Don Cross
9/11/2015
A few weeks ago you provided me with answers about the use of the 1 x 4 lathing, saying to run vertical and then horizontal 1 x 4's to provide a vertical gap between the decking /underlayment and the standing seam metal roof. I went back to the contractor and was told that they don't do that, and currently the contract has only the 1 x 4 horizontal lathing. The lathing is yellow pine, untreated because of the issue of treated lumber corroding the galvalum/metallic paint coating. So, what my worry is is that the contractor is doing what I have told him, but what may not be the right thing to do. The problem being that condensation from the bottom side of the metal roof will wet the yellow pine and cause rot. Their reason for the 1 x 4's is strength, the contractor's reason being that the plywood decking does not provide sufficient holding power for the clips of the standing seam metal roof. So, the issue may be one of the contractor having a shorter term view of what is best versus my concern of a longer term view of what is best. They have also said that they build the roofs both ways, with and without the 1 x 4's, but I think that is a cost issue more than what is the best issue. So, the question is will the 1 x 4's rot in a few years or last as long as the standing seam metal roof? Another related question is there a gap between the bottom of the sheet metal panel and the 1 x 4's, that the clip actually holds the panel up about an 1/8 inch so it is not in contact with the wood and there is some ventilation, and for the condensation to run down? A third question is does anyone ever paint the 1 x 4's or just leave them bare?
Todd Miller
Isaiah Industries, Inc.
9/11/2015
Thanks. Good questions. I have never seen anyone paint the 1 x 4's. I also rarely see condensation / rot issues though your concerns are valid -- the horizontal 1 x 4's will block water from draining off the roof. Provided the roof is installed properly and there is good ventilation in the attic space, you should not see large amounts of rain water or condensation beneath the metal panels. Will you have good attic ventilation? As far as strength of fastener pull out resistance into pine vs plywood, I suspect others have heard that argument before but I have never heard it. Most metal roofing uplift tests (to simulate wind pressures) are performed on installations over 1/2" plywood decking. I really do not think there will be an appreciable, or necessary, improvement with going over the 1 x 4's unless for some reason the roof deck is already in a weakened or jeopardized condition due to previous roof leaks or attic condensation problems (which would indicate a lack of attic ventilation).
Guest User
9/17/2015
Thanks very much for the information. I will stay with using the 1 x 4's since the contract is already signed, and it is best to not make unnecessary changes. The attic has good ventilation with soffit vents running continuously at the bottom of the attic and a ridge row ventilator running full length of the peak of the roof. And it is well insulated above the ceiling. The attic is comparatively cool to most attics in the summer. I have looked at sheet metal around here and it appears that sheet metal that is open to the atmosphere (such as carports) get heavy condensation, but the interior side of enclosed sheet metal is dry (such as the inside of a metal building). So, I will assume that the interior space created by the 1 x 4's and decking will be enclosed and dry. That would leave a leaky standing seam metal roof being the main culprit, and my guess that would be a low probability since this roof is a very simple design. Thanks very much for your help.

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