20' timber should not be a problem, 40' could be but still possible.
Roof Top Guard is a brand, I believe, and yes it replaces tarred paper/felt. I am about to put some down in stead of tp, and a metal roof directly on top of that. I am after the additional mass of the roof surface to keep the metal from changing temp too quickly and easing the sweating problem. I don't see any problems with doing this, maybe someone will point out a reason.
Collar beams, even so, someone may one day, and what a bugger it would be.
Tim
