What Is a Hardwood Floor Shim?
A hardwood floor shim is a thin, dry, stable piece of wood used to fill a small gap between a subfloor and a joist, or to correct a minor low spot in an approved wood-floor assembly. It must fit firmly and be secured. A shim is not a fix for broad unevenness, a weak joist, or a concrete-floor problem.
When you upgrade a floor, a small dip or squeak can make the whole job feel uncertain. The word shim may sound like a quick fix, but where it goes matters. A loose or misplaced piece can leave the floor unstable, while a well-fitted shim can restore firm support at one small spot.
I use one basic test: identify what is moving or out of level before adding material. A gap between a sound joist and subfloor is different from a sagging joist or a wide low area. The right repair depends on that difference.
What a hardwood floor shim is, and where it belongs
A hardwood floor shim is a solid piece of wood that fills a specific gap in a wood-framed floor. It may sit between a joist and the subfloor, or form part of a repair approved for a small, local low spot. It is not a loose layer beneath the finished flooring.
For a common repair, the shim fills a gap where the subfloor does not sit firmly on a joist. The subfloor is then mechanically fastened to that joist. The goal is firm, even support, not lifting the floor higher than it was.
| Situation | Is a shim appropriate? | What the repair needs |
|---|---|---|
| Small gap between sound joist and subfloor | Often, if the floor system permits it | Snug, stable wood; secure refastening |
| Minor local low spot in an approved wood assembly | Sometimes | A method approved for that flooring system |
| Broad low area across several joists | Usually not as a scattered-shim fix | Joist correction, supported overlay, or compatible patch system |
| Weak, rotted, or sagging framing | No, not as the repair | Structural assessment and framing repair |
| Concrete slab beneath hardwood | No wood shim beneath a wood subfloor | Compatible patch or leveler and required moisture control |
A shim is different from underlayment. Underlayment is a layer specified as part of a flooring system; it does not replace sound, flat support. Shims also differ from flooring wedges used during installation to hold an edge gap. Those wedges are temporary and must not remain in a way that blocks floor movement.
Inspect the Subfloor, Joists, and Moisture Before Choosing a Shim
A shim only helps when the problem is a small, understood gap. Before using one, check whether the subfloor and joist are sound, whether the floor is flat enough for the chosen product, and whether moisture is within the manufacturer’s limits. These checks help separate a local support repair from a larger framing or moisture problem.
Start by locating the low spot or movement. Remove or lift flooring only as needed to see the subfloor and framing. Look for loose fasteners, gaps, cracked or soft wood, stains, and signs of rot. A squeak can come from movement between flooring layers or at a joist; it does not prove that a shim is needed.
Check flatness and moisture
Use a straightedge or other method recommended by the flooring maker to check the subfloor. A common hardwood-preparation benchmark is no more than 3/16 inch variation over 10 feet or 1/8 inch over 6 feet. The selected product may require a tighter surface, so its instructions take priority.
Measure wood moisture with a calibrated meter and record the readings. As an NWFA installation guideline, the difference between the wood subfloor and flooring should generally be no more than 4 percentage points for boards under 3 inches wide or 2 points for flooring 3 inches wide or more. Species, product, and regional conditions can affect the required limits, so confirm them for your job.
Concrete needs a different approach. There is no single moisture-vapor emission rate (MVER) or in-situ relative humidity (RH) limit that applies to every hardwood and adhesive. Test by the method specified by the flooring or adhesive manufacturer. Proceed only within its published limit, and use an approved moisture-mitigation system when required.
- If the joist is damaged, the subfloor is soft, or the floor has significant sag, stop and address the structure first.
- If the surface is broadly uneven, do not scatter shims across it as a shortcut.
- Do not apply tile-specific deflection rules to hardwood. Use the hardwood product’s requirements and have structural concerns assessed appropriately.
Fit and Secure Hardwood Floor Shims Without Creating High Spots
A correctly fitted shim fills the gap without forcing the floor upward. Use dry, sound wood that suits the framing. Make firm, continuous contact, trim it flush, and mechanically refasten the subfloor to the joist. A loose wedge can move, create a point of support, and lead to noise or renewed movement.
For a small joist-to-subfloor gap, work through these steps:
- Confirm the gap. Check that the subfloor and joist are sound and that the gap is local. Do not mistake a sagging joist for a gap that needs filling.
- Fit the shim. Choose a dry wood shim or strip compatible with the framing. Fit it snugly so it supports the subfloor along the gap. Do not hammer it in to jack the floor upward.
- Secure the assembly. Mechanically fasten the subfloor to the joist with suitable fasteners. Follow the flooring and subfloor instructions for fastening details.
- Trim and check. Trim the shim flush. Confirm that the repair is rigid, does not create a high spot, and leaves the surface within the flooring system’s flatness limits.
- Recheck moisture and layers. Before installing the hardwood, confirm moisture readings and install any specified underlayment or moisture-control layer with compatible seams, fasteners, and adhesive.
