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Metal over shingles, no typical attic

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Guest User
8/5/2009
I have read several forums and the are a lot of questions about installing a metal roof over asphalt shingles in a typicle attic situation. I have a log home and I have no attic. Roof consist of T/G pine ceiling. 4" studs on edge with 4" stirofoam between them for insulation. 1/2" sheeting, underlayment and then shingles. There is no ventilation in this roof. I have had no problems other than the underlayment was installed poorly and I have some leaks. I want to install metal, but am worried about condensation.
Ken Buchinger
NCI Building Systems, Inc.
8/5/2009
I would consider removing the shingles and installing a new underlayment, possibly a radiant barrier type. Then, I would install 1 x 4's parallel to the roof slope, followed by 1 x 4's perpendicular to the roof slope on top of the previous 1 x 4's (the spacing of these 1 x 4's will be controlled by the requirements of the metal roof panel). This will create a 1-1/2" air cavity between the metal panels and the wood deck. I would install air intake at the eave and exhaust at the ridge. This set-up should help prevent condensation, provide a cooler roof deck in the summer (which should translate into lower cooling costs)and help prevent ice dams in the winter.
Guest User
8/5/2009
Could I just install the new underlayment over the shingles? My roof is only seven years old. Also would the shingles serve as a radiant barrier? I have a wrap around coverd porch. Of course the porch roof is not insuiated and has 3/4 pine sheeting. The 1 1/2" air cavity would be open all the way around the house so I should have normal convection as long as my ridge cap is vented. Do you have any reserves about doing it this way. Would save some money by not removing shingles. Also have you ever heard of a condensation problem on roofs like what I have. Thanks
Ken Buchinger
NCI Building Systems, Inc.
8/6/2009
Yes, you can install a new underlayment on top of the shingles and under the lathing. The shingles will not act as a radiant barrier. I would suggest using an underlayment that also serves as a radiant barrier. I have not heard of condensation problems with your roof design. However, any time warm moist air can come into contact with a material that is significantly lower in temperature, the potential for condesation will exist.
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