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Decking and/or lathe with hot roof construction?

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Tommy Whiddon
2/8/2009
Hi, I am going to build a combination apartment/workshop with R panel metal roofing, unvented attic and spray foam insulation throughout (walls & roof). The rural area I'm building in has no permitting or inspection process, so I have a little bit of flexibility. I want to be as cost effective as possible but still do things correctly. Can someone comment on the following roof installation options: 1) Plywood or OSB decking with felt (or other) underlayment, metal on top of wooden lathe on top of underlayment. 2) Same as option #1 without the wooden lathe, i.e., metal attached directly to felt underlayment 3) Lathe attached directly to rafters (no decking or underlayment) with spray foam directly against the underside of the metal roofing I would like to go with #3 to save cost but am not sure if it is structurally correct. Whichever option I use on the shop will probably also be used to build a small house. Thanks, Tommy
Guest User
2/8/2009
Tommy, Option number 2 is the best. On option #1 you have the extra expense of the lathe and also your flashings have to wider to cover the extra wood. Option #3 leaves you without the peace of mind that you have a solid deck. Also, your foam insulation will be against the metal instead of against the decking. If you do #3 and ever had to take a sheet back off, the insulation will probably have to be replaced in that area. I have never done a roof this way but on some that we have repaired, the insulation is tightly adhered to the metal panel.
Guest User
2/9/2009
Wade, thanks very much for the information, I will be doing option #2.
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