I just had the warping issue for the first time today. I used masonite 5/8" thick presswood for the shelves and 2x2's for the rest. I have had it functioning for three years with no issues, but today i decied to see why the top tub was so hard to slide, sure enough the top had swollen. non of the others show any signs of sweeling. I think i missed the 2nd coat of exterior valspar paint when i was coating them. I say as long as the shelves are sealed up correctly there should be no problem with swelling or bowing. make sure you paint before you build though. my first rack was so tight i couldn't put a tub in it let alone heat. all because i built it then painted it well (like 5 coats).