Short definition
A toilet shim is a thin plastic wedge inserted between the toilet base and the floor to level a wobbly toilet. After leveling, you cut excess shim flush with the base and conceal the gap with caulk. Ten minutes with shims prevents a $300 wax-ring repair down the road.
What it is
Standard plastic toilet shims are about 6 inches long and tapered from 1/8 inch to nothing. You slide them under the toilet base wherever it rocks, working in small adjustments until the toilet sits firm. Hercules and Oatey are the common brands; a pack runs $3 to $6.
Plastic is the modern standard. Older repairs used cardboard or wood scraps — both absorb water and rot, breaking the seal a year or two later.
Why it matters to a homeowner
A toilet that rocks even slightly is a slow-motion leak. Each rock flexes the wax ring; over months the seal breaks down and water starts seeping at the base — exactly the wax-ring failure that costs $200 to $400 to fix professionally. Ten minutes with a $5 pack of shims at install time prevents the repair entirely.
If your inspector flags a wobbly toilet on a pre-purchase walk, this is the easy fix, not a deal-breaker: lift the toilet, check the closet flange isn’t damaged, replace the wax ring, set the toilet on a fresh seat, shim level, and caulk the base.
The two most common DIY mistakes are skipping the trim step (visible plastic edges look bad) and caulking over uncut shims (leaves a hidden leak path). Cut the shims flush with a utility knife before caulking.
Common variants
- Plastic vs. cardboard or wood. Modern plastic doesn’t rot; wood and cardboard do. Don’t use either as a substitute.
- Shim vs. closet flange extender. A shim levels the toilet on top of the flange. A flange extender raises a sunken flange up to floor level — a different repair when the flange itself sits too low.