Hi all,
I’m putting a Galvalume roof over T&G decking. I had the impression that a metal roof needed paper under it whenever it was put over solid sheathing but my local inspector says the paper isn’t really needed. I respect this guy but I wouldn’t mind getting a few more opinions on this one. How many of you put paper under metal, and what are your reasons for doing so?
This is on a 12/12 pitch so I’d rather not have to worry about paper tearing underfoot anyway, but I want this roof to last and as much as I’m paying for the decking I want that to last as well.
Replies
The reason for paper under the metal is to act as a vapor retarder or sweat sheet due to condensation that can form under the metal. I would also make sure that if possible that the metal be installed with clips. Anchored directly to the sheating, and with temperature changes this will sould like a locomotive traveling through the house at night. Standing seam is the best answer.
I've done it both ways, and prefer having the paper for insurancce, as well as a friction layer to protect the back of the metal when it is moving from thermal expansion.
But we talked about this issue a while back and one guy mentioned that his manufacturer had reasons to exclude tarpaper from it recommendations. So checking in with them is a good idea.
From an installers viewpoint, and this would be good in the heat there, too, is to paper, then lay 3/4" furring strips @ 24-30"OC horizontallly. You have a built in ladder to work from and a better roof with the metal not in direct contact with the paper or deck. Edge trims cover the space from view.
Excellence is its own reward!
Thanks all. I called the manufacturer and they recommended paper. Since the sheets are 3' wide I may try to run the paper down the roof with each sheet and screw through the whole thing (the existing house is screwed on, Rainguard crimp.)
Piffin, I'm still thinking about your idea of laying down furring. It does help cool things. I just need to talk myself into it.
We always refered to the paper as a "slip sheet". My assumption was that it performed the same purpose as rosin paper below a wood floor-that is to prevent the two materials from sticking to each other thus reducing noise and controlling the expansion. That said, there must me a manufacturer's system that you are using. You answer should be coming from the manufacturer.
BTW-if this is on a residence or a commercial project, I hope that you are using a concealed clip system. The systems that screw through the metal leak. Carefull installation will reduce the leaks, but they do leak.