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TOPIC: I Have An Existing Metal Roof and Have A Question

White rust from acoustical decking

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I have from bottom to top - acoustical galvanized coil coated decking, plastic covered insulation in the low points, 5/8" treated plywood screwed down with SS screws, rubber membrane flashing, standing seam metal roof.

I am getting white rust from the acoustical holes on hot muggy days.
Why?

jason helton

11/1/2021
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This sounds like condensation which is the result of warm moist air originating inside the building trying to drive its way out and encountering a cooler surface along the way.

Todd Miller
Todd Miller
Isaiah Industries, Inc.

11/2/2021
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It is an outside pavilion. And I was told there should be no white rust. Just trying to figure out where the mistake in design may be.

jason helton

11/2/2021
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Can you send me a photo of what you're seeing please? [email protected]

Todd Miller
Todd Miller
Isaiah Industries, Inc.

11/2/2021
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12682.jpeg

Roof detail.pdf

jason helton

11/2/2021
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Thanks. I have a better understanding now. Have you talked to the manufacturer of the acoustical panel? My best guess is water vapor is passing through it and condensing someplace in the assembly and the corrosion is occurring. I'd ask the panel manufacturer if they can come do a site inspection.

Todd Miller
Todd Miller
Isaiah Industries, Inc.

11/2/2021
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Yes, they have done evrything according to specs.
We believe it could be the 1000 holes they drilled in it that once drilled it created a exposed galvanizing. But simply, that it holds too much condensation and the design should have a rubber barrier between the decking and wood, and there should have been 2 inches of insulation to help control the condensation.

Simply: decking supplied per specs, deck installation per specs, wood installation per specs, standing seam roof supplied and installed per specs.

Everybody is at a loss.

jason helton

11/2/2021
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I agree that I was leading toward it coming from moisture dripping down against the perfed holes and then causing corrosion around the holes. This is an unusual application and normal dynamics don't come into play which would protect the exposed steel edges.

Todd Miller
Todd Miller
Isaiah Industries, Inc.

11/2/2021
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