior or light exterior work.

Nelson Jumbo Treated Shims: For Big Adjustments

Sometimes a framing member is significantly out of square or a foundation wall has a deep dip, and a standard shim simply isn’t thick enough. Jumbo shims provide the additional height needed to bridge these gaps without having to stack multiple thin shims, which can become unstable.

Stacking too many shims creates a “slippery” connection that can shift under a load. Using a single, larger shim maintains the structural integrity of the gap fill and keeps the load path direct.

Always check the bearing surface when using jumbo shims to ensure they are fully supported underneath. A thick shim hanging in the air is a recipe for failure; ensure the gap is filled consistently across the entire footprint of the shim.

CMPC Treated Bed Shims: For Sill Plate Leveling

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Sill plates must be perfectly level to ensure the entire house frame sits square, which is why specialized bed shims are preferred. These are engineered specifically for the interface between the concrete foundation and the wood sill plate.

These shims are manufactured to handle the specific compression loads of a building’s weight. They provide a wider surface area, which helps distribute that weight evenly across the foundation wall rather than creating concentrated pressure points.

For any project involving the base of the framing, use these over standard shims. They are specifically graded for the high-moisture, high-load environment found where the house meets the earth.

When to Use Treated vs. Standard Wood Shims

Treated shims must be used anywhere the shim will be in contact with masonry, concrete, or exterior-grade lumber. Any wood that touches a foundation, a slab, or resides in an exterior wall cavity is subject to moisture absorption, making treatment a code requirement in most jurisdictions.

Standard, untreated wood shims are reserved for interior, dry-conditioned spaces, such as adjusting an interior door jamb. Using untreated wood in an exterior application is a liability, as the shim will eventually rot, leading to wall movement and water infiltration.

A simple test: if the area is subject to a dew point or potential moisture wicking, always spec treated. If the area is climate-controlled and away from any concrete or ground contact, untreated cedar or pine is acceptable.

How to Properly Set and Secure Framing Shims

Shims should always be installed in pairs, sliding them against each other to create a perfectly flat, parallel surface. Using a single wedge creates an angled bearing surface that can slide under load, leading to a loss of height and structural instability.

Once the shim is positioned, fasten it in place if possible, or ensure it is “trapped” by the weight of the structural member above. If the shim is exposed on an exterior wall, seal the perimeter with a high-quality exterior caulk to prevent water from wicking into the gap.

Never use shims to correct extreme structural deficiencies without consulting a structural engineer. They are intended for fine-tuning levels and plumb, not for shoring up failing load-bearing supports.

Composite vs. Treated Shims: Key Differences

Composite shims, made from high-density plastic or polymer, are virtually immune to rot and insect damage. They are the top choice for plastic window installations or exterior doors where the shim will be exposed to the elements for years.

Treated wood shims are preferred when the shim needs to be fastened or nailed directly into the framing. Wood shims grip better under friction and don’t expand or contract at a rate that compromises the hold of a standard nail or screw.

Consider the application: if the shim needs to carry heavy structural weight, wood is often preferred for its compressive strength and compatibility with wood framing. If the shim is for exterior trim or a non-structural gap, composites offer better long-term moisture resistance.

Understanding Shim Sizing for Precise Leveling

Selecting the right shim thickness is the difference between a professional finish and a structural nightmare. Always measure the gap first and select a shim that provides the needed height with the thinnest possible profile.

Over-shimming forces the structural member out of alignment and can cause bowing or twisting in studs or joists. If a gap requires more than an inch of shimming, consider using a piece of pressure-treated plywood or a structural leveling grout rather than stacking excessive shims.

Take the time to verify the level in two planes. A shim that levels the member from front-to-back but leaves it tilted side-to-side will eventually manifest as a hinge point, causing cracks in drywall and door alignment issues later on.

Choosing the right treated shim ensures that the hidden adjustments behind your drywall or siding remain stable and protected for the life of the structure. By respecting the nuances of moisture resistance and load-bearing capacity, you provide a foundation of quality that supports everything built on top of it.