When moisture condenses on plywood roof sheathing repeatedly or continuously the wood will turn black. This can be caused by the action of moisture interacting with the tannins in the plywood.
The picture above is from a long-standing moisture condition in an attic where much of the roof sheathing has turned black. There are some areas with obvious mold or mold-like fungal growth present but most areas just appeared to be stained.
Any time a home inspector finds this kind of ongoing staining the prognosis for the roof is not good if repairs are not made to eliminate the moisture. Of primary concern is figuring out why there is so much moisture in the attic.
In this case, all of the existing soffit vents, of which there were too few to begin with, were all blocked with extra insulation that had been blown in over top of the original construction layer of insulation. Improving the soffit ventilation will solve most of the problem as long as there are adequate roof vents. On the other hand, you need to be sure there are no other contributing factors adding to the moisture problem.
Here is a partial list of some of the “common” pathways for moisture finding its way into the attic:
- Non-airtight lighting fixtures,
- Poor attic hatch seal,
- Missing fire stops around wires and pipes running into the attic space,
- Missing fire stops around HVAC equipment vents,
- HVAC equipment venting directly into attic,
- Standing water in condensate trays,
- Missing fire stops around chimneys,
- Dryers venting into attics,
- Bathroom, laundry and kitchen exhaust fans venting into attics,
- Missing ceiling vapor barriers.
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