how thick should shed walls be Garden shed slab thickness; archive view.. make it higher and close as possible to a level 100mm thickness? the shed is going over a sewer so i was trying... Roof shed types,big sheds brisbane,how to build shed door frame,lean.. I am thinking of building a shed of 12x21 size... what is the best thickness of plywood for walls,.. the floor should be 3/4" thick.. Garden shed slab thickness; archive view.. make it higher and close as possible to a level 100mm thickness? the shed is going over a sewer so i was trying... How to build shed walls... you can use the surface of your shed floor to build either wall first. first, measure and cut your top and bottom plates.... ...