Why American Toilets Hold So Much More Water
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American toilets keep a deep water pool because of siphonic trapway design. See why European bowls stay nearly dry and what that…
Read the guideWhat to Use to Shim a Toilet (and What to Avoid)
A close look at which shim materials actually hold a toilet steady for years and which ones quietly rot, swell, or crush under the base.
Quick Answer
Use plastic toilet shims, thin strips of PVC or plastic composite, or plumber's plastic wedges to stabilize a rocking toilet, since these materials will not swell, rot, or compress under the seal. Avoid wood shims, cardboard, coins, and folded paper, because they absorb moisture from condensation or slow leaks and eventually crumble or expand, letting the wobble come back. Insert shims at the base on the low side, tap them in until the bowl sits flat, then score them flush with a utility knife before caulking around the base. If the floor gap is uneven or large, plastic washers or a shim kit with multiple thicknesses works better than a single wedge. The goal is a permanently rigid, waterproof shim that stays flat under the porcelain for years.
A rocking toilet almost always needs a shim, not a full reseat. The bowl sits slightly off the floor on one side because most bathroom floors are not perfectly flat, and a thin gap under the base lets the porcelain flex every time someone sits down. The fix is simple in concept: fill that gap with something rigid and waterproof. But the material matters more than most people expect, because whatever goes under the base gets covered in caulk and never checked again for years.
Why the shim material actually matters
Once a toilet is caulked around the base, the shim is sealed in place. Nobody pulls it back out to check on it. That means the material has to survive years of humidity, occasional condensation on the tank, and the constant small flexing every time someone stands up or sits down. A material that seems fine on day one can fail quietly over the following months. If the toilet starts rocking again six months after a repair, the shim material is usually the reason, not the repair method itself.
Best materials to shim a toilet
Plastic toilet shims
Purpose-built plastic shims are sold in hardware stores specifically for this job, often in multi-packs with a few different thicknesses. They are usually made from rigid polypropylene or similar plastic, and they will not absorb water, swell, or compress under the porcelain. Because they come pre-shaped as thin wedges, they slide under the base edge easily and can be scored off with a utility knife once they are snug.
PVC or plastic composite strips
Cut strips of scrap PVC trim, plastic decking composite, or rigid plastic sheeting work just as well as purpose-made shims. Plumbers commonly recommend these when a wobble needs a slightly thicker fill than a standard shim kit provides, since a strip of PVC can be stacked or cut to whatever thickness closes the gap.
Plastic washers
For small, uneven gaps, stacking a few plastic washers under one edge of the base can work as a low-profile shim. They are already flat and rigid, and they will not shift once caulk holds them in place.
Materials to avoid
- Wood shims or paint stir sticks. Wood absorbs moisture from bathroom humidity and slow tank condensation, then swells, warps, or eventually rots. A wobble that seemed fixed can return once the wood softens.
- Cardboard or folded paper. These compress under weight almost immediately and disintegrate the first time they get damp, which is common in a bathroom.
- Coins. Coins are rigid, but they are round, thin, and slide out from under the base easily since nothing holds them in a fixed spot the way a wedge does.
- Rubber or foam scraps. These compress under body weight, which reintroduces the same flex the shim was supposed to eliminate.
How to shim a toilet correctly
- Confirm the toilet actually rocks by pressing down gently on opposite corners of the bowl. If you have not already ruled out a loose closet flange or broken wax ring, it helps to work through how to fix a toilet that wobbles first, since a shim will not solve a structural flange problem.
- Identify exactly which side and corner is low. Most toilets only need shimming at one or two points, not all around the base.
- Slide the plastic shim under the base at the low point, thin edge first, until it fits snugly. Do not force it hard enough to lift the opposite side off the floor.
- Check for level using a small level placed across the tank lid or bowl rim, similar to the method described in fixing a toilet that is not level.
- Once the bowl sits flat and stops rocking, score the exposed shim with a utility knife so it sits flush with the base edge, then snap off the excess.
- Caulk around the base to lock the shim in place and keep water from reaching it. A full walkthrough of technique and gap placement is covered in how to caulk around a toilet.
When a shim is not enough
Shims fix a floor that is slightly uneven under an otherwise sound installation. They do not fix a broken flange, a rotted subfloor, or a wax ring that has failed and is letting the toilet sink. If the base feels spongy, if there is any water staining at the floor line, or if the rocking got worse quickly rather than developing slowly, the flange or subfloor likely needs repair before shimming does any good. Shimming over a soft or damaged subfloor just hides a bigger problem temporarily.
Choosing a toilet that stays stable from the start
Some toilet bases are more prone to rocking than others, especially models with a narrow footprint or a base shape that concentrates weight on a small contact area. If you are replacing the toilet anyway rather than repairing the current one, it is worth looking at options built for straightforward, stable installs, covered in this guide to toilets that are easy to install.
Frequently asked questions
? What household item can I use to shim a toilet in a pinch?
A cut strip of rigid plastic, like a piece of PVC trim or a plastic putty knife handle, works better than wood or cardboard as a temporary fix. Replace it with a proper plastic shim kit as soon as possible, since improvised plastic scraps are often not shaped to hold their position under the base.
? Can I use wood shims to level a toilet?
Wood shims are not recommended because bathroom humidity and occasional condensation cause wood to swell, warp, or rot over time. A toilet that seems stable right after installation can start rocking again months later once the wood degrades under the sealed base.
? How many shims does a toilet usually need?
Most rocking toilets only need one or two shims at the specific low point, not shims spaced evenly around the entire base. Adding shims where the bowl already sits flush can create a new rock instead of fixing the original one.
? Do I need to caulk over toilet shims?
Yes. Caulking around the base after shimming keeps the shims from shifting out of place and stops water from seeping underneath and reaching the subfloor. Skipping the caulk leaves the shim exposed to moisture and foot traffic, which can knock it loose.
? Why does my toilet rock even after I shimmed it?
If a toilet still rocks after shimming, the shim material may have compressed or the actual cause may be a loose closet flange, a broken wax ring, or a soft subfloor rather than an uneven floor surface. Checking flange tightness and subfloor condition is a necessary next step before reshimming.
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