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The Home Smithy
The Home Smithy, Home Builder
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What underlayment should be used under a 22g metal roof? The

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What underlayment should be used under a 22g metal roof? The roof is 3/4" t&g glued and sealed on the ends of the sheets with siga flew 1a. It was then coated with rolled on fibered aluminum asphalt, air tight. 12" TJI ceiling joists with bat fiberglass r30 insulation with 1 1/2" airspace right below the roof decking, continously vented at top and bottom eves and 5/8" sheet rock on the bottom the ceiling joists. Weather fairly temperate on sunny hill in Marin county a little north of San Francisco, Ca
Submitted: 4 years ago.
Category: Home Improvement
Expert:  The Home Smithy replied 4 years ago.
Hi. Welcome to Just Answer.
30# XXXXX felt will be more than sufficient to complete the job.
Be sure that any metal that has sharp edges is prevented from coming in contact with the felt.
Anywhere that the metal is going to bear heavily on the felt should be doubled to allow for the extra weight during expansion and contraction.
I was raised in Napa and am familiar with the weather up there. The only real concern is blowing rain getting up under the felt. As long as the bottom edge is flashed properly you shouldn't have any problems with your roof, except for the odd woodpecker trying to stuff some acorns in it.
Let me know if you have any other concerns.
Best, Smitty
Customer: replied 4 years ago.

Do I have to worry about condensation under the standing seam steel roofing, because several roofers have told me they woulod have to use a stick down moisture barrier like bituthane (sp?).


Expert:  The Home Smithy replied 4 years ago.

The only time you would need to use direct contact underlayment is when there is going to be a significant temperature difference between the two sides of a roof. In your case you haver placed insulation in the roof system to avoid that problem.

The space between the top of the roof, and the top of the insulatioon is where you need to be concerned about condensation. As long as there isnt a significant temperature difference within that area you are good to go.

As you state that there is a 1-1/2" vented space just above the insulation. That means that the material above that will be in the same temperatrure range on top as well as on the bottom.



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Customer: replied 4 years ago.

Thank you, XXXXX XXXXX my theory by which I designed the roof system.