insulated metal roof over 2x6 cathedral ceiling/deck

Guest User
3/21/2008
Our church ran into a problem when replacing the shingles on the 5000 sq ft cathedral ceiling of the sanctuary. We discovered that the low pitch cathedral ceiling had no insulation and that the finished wood ceiling and roof deck were one in the same: 2x6 tongue and groove(really about 3" thick)We have decided to install a metal roof with iso board for insulation. we are considering 3" minimum r-18 up to 4.5" r-27. we have had conflicting advice on how to gow about this. 1. The shingle roof was over tar paper directly on the deck, no moisture barrier. There never has been any condensation on the ceiling of the sanctuary. 2. we installed a tarp over the bare deck and it balloons up from air leaking through the deck joints. 3. we want to put down 6 mil plastic as a air/vapor barrier, then put down the iso board directly on the plastic. 4. Next we want to install furring strips horizontally every 24" and install the metal roof to them, using long fasteners to screw it all to the wood deck. 5. Is this an acceptable method, or are we missing anything important. I assume that if the ceiling didn't form condensation, then this assembly won't either, except under the metal roof. What can we do to enhance the insulation qualities, minimize moisture problems, and eliminate heat loss due to air escaping.
Guest User
3/23/2008
Mike, We have done several buildings of this type. We do it one of two ways and I prefer this first method the most. Tear off the old shingles and install a peel and seal underlayment. This keeps the building in the dry while work progresses. We use Tamko's Metal and Tile Underlayment because it is easily obtainable in our area. Install a 2x4 laid flat around the perimeter--all eaves and rakes--screwed firmly in place. Screw length is important, you don't want to penetrate through your 2x6 deck. Install another 2x4 directly on top of this one. This gives you a height exactly the same as the thickness of 3" ISO board. Next, install a row of ISO board butted into the double 2x4. We then install 1/2" OSB even with the outside of the 2x4's. We nail the decking into the 2x4 but in the field we use stress plates and screws. Screw length is critical--you don't want to penetrate your decorative ceiling. We use stress plates because they spread the load and you don't have to worry so much about running a screw through the decking. We now install our underlayment--usually 30 lb. felt and the metal roof. Another way is to do the same thing up to the point of installing the ISO board. We use a table saw and cut the ISO board in half the long way. We butt the ISO into the eave 2x4 from end to end. We then install a doubled 2x4 butted into the ISO. We repeat this process all the way up the roof. This will give you a solid surface to fasten the metal to at 2 foot intervals. We then install our underlayment and metal. You can buy ISO with OSB already glued to it but our problem with this is the manufacturers don't always get the ISO and OSB squared up precisely with each other. It also costs more than buying the material separately and doing it yourself. I hope this helps you. [email protected]
Todd Miller
Classic Products, Inc.
3/26/2008
Thanks Wade. This sounds like a good solution. I do get concerned about condensation up here in Ohio where this person is located but I think the self adhering underlayment would act as a vapor barrier.
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