Shimming a Toilet: The Right Way to Fix a Rocking Bowl

A rocking toilet isn’t just annoying — every wobble breaks the wax ring’s seal and slowly soaks the subfloor below. By the time you smell anything, you’re often looking at $400-$800 in joist and tile repair. Shimming a toilet properly takes about 20 minutes and uses materials that cost less than $5. The trick is using the right shim material, placing them in the correct spots, and finishing with the right sealant. Here’s how to do it without pulling the toilet.

Part of our Bathroom Remodeling & Flooring guide — explore the full guide →

Why Toilets Rock in the First Place

Floors are never perfectly level. Tile installations especially leave slight ridges between tiles and dips along grout lines. The toilet base, usually flat porcelain, can only rest on three contact points. Any movement under your weight pries the wax ring loose, breaks the caulk seal, and gradually loosens the closet bolts.

Quick test: sit on the toilet and rock side to side gently. If you feel any movement at all, it needs shims. A solid toilet feels like part of the floor.

The Right Shim Material

Skip wood shims. Tapered cedar shims work for door frames, but in a bathroom they wick water, rot, and grow mold within a year. The job requires plastic shims designed for toilets.

• Hercules Johni-Shims — $4 for a pack of 8, made of stiff polypropylene, snap-off tabs let you trim them flush ($4)
• Oatey Toilet Shims — similar design, often stocked at Home Depot
• Generic plastic horseshoe shims — work in a pinch, sold at any plumbing supply

You’ll also need a tube of 100% silicone caulk in white or clear (not latex), a utility knife with a fresh blade, and a putty knife for cleanup. A small mirror helps inspect the back of the bowl.

Step 1: Identify the High and Low Points

Press down on the front, back, and both sides of the bowl one at a time. Two of these spots will have no give — those are your high points where the toilet already touches the floor. The other two are low spots that need shims.

Mark the low spots with a piece of painter’s tape pointing to the exact location. Most rocking toilets need shims at either the front corners or the back corners, rarely all four.

Step 2: Loosen the Closet Bolts Slightly

Pop the bolt caps off (gentle pry with a flathead). Loosen each nut a half-turn — not enough to break the wax seal, just enough to let the bowl flex slightly. This makes shimming much easier and prevents you from hairline-cracking the porcelain by forcing a shim under a bolted-down base.

Skipping this step is the number one reason DIYers crack their toilet during this job. A cracked base means a $200-$400 replacement.

Step 3: Slide Shims Into Place

Push a shim under the bowl at each low point with the thin edge first. Stop when you feel firm resistance — don’t hammer or wedge with pliers. The goal is contact, not lifting.

Test the rock again. If it still moves slightly, add a second shim stacked on top of the first, but offset by 1/4 inch so the tips don’t bottom out together. Two shims max per location. If you need more than that, the floor is severely out of level and you have a bigger problem (rot, settling, or improper subfloor) to address first.

Step 4: Retighten the Bolts

Bring each closet bolt back to snug, then add a quarter turn. Hand-tight plus a quarter is the sweet spot. Cranking down with a socket wrench cracks porcelain — I’ve replaced too many bowls because the previous installer kept tightening to “fix” a rock that needed shims instead.

Sit on the toilet and rock again. If it’s solid, you’re nearly done. If not, recheck shim placement.

Step 5: Trim the Shims Flush

Score the protruding plastic with a sharp utility knife, then snap it off along the score. Johni-Shims have a pre-scored breakaway line that snaps clean. For other brands, two passes with the blade gives you a clean break.

The shim should end exactly at the bowl’s outer edge — neither sticking out nor recessed underneath. If it’s poking out, the caulk won’t hide it.

Step 6: Caulk the Base (Mostly)

Run a continuous 1/4 inch bead of 100% silicone around the bowl base where it meets the floor. Skip a 2-3 inch gap at the back. This intentional gap is required by most plumbing codes — it lets you see water if the toilet ever leaks from inside, instead of trapping it under the bowl where it ruins the subfloor.

Smooth the caulk with a wet fingertip or a $3 caulk tool. Wipe excess immediately with a damp rag; silicone is brutal to clean once it skins over (about 10 minutes).

Common Shimming Mistakes to Avoid

A few errors I see constantly on service calls:

• Using wood shims that rot within a year
• Over-tightening bolts to compensate for rock (cracks porcelain)
• Stacking 3+ shims in one spot — your floor needs structural work, not more plastic
• Caulking 100% around the base, hiding leaks until rot is severe
• Forgetting to loosen bolts before inserting shims

If a toilet keeps rocking within a few months after shimming, the floor underneath is likely soft. Pull the toilet, check the flange and subfloor, and budget for plywood repair or a new flange. No amount of shimming fixes a compromised substrate.

When to Pull the Toilet Instead

If the rock is more than 1/4 inch of travel, if water seeps from the base after flushing, or if the closet flange is broken or below the finished floor, shimming is a temporary patch at best. Pull the bowl, replace the wax ring, repair the flange, and reset it level. Total cost for parts: $15-$30. Time: about 90 minutes for a first-timer.