We installed a shower floor kit to be used with tile which includes specific instructions for proper installation. After installing with the pitched level bars in place, we noticed that the areas along where the wall meets the floor along the widest portion of the shower floor has a lower pitch than the interior corner sides. Even though the mortar mix is down, the center widest portion is about 1/2 lower than the inside corners along the walls. Is there a way to add thicker thin set to those low areas to compensate for this so all is level along the wall edges while maintaining the proper pitch to the drain? If not what can we do to fix it?

Level with thinset.

Would you recommend putting some kind of polymer or apoxy additive to the thinset to be sure it doesn't give under the weight of stepping on the shower floor? If so, what kind?

Thank you!

If you leave the floor for 2 days, you should have no problems using the thin set as a leveling agent.

Chuck