On Tue, 2 Jan 2018 20:37:19 -0500, Ed Pawlowski wrote:

What can be used to shim a toilet on a ceramic tiled floor.

My neighbor has a toilet that rocks from front to back, the fulcrum
point being roughly where the bolts are. It sits on tile on top of a
concrete slab so correcting the floor is out of the question.

Gravity keeps the toilet sitting to the back, when you sit on it, it
rests on the front and goes back when you get off.

Temporarily, I was able to slide a paint stirrer under the rear to
support it and it does not move when you sit on it. I'm thinking or a
grout type material that can be squeezed in the gap and then will set
hard. Wood will eventually get wet and rot. Silicone would be good to
put under but may be too soft to support the weight. What about a few
washers with silicone?

The nuts on the hold down seem snug and I don't want to crank down on
them and break the ceramic. I don't mind helping, but I don't want to
buy him a new toilet.

You can use a hardwood shim and silicone bathtub caulk to neaten it up.
I've done it a few times. The shim won't rot unless you've got a bigger problem.