Bathtub on Wood Shims?

wood rots

tubs with feet (claw tub) have adjustable feet so you should never need shims. If the feet are on wood shims it was installed improperly. NEVER USE WOOD TO SHIM A BATHTUB

Very frequently, with overflows that make their way to the wood below, with the weight of the tub, the wood sags a bit. Sometimes, quite a bit. So the new tup needs to be set level, and so it is shimmed. Unless you are in an area of specific municipal codes calling for construction otherwise, your understanding regarding earthquake preparedness is not correct.

that is only for fiberglass tubs

plaser of paris or mortar will work better than thin set. thin set disolves when wet.

according to you it was perfect, it's too bad you used the WRONG stuff (thinset)

As far as I know, most manufacturers will indicate that the shimming is to be done by bedding the unit into Quikcrete or similar product. Keeps it from having single points of support that can cause fracturing later. Even nicer, IMO, to bed not only the outer edge in concrete, but also the main part of the tub if it's fiberglass. Makes for such a secure feeling when you're taking a shower. When I did my remodel, I used thinset for under the unit, but you really have to work quickly. I built the levels up till they were almost there, then a last glop over it all and put the unit in to be pressed into position. The job was perfect, but it's not a job for the faint of heart.

No it does not. Thinset is mortar -- Portland cement, sand, lime. It sets by chemical reaction and dose not dissolve in water. It is what you use on cement board to set tiles in wet locations, among other uses.

TRY THESE TESTS

TEST 1 put thinset in a bucket let dry fill with water see what happens?

TEST 2 put mortar in a bucket let dry fill with water see what doesn't happen?

thinset is NOT mortar.

that is correct thinset is usually NOT inches thick though mortar can be!