Originally Posted by
socket
Welcome and thanks!
The gym build was a success. There wasn't much lateral slope, so we decided to try shimming with hardboard before pouring the self-leveling underlayment. We were able to get it level just with the shims, so I didn't end up needing the underlayment. There are some spaces between the shims that have potential for bowing, so if that happens, I'll go back and do it "right" with the underlayment. In retrospect, I wish I had just manned up and poured it, but I'm still happy with how everything turned out. Thanks for all of your help!