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Stains on ceiling


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I was hired by a condominium management company to look at stains on the ceilings of two units. They are not water stains. The stains go all the way through the drywall. Some wood adjacent to the drywall is also stained, and a substance has dripped on some ductwork in the attic.

The units had FRT plywood that was replaced. I am guessing that this caused the stains, but I have never seen this before. Also, the FRT was replaced with an OSB sheathing that has a fiberglass reinforced Magnesium Oxide layer on the underside. This material was stained at some areas, but I would not think it could have caused the staining on the drywall.

Any thoughts?

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No odor and not a single mouse dropping.

I think condensation is a factor, but the stains are not just from water. I think the treatment chemical leached out of the FRT. This would explain the corrosion I observed at some nails and the somewhat oily nature of the stains. The treatments are primarily salt based so they absorb water.

BTW, The stains peg a moisture meter, but directly adjacent to the stains the meter read zero.

Finding the exact cause of the stains is beyond what was required, but I always like to learn, especially when I see a condition I have never seen before.

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It looks like an oil-based substance dripped from the roof sheathing. The stains on the wood in the third pic show that. As you mentioned in your post, it seems to be a result of work done on the roof, or from the sheathing itself, some product that soaks through drywall. It needs the scratch and sniff, taste test. Now that mouse pee is ruled out. [:)]

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