I'm planning to build a 3 season room on the back of my house and was wondering if I can us I joist for the flooring? The roof will be framed and supported buy 4x4 with concrete pillars 42" in the ground(frost line). The floor will span from my house to the post and will be opened to the ground and old patio 8+" of clearance. Can they be used being that close to the ground? Using I joist due to I want to run 18' with no xtra supports.