6 Best Structural Shims For Uneven Surfaces That Last

Nothing halts a project faster than a framing member that sits out of plane or a subfloor that refuses to level out. Shims are the unsung heroes of a structural assembly, transferring massive loads while correcting the inevitable inconsistencies of wood and masonry. Using the wrong material—like a pine scrap or a brittle plastic sliver—can lead to point-load failure or gradual creeping. Proper shimming is the difference between a door that swings true for decades and one that settles, binds, and eventually fails the homeowner.

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Grove G-Pro Plastic Shims: Best All-Purpose Choice

Grove G-Pro plastic shims are a staple for a reason: they strike the perfect balance between rigidity and ease of use. These are typically composed of high-impact polystyrene, which resists splitting when driven into tight gaps.

They shine in general carpentry and interior framing where high-moisture exposure isn’t the primary concern. Since they don’t compress under standard dead loads, they maintain the precise spacing required for drywall installation or door jamb leveling.

The main takeaway here is versatility. If the project involves standard residential framing, keep a box of these on hand to handle the minor deviations found in almost every wall cavity.

Dayton Superior T31 Steel Shims: For Max Load Bearing

When the project moves from wood framing to structural steel or heavy timber, plastic simply won’t cut it. Dayton Superior T31 steel shims are designed for scenarios where the compression strength of plastic would be a liability, such as under heavy steel columns or ledger boards supporting a deck.

Steel provides an unyielding interface that won’t creep or deform over time under the immense pressure of structural loads. These shims allow for precise height adjustments on base plates without the risk of “squashing” that plagues lesser materials.

Always use these in high-load applications where the structural integrity depends on the shim’s ability to maintain a consistent plane. If the load is calculated in tons rather than pounds, steel is the only reliable choice.

EZ-Shim Interlocking Shims: Foolproof & Secure Stacking

EZ-Shim interlocking shims solve the age-old problem of shims sliding out of place while trying to drive a fastener. Their interlocking design allows for secure stacking, ensuring that the shim pack stays exactly where it was placed during the initial alignment.

This is particularly useful when installing exterior doors or windows, where maintaining a plumb, level, and square opening is critical. Because they lock together, they behave like a single, solid unit rather than a slippery stack of individual pieces.

The bottom line is efficiency. These reduce frustration by keeping the shim assembly stable while the final fasteners are driven home.

Nelson S-Series Solid Plastic Shims: Uncrushable Pick

Nelson S-Series solid plastic shims are engineered to offer extreme crush resistance that rivals wood without the rot-prone nature of organic material. They are specifically formulated to withstand the direct pressure of fasteners passing through them.

These are the go-to for heavy-duty window installations where the window unit needs to be supported directly under the mullions. Unlike cheaper hollow-core alternatives, these will not collapse when the installer torques down the mounting screws.

For any application where the fastener must pass through the shim itself, the Nelson S-Series provides the necessary density to prevent local deformation. It is an investment in long-term stability for any window or door assembly.

Pro-Lok Plastic Horseshoe Shims: For Masonry & Bolts

Pro-Lok horseshoe shims are a specialized tool designed specifically for shimming around bolts and anchoring systems. The U-shaped design allows them to slide around an anchor bolt or rebar without the need to remove the fastener or clear the entire hole.

These are essential in masonry work or when leveling metal base plates on concrete. They are color-coded by thickness, which makes keeping track of the shim height simple and error-proof during a fast-paced installation.

If the job requires plumbing a post or anchoring a steel plate to a slab, these horseshoes are indispensable. They save time and ensure that the anchor remains centered in the plate’s mounting hole.

Wobble Wedges Tapered Shims: For Fine-Tune Adjustments

Wobble Wedges are the precision instruments of the shimming world, featuring a distinct taper and a textured surface. These are not meant for structural heavy lifting but are perfect for fine-tuning the last sixteenth of an inch.

They excel in cabinetry, furniture leveling, and fine finish work where a slight rock or a tiny gap needs to be closed. The rubberized material provides a high coefficient of friction, meaning they won’t slide out of place on slick surfaces like tile or polished concrete.

Keep a variety pack on the truck for those final touches that make a project look professional. They are the ideal choice when the shim needs to grip the surface rather than just sit under it.

How to Read a Shim’s Compressive Strength Rating

Compressive strength measures how much weight a shim can hold before it permanently deforms or cracks. This rating is typically provided in PSI (pounds per square inch) and is the most important spec to check when dealing with heavy structural loads.

For light framing, a lower PSI rating is acceptable, but for structural support, ignore any product that does not explicitly list its compressive capacity. Always ensure the calculated load on the shim is significantly lower than its maximum rated capacity to account for safety factors.

• Low load: Up to 1,000 PSI (general carpentry).
• Medium load: 1,000 to 5,000 PSI (windows/doors).
• High load: Above 5,000 PSI (structural steel/masonry).

Plastic vs. Steel vs. Composite: Picking Your Material

Choosing the wrong material can lead to premature failure, rot, or compression loss. Understanding the trade-offs between plastic, steel, and composite is vital for the longevity of any construction project.

• Plastic: Excellent for moisture resistance and non-load-bearing or light-load applications.
• Steel: Required for heavy structural steel and high-load compression zones.
• Composite: Offers a middle ground with high rigidity and immunity to the moisture-driven decay that destroys wood shims.

In exterior applications, always prioritize non-organic materials. Wood shims, even when treated, are prone to shrinking, swelling, and eventually rotting over the life of the structure.

Proper Shimming Technique for Windows and Doors

Properly shimming a window or door is not just about filling a gap; it is about transferring the weight of the unit to the rough opening without distorting the frame. Improper technique leads to operational failures, such as doors that rub or windows that refuse to lock.

Always place shims at every fastener location to ensure the jamb remains perfectly flat when the screws are tightened. If the fastener is driven through the shim, ensure the shim is solid and of the appropriate thickness to fill the entire gap.

Do not over-shim. The goal is to provide firm support; forcing a thick shim into a tight gap can bow the frame, which causes the very operational problems you are trying to avoid.

Do Structural Shims Need to Be Glued or Fastened?

In most structural scenarios, the friction created by the compressive load is sufficient to hold shims in place. However, if the shim is in a location prone to vibration or if the load is intermittent, additional security is necessary.

For high-load steel connections, shims are often welded or tacked into place to prevent movement during seismic events or thermal expansion. In wood framing, if a shim pack is loose, a small bead of construction adhesive or a well-placed finish nail can prevent the shims from migrating over time.

Always assess the long-term stability of the application. If there is a risk that the shim might shift due to building settling, take the extra step to lock it in place mechanically.

Proper selection and placement of structural shims represent the difference between a project that requires constant maintenance and one that remains rock-solid for the long haul. By matching the material and load rating to the specific demands of the framing, you ensure that the structure stays plumb, square, and secure regardless of the site conditions.