Shims

What Are Shims?

Shims are small, precisely sized spacers used to fine-tune alignment, level surfaces, and take up small gaps during building, installation, and repair work. They can be tapered (wedge-shaped) for gradual adjustment or flat for uniform spacing. Common materials include cedar or pine (wood), polypropylene or composite plastic, and metals like steel, brass, or stainless steel. By slipping a shim between two parts—such as a door jamb and a stud—you can nudge something into plumb (perfectly vertical), level (perfectly horizontal), or square (90-degree corners) without tearing things apart.

Shims are invaluable for two reasons: they allow tiny, controlled adjustments, and they distribute load so parts stay put after you fasten them. Think of them as the quiet helpers that make doors latch cleanly, cabinets line up, floors feel solid, and machines run true.

Common Uses and Applications

• Hanging doors and windows: Placing shims around the frame sets the unit plumb and square so it opens and closes smoothly.
• Setting cabinets and countertops: Shims bring base cabinets into level, align faces, and support countertop seams.
• Flooring and stair fixes: They fill small gaps under squeaky subfloors or treads and help level transitions.
• Decks and exterior framing: Composite or metal shims adjust post bases, ledger alignment, and rail posts where small corrections are needed.
• Appliances and furniture: Shims stabilize wobbly furniture and help level laundry machines or fridges when feet aren’t enough.
• Toilets and plumbing fixtures: Plastic shims stop a toilet from rocking on uneven tile.
• Equipment and machinery: Metal shims align motors, pumps, and saws for smoother operation and reduced wear.

Types of Shims

• Wood shims: Usually tapered cedar or pine, easy to cut or snap. Great for interior trim, doors, and windows. They can compress or swell with moisture.
• Composite/plastic shims: Stable in damp areas, won’t rot, many are rated for exterior use. Often ridged to prevent slip.
• Metal shims: Steel, stainless, brass, or aluminum. Used for high-load, high-precision work (machinery, structural bearing points). Available as flat shim stock or pre-cut packs.
• Horseshoe/slot shims: U-shaped, stackable pieces (often plastic or metal) that slide around bolts or anchors without removing the fastener. Ideal for cabinets, ledger adjustments, and equipment.
• Laminated/peelable shims: Built from thin layers you peel off to dial in thickness. Good for machinery and door hardware mortises.
• Shim stock rolls: Thin metal sheets you cut to size for custom fits, common in automotive and fabrication.

How to Choose and Use Shims

Selection tips:
- Match the environment: Use composite or stainless shims where moisture is present (bathrooms, exterior, basements). Wood is fine in dry interior spaces.
- Consider load and precision: For structural bearing points or machinery alignment, choose metal. For trim and light framing, wood or composite works well.
- Shape matters: Tapered shims are best for gradual adjustments behind frames. Horseshoe shims excel where you need to slide around a bolt or stack exact thicknesses.
- Keep an assortment: A small kit with wood wedges and a range of plastic horseshoe sizes (1/16, 1/8, 1/4 inch) covers most household needs.

Using shims effectively:
1. Set reference: Use a quality level, laser level, or square to establish what “correct” is before shimming.
2. Work in pairs when possible: Opposing tapered shims (points facing opposite directions) can create a stable, parallel stack that won’t drift.
3. Tap gently: A light hammer or mallet is enough. Don’t drive a shim so hard that it bows the material you’re adjusting.
4. Secure the position: Once aligned, fasten through or next to the shim with screws or nails. A dab of construction adhesive helps prevent creep.
5. Trim cleanly: Score wood or composite shims with a utility knife and snap, or use a flush-cut saw. Cut only after fastening.
6. Verify again: Re-check level, plumb, and operation (door swing, drawer alignment) after fastening and trimming.

Care and storage:
- Store shims flat and dry. Keep plastics out of prolonged sun exposure to avoid brittleness.
- Label shim thicknesses or keep them in divided containers for quick grabs.

Safety:
- Wear eye protection when cutting or snapping shims.
- Don’t place hands beneath a load that isn’t fully supported.

Common Mistakes to Avoid

• Using wood shims in wet or exterior locations: They can swell, rot, or compress. Choose composite or metal instead.
• Over-shimming a single point: Too many stacked shims can create a weak fulcrum. Spread the load across multiple points or use a thicker, solid shim.
• Bowing the frame: Pounding a shim too hard can distort a door or window jamb, causing latch and binding issues.
• Cutting flush before fastening: Trimmed shims can slide out if not secured first.
• Shimming only one plane: Check plumb and level in both directions and confirm square. Doors often need shims at hinges and strike side locations.
• Using improvised scraps: Cardboard or drywall pieces compress and degrade; use proper shims.
• Ignoring code or manufacturer guidance: Structural or exterior elements may require non-compressible or corrosion-resistant shims.

Related Terms and Concepts

• Level, plumb, square: Reference alignments.
• Reveal: The visible, uniform gap between parts (such as a door and its jamb).
• Furring: Strips used to build out or level a surface, similar in purpose to shimming but across larger areas.
• Spacers/packers: Uniform-thickness pieces used like flat shims.
• Ledger, sill, and jamb: Places where shims are frequently used during installation.

Practical Examples

• Hanging a prehung door: Place tapered wood shims behind each hinge location and at the latch area. Use opposing pairs to hold the jamb parallel to the stud. Check the reveal around the door, adjust by tapping shims, then screw through the jamb and shims into the studs. Trim the shim tails before installing casing.
• Leveling a run of base cabinets: Snap a chalk line at finished height. Starting at the high spot of the floor, set the first cabinet level using plastic horseshoe shims at load points. Connect adjacent cabinets, checking the faces for alignment, and add shims at corners as needed. Anchor to studs only after everything is level and flush.
• Stopping a rocking toilet: After setting the wax ring and placing the toilet, test for wobble. Slide plastic shims at low points until stable, then snug the flange bolts without over-tightening. Trim shim excess and apply a neat bead of caulk along the front and sides, leaving the back uncaulked for leak detection.
• Quieting a squeaky subfloor (from below): Have a helper step on the squeak. From the basement or crawlspace, locate the gap between joist and subfloor. Gently tap in a tapered wood shim with a dab of construction adhesive until the squeak stops. Don’t force it so far that you lift the floor.
• Aligning a deck post base: Where small height differences exist on a concrete pier, use non-corroding composite or stainless shims under the post base plate, stacked to uniform support. For larger discrepancies, switch to a grout pad or reframing, not a thick pile of shims.

With a small assortment and a steady hand, shims help you achieve clean fits, smooth operation, and long-lasting installations across the house.