How do I level a new fiberglass tub installed on an uneven slab ?

Do I shim the front of a fiberglass tub in order to compensate for an uneven slab ? Shims made out of what ?
Long version -
Replacing an old worn cast iron tub with a new American Standard Americast fiberglass tub. (Old one was 60 years old, nasty, and of course cold). This brand of tub does not get a mortar bed under it, the weight of the tub rests on the stringer and the fron edge of the tub basically.
The plumbing is real simple, I'm reusing most parts and have no rough ins to change or anything.
BUT the slab is crooked. The tub is a right hand drain and the floor slopes down half an inch (left side is half inch higher than the right side where the drain is).
The back of the tub will rest on the stringer so it will be level no matter what but the front lip of the tub will be a problem.
Do I use shims ? Made of what ? Plywood ? How many shims, like one every foot ? I'm concerned about the weight of the tub (and me and water) resting on that front lip.
Also thought about leveling the floor but I've already done that to the extent I could, any more leveling to fix the tub area will cause there to be a half inch step to get into the bathroom. Ans Level Quik ain't cheap.
Thank you.
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snipped-for-privacy@yahoo.com wrote:

of scribed trim.(The plastic faux wood trim made from old pop bottles is great for bathrooms.) Don't need to shim the whole space, just where the hardpoints are on the bottom. Look at the bottom, and at the digram usually printed on the box. Just like your car has special spots to place a jack, a glass tub has pads to catch the weight. Make sure you don't raise it too high- stepping down from tub to bathroom floor confuses people and makes them fall. (My sister's house guest suite is like that- I added a grab bar before I left.)
aem sends...
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Yes, shim the front with something plastic, pvc boards if seriously off, if not too much look at the recycleing marks on some used plastic containers, try to find ones with the number 2 on them which is High Density Polyethylene, usually laundry soap containers, #1's would be the second choice, I.E. soda bottles. The tub should be level on the edge. Most fiberglass tubs these days have the pitch built in to them, you can verify on your brand with a level. Also, while it does not get a mortar bed, you should use plaster as a bed, not the premix drywall compound, use a setting type plaster. If you don't, you will have a "spoungy" tub. Most manufacturers also recomend this for rigidity.
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