A new toilet should sit flat before final installation.
If it rocks before the wax ring is crushed, stop and check the base.
You bought two Kohler Elliston toilets.
One has support ribs and a horn about 1/8 inch higher than the rim.
That can cause a small wobble on the floor.
The toilet horn often hangs lower than the base rim.
That part can be normal on many toilets.
The horn helps guide waste into the flange opening.
But the support ribs are a different concern.
They should not be the main contact point on the floor.
If the ribs touch first, the toilet may not sit firmly.
It can also stress the china when bolts are tightened.
That stress may lead to cracks later.
A rough-looking underside can also be normal.
Toilet bottoms are not finished like the visible bowl.
Still, rough casting should not create rocking.
Before installing, set the toilet on a flat surface.
A piece of plywood or a level floor works well.
If it rocks there, the toilet may be defective.
Compare it with the second toilet you bought.
If only one rocks, that is a strong clue.
You can also contact Kohler support.
Give them the model number and photos of the base.
You can also review Kohler’s toilet installation help.
Start with the Kohler installation resources.
Question
Should I use plaster of Paris under this toilet, or replace it?
Answer
Exchange it if the support ribs sit higher than the rim and cause rocking.
Do not use plaster of Paris to hide a bad toilet base.
Use toilet shims only for a sound toilet on a slightly uneven floor.
Plaster of Paris can crack over time.
It may also make future removal harder.
Plastic toilet shims are safer and easier to trim.
Use them after the toilet is set on the wax ring.
Then tighten the closet bolts evenly.
Do not overtighten the bolts.
Porcelain can crack with too much force.
After the toilet is firm, caulk around the base.
Many installers leave a small gap at the back.
That gap can show leaks sooner.
For more installation basics, see this toilet installation guide.
The best choice is simple.
Return or exchange the toilet before setting it.
A new toilet should not need patching to sit right.
Fix the floor with shims, not a flawed toilet.
I’m Chris Mayer, writing for Plumbing 101.
