I am building a screen house with a metal roof -- do I need roof felting, and if so, is there a non-asphalt nontoxic alternative? - Green Home Guide

I am building a screen house with a metal roof -- do I need roof felting, and if so, is there a non-asphalt nontoxic alternative? - Green Home Guide: "The roof will not need to be insulated. it seems to me I can screw the metal directly onto a simple wooden framework. I have chemical sensitivity and would like to build without asphalt, rubber or plastic in the roof, all of which heat up in the warm weather and cause outgassing. I'd prefer no felting if possible."