What Is a Floor Leveling Shim?
A floor leveling shim is a load-bearing wedge or packer that corrects a small, local height difference beneath a supported floor part. It must fit the assembly, bear firmly, and be fastened as required. A shim is not a fix for a broadly uneven floor, weak framing, damaged subfloor, or moisture problem.
A floor can look level and still have a dip, much like a table that rocks on one short leg. That small gap may call for a shim, but the right fix depends on what supports the floor and why the gap is there. I often find that the key question is not “Which shim should I buy?” but “What is moving, and where is it meant to bear?”
What a floor leveling shim does
A floor leveling shim is a firm, shaped piece placed at an intended support point to make two parts meet at the right height. It fills a local gap under a supported subfloor or floor component. The shim works only when the assembly can safely carry the load and the shim stays in place.
A shim may be a tapered wedge or a flat packer of a set thickness. Its job is to create stable contact, not to lift an entire room’s floor or make a weak structure sound. Use only a shim material and fastening method approved for the specific assembly.
For example, a suitable packer may help close a small gap where a subfloor panel meets a planned support. It does not fix joists that are too small, too far apart, damaged, or unsupported. Nor does it replace an underlayment, tile membrane, moisture barrier, or properly designed subfloor.
| Term | Plain meaning | What it does not mean |
|---|---|---|
| Shim or packer | A firm piece used to fill a local gap at a support | A broad floor-leveling compound |
| Flatness | How much a surface varies beneath a straightedge | Whether the floor is level to the horizon |
| Deflection | How much a floor bends under load | A surface dip that can be hidden with finish material |
| Underlayment | A layer required by a flooring system | A repair for weak framing |
Inspect the Substrate, Framing, and Subfloor Before Shimming
The substrate is the surface that supports the flooring assembly, such as a wood subfloor or concrete slab. Before choosing a shim, identify the substrate and inspect its condition. Measure the high and low areas, then find out whether the problem comes from a local gap, surface variation, movement, or a structural defect.
Start by checking whether the floor is wood-framed or concrete. On a wood floor, inspect the joists and panels for rot, delamination, loose fasteners, cracks, and movement. Check joist spacing, span, condition, and support against the plans or a qualified building professional’s advice. Do not shim over damaged wood or treat a weak joist as a height problem.
Use a long straightedge or laser to map high and low areas. Note where the variation occurs and how much it changes over the distance. A small gap at one support point is different from a low area that runs across a room. The latter usually calls for an approved surface repair or structural correction, not loose shims.
For tile, flatness requirements are not the same as levelness. Under ANSI A108.02, tile with all edges shorter than 15 inches has a maximum substrate variation of 1/4 inch in 10 feet and 1/16 inch in 1 foot. For tile with at least one edge 15 inches or longer, the limits are 1/8 inch in 10 feet and 1/16 inch in 2 feet. These are flatness tolerances, not a requirement that the floor be perfectly level.
On a concrete slab, do not place structural shims on top to correct broad unevenness. Check for cracks, loose or weak areas, and moisture concerns. Use a compatible patching or leveling system for surface variation, and follow the flooring system’s instructions. If the slab’s condition or cause of movement is unclear, get it assessed before covering it.
Fit and Fasten Shims for Stable, Precise Support
A fitted shim makes full, stable contact at an intended bearing point and cannot slide or crush under the expected load. Its thickness should match the measured gap, and its material must suit the assembly. A shim that rocks, projects unevenly, or relies on friction alone may shift and let the floor move again.
- Measure the gap. Use a straightedge or other suitable tool to locate the actual support point and the size of the gap. Do not estimate from the finished floor surface alone.
- Choose a compatible shim. Confirm that the material can support the load without crushing, creeping, absorbing harmful moisture, or interfering with the specified assembly.
- Fit it at the support. The shim belongs where the floor component is designed to bear. It should provide stable contact, not create a new point load or hold up a loose, deteriorated panel.
- Fasten it as required. Follow the assembly’s approved fastening method so the shim cannot shift. Do not assume adhesive or a nail is suitable without checking the system instructions.
- Recheck before covering. Confirm the support is firm and the floor no longer moves at that point. Then verify that the surface still meets the flooring system’s flatness and structural requirements.
| Situation | Appropriate direction | Avoid |
|---|---|---|
| Small gap at a sound, intended wood bearing point | Fit and secure a compatible shim if the assembly allows it | Loose scraps or a stack of unsecured pieces |
| Broad dip across a wood floor | Inspect framing; repair structure or use an approved surface system as suitable | Filling the whole area with scattered shims |
| Uneven concrete slab | Use a compatible patch or leveling system after checking slab conditions | Structural shims on the slab |
| Soft or moving subfloor panel | Repair or replace the panel and address its support | Wedging under the weak panel and covering it |
Illustrative cases: local support versus broad unevenness
Consider a sound wood-framed floor with a small, measurable gap at a planned bearing point. If the design and flooring system permit it, a fitted and secured shim can close that gap. The key is that the surrounding framing and subfloor are sound and the support remains stable.
