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Air gap needed

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Guest User
6/14/2009
I am confused about metal roofing and air gaps. I have mostly seen galvanized sheet metal roofing panels put on sheds and such. Standard way is to lay purlins on the roof beams and fasten the metal sheet to the purlins. Plywood decking is never used to directly fasten metal roofing to. Now I watch online videos of roofers installing metal roofing directly over asphalt shingles for homes that were previously asphalt. I thought there was supposed to be an air gap between the roof deck and the underside of the metal roofing-to ventilate it and prevent condensation. I also see them installing it directly over vapor barrier where there was no asphalt shingle (new house), again no purlins or air gap is installed it seems. I have a repair job to do and I need to know-air gap or no? Sincerely confused. Thanks.
Todd Miller
Isaiah Industries, Inc.
6/15/2009
If going over old shingles always install a layer of underlayment over the old shingles. Always adhere to manufacturer instructions for the roofing but most will not mandate an airgap. Far more important in my experience than airflow under the panels themselves is attic ventilation. Make sure that your attic is vented with intake and exhaust vents.
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