What could the discolored areas on the underside of my roof be?

Question

I had an inspection done on my house last year before I bought it, and was told by the inspector that the dark discolored areas of the plywood on the bottom side of my roof was likely "dark staining cause by poor ventilation and air circulation" and that it did not appear to be mold.

I guess my question(s) are what would cause the dark stains? Moisture? What else could it be besides mold?

You can also see dark areas on the plywood on the sides too.

I've seen several pictures online that looks similar to mine that claim they have mold.

I just wanted some ideas as to what this might be before I spent a lot of money on mold testing and treatment.

Answer 1

I doubt if it's mold, but don't trust me or any other so called expert. Have it tested.

The dark areas seem to be along plywood joints and not in "irregular splotches". Mold tends to grow in random shapes...

However, it appears there is a lack of attic vents. There should be vents along the eaves and I don't see any daylight along the eaves in the picture. When moisture gets in the attic and then condenses, it will form on the plywood sheathing. After a few years of this it will discolor the plywood, especially at plywood joints (where there is less resistance from outside air to attic air) and around roofing nails. Are there dark areas around protruding nails?

Mold can form anywhere, but I think this is moisture and a lack of ventilation.

Answer 2

Mold generally forms where there is moisture and a lack of ventilation. This, however, looks old because your roof is clearly dry.

Which suggests that at one point the roof was leaky, got wet, didn't dry out quickly, got wet again, etc but if the shingles on the roof (assuming shingles) are newer then the new roof addressed the water penetration issue. Eventually the roof dried out, the mold died, and left behind that staining.

I would monitor the roof over time to see if it spreads/grows - and look for any wet spots. However, if you find none, then I'd LEAVE IT ALONE. As long as you don't disturb it, you won't spread any spores, and if it's dormant (dry/inactive) it won't spread spores either.