Now consider a room where a straightedge reveals a broad dip and the floor flexes as someone walks across it. Shimming one spot may hide part of the symptom, but it does not establish why the floor bends. The framing and subfloor need assessment, and the right repair may involve structural work rather than a shim. These examples explain the decision process; they are not substitutes for site-specific design.
Check Deflection, Transitions, and Required Movement Joints
Deflection is the amount a floor bends under load. Floor coverings can have limits on how much movement they can tolerate, so a shim cannot stand in for adequate joists, bracing, or support. Check the full assembly requirements, including the selected flooring, underlayment, mortar, and membrane.
Common minimum structural design criteria are L/360 for ceramic or porcelain tile and L/720 for natural stone under applicable loading. “L” is the joist span; the fraction describes the maximum permitted deflection. For example, a 12-foot span is 144 inches, so L/360 equals 0.4 inch. These criteria are not a field test or permission to shim a weak floor. Confirm the complete assembly’s requirements with its manufacturer or a qualified design professional.
A loose or deteriorated panel is a critical warning. A shim placed beneath it can concentrate the load, loosen, or crush. The renewed movement may crack tile or grout or cause flooring to debond. Repair or reinforce the structure rather than hiding the problem with shims or an uncoupling membrane.
Also check height changes at doorways and flooring transitions. A local shim may affect how two surfaces meet, but it must not interfere with the specified transition piece, required clearance, or the flooring manufacturer’s instructions. Plan these details before installing the finish floor.
Tile needs movement joints where the selected system requires them. TCNA EJ171 provides guidance for movement joints; follow the project and system requirements at perimeters and other locations. Do not fill a required movement joint rigidly with grout. Use the specified flexible sealant or joint profile.
Protect the Floor from Moisture and Long-Term Movement
Moisture can damage a floor assembly or undermine the bond of flooring materials. A shim does not stop moisture, and it must not block a required underlayment, membrane, adhesive, or expansion detail. Resolve moisture sources and verify the flooring system’s limits before installation.
For concrete, ASTM F2170 measures relative humidity inside the slab. ASTM F1869 measures moisture vapor emission rate. Use the test method the flooring system permits, and follow its instructions. There is no single RH percentage or MVER limit that works for every floor. Use the strictest limit set by the flooring, adhesive, and moisture-mitigation manufacturers.
Before installing flooring, confirm that the substrate meets the system’s flatness, deflection, and moisture requirements. Use specified primers, fasteners, mortar, adhesives, and coverage rates. For tile in typical interior dry areas, target at least 80% mortar contact; wet areas and exterior applications require at least 95%, with edges and corners supported. Periodically lift a tile to check coverage. Do not set tile on isolated dots of mortar.
For level interior surfaces expected to be walked on when wet, DCOF of at least 0.42 is a commonly cited ANSI A326.3 criterion for certain applications. It is not a guarantee of slip safety in every setting. More importantly for shimming, the finish surface and its support must both meet the selected system’s requirements.
Next step: identify the substrate, measure the variation, and correct the cause before choosing a shim or leveling product. If the floor moves, framing is damaged, or the repair affects structural support, seek a qualified assessment.
Frequently asked questions
These quick answers explain when a floor shim is useful and where its limits lie. The right choice depends on the substrate, support location, flooring system, and condition of the floor. Always check the approved assembly instructions before installing a shim or covering the substrate.
Can I use a floor leveling shim under any floor?
No. Use a shim only at an appropriate support point, with a material and fastening method approved for that assembly.
Can shims level an entire room?
No. Shims address local height differences at support points. Broad unevenness needs an approved surface repair or a structural correction, depending on its cause.
Can I put shims on a concrete slab?
Do not use structural shims on a slab to correct broad unevenness. Assess the slab and use a compatible patching or leveling system as needed.
Will a shim fix a bouncy floor?
Not necessarily. A bouncy floor may have inadequate or damaged framing. Check the structure; do not use shims to mask excessive deflection.
Can I stack shims to fill a large gap?
Do not stack loose, compressible, or unsecured scraps. Use only a designed, stable support detail approved for the assembly.
Do I need the floor to be level before installing tile?
Tile needs a substrate that meets the required flatness and structural limits. Flatness and levelness are different, so check the applicable system requirements.
What deflection limit is common for tile?
Common minimum design criteria are L/360 for ceramic or porcelain tile and L/720 for natural stone under applicable loading. Confirm the full assembly requirements.
Does a shim replace an uncoupling membrane?
No. A shim provides local support. It does not replace a required membrane, underlayment, moisture system, or properly designed subfloor.
Can a shim solve a damp-floor problem?
No. Test concrete moisture using a method permitted by the flooring system, and follow the strictest manufacturer limit. Address moisture before installation.
Should grout fill the gap at the floor’s edge?
Not where a perimeter or movement joint is required. Keep that joint open for the specified flexible sealant or joint profile.
(This article was written by one of our staff writers, Marcus Vance. Visit our Meet the Team page.)