The shim should not be left unsecured in the hope that flooring fasteners will hold it in place. Nor should it be wedged between the finished floor and subfloor. That can create unwanted pressure rather than repair the support below.
A simple fit check
With the floor exposed, press near the repaired area and check for movement. The subfloor should sit firmly on its support; the shim should not shift. Then use the straightedge again. If the repair makes a ridge or the surface still exceeds the product’s flatness limit, correct the underlying assembly before laying flooring.
Real-World Shim Scenarios: Two Illustrative Cases
These examples show how the same word, “shim,” can point to very different repairs. They are illustrative situations, not claims about a particular job. In each case, the diagnosis comes first; the shim is appropriate only if the floor system and product instructions allow it.
Case 1: A small gap at a sound joist. A homeowner finds a short section of subfloor that moves slightly above an undamaged joist. The gap is local, and the surrounding floor is flat and dry. A snug wood shim can fill the gap, followed by mechanical refastening. The repair is checked for movement and high spots before hardwood goes down.
Case 2: A dip across several joists. A room has a broad low area, and a straightedge shows the deviation exceeds the selected flooring’s limits. Adding separate wedges at each joist could create uneven support and high spots. The installer instead investigates framing and considers an approved correction, such as joist work, a properly supported overlay, or a compatible patch system.
Case 3: A sagging or weakened joist. A floor dips and continues to move under load, or the framing shows rot. A shim that forces the subfloor upward can hide the damage and concentrate load. The joist needs appropriate structural repair or reinforcement before flooring installation; a shim alone is not a safe substitute.
Detail Transitions, Perimeters, and Structural Deflection
Hardwood needs firm support, but it also needs the movement allowance required by its manufacturer. Shims belong in the support assembly only where permitted. They must not be used to pin the flooring at a wall, doorway, fixed object, or transition where the product requires expansion clearance.
Once the subfloor repair passes its checks, install the hardwood only by its approved method: fastening, gluing, or floating as specified. Keep required perimeter and fixed-object clearance. Detail transitions according to the flooring instructions, and reinstall trim without fastening it through or trapping the moving floor.
Structural deflection means a floor bends under load. A shim cannot correct a weak or overly flexible structure, even if it hides a gap at one point. If there is significant sag, movement, rot, or damage, have the framing assessed and repaired before installing flooring.
For a concrete base, do not drive wood shims into the slab or place them beneath a wood subfloor as a leveling fix. Use a patch or leveling product and moisture-control system compatible with the flooring and adhesive. Follow their substrate preparation requirements.
Maintain the Repair and Protect the Floor Long Term
A lasting repair depends on stable support, suitable moisture conditions, and correct installation. After the work, keep the repair information and moisture readings with the flooring records. If movement, new squeaks, or a change in floor level appears, investigate the cause instead of adding another loose shim.
- Keep the shim dry and firmly secured as part of the repaired assembly.
- Do not block expansion clearance with shims, trim, or fasteners.
- Avoid loose shims, cardboard, and compressible scraps beneath floating flooring.
- If moisture readings or the floor’s movement change, check the source before making a new repair.
The practical takeaway is simple: use a hardwood floor shim only for a small, verified support gap in a compatible wood-floor assembly. For broad unevenness, concrete, or damaged framing, choose a repair designed for that condition.
Hardwood Floor Shim FAQ
These quick answers distinguish a proper shim repair from common misuses. Always confirm the selected flooring and adhesive instructions, since their flatness, moisture, and installation requirements govern the finished assembly.
What is a hardwood floor shim?
It is a thin, dry piece of stable wood used to fill a small gap in an approved wood-floor support assembly.
Where does a wood floor shim go?
A common location is between a sound joist and the subfloor, where it can restore firm contact before the subfloor is refastened.
Can I put a shim under hardwood flooring?
Not as a loose piece beneath the finished floor. Use a shim only in a suitable support repair approved for the floor system.
Can I use a shim to level a concrete floor?
No. Do not place wood shims beneath a wood subfloor on concrete as a leveling fix. Use compatible patching or leveling and moisture-control products.
How tight should a shim fit?
It should fit snugly and provide firm, continuous contact without lifting the floor or creating a high spot.
Should a shim be fastened?
The repair must be secured, and the subfloor should be mechanically refastened to the joist. Do not leave a loose wedge that can shift.
Can shims fix a sagging joist?
No. Significant sag, rot, or structural movement calls for assessment and repair of the framing, not a shim that masks the problem.
How flat should a hardwood subfloor be?
A common preparation benchmark is within 3/16 inch over 10 feet or 1/8 inch over 6 feet. Check the flooring maker’s instructions for its exact requirement.
Can a shim block hardwood expansion space?
No. Keep the manufacturer’s required clearance at perimeters and fixed objects so the floor can move as designed.
What moisture readings are acceptable?
Use the flooring and adhesive manufacturer’s limits and test method. For wood, compare calibrated-meter readings for the subfloor and flooring; for concrete, follow the specified test and limit.
(This article was written by one of our staff writers, Marcus Vance. Visit our Meet the Team page.)