When moisture condenses on plywood roof sheathing repeatedly or continuously the wood will turn black. This can be caused by the action of fungal growth and molds and the affect they have on the tannins in the wood.
The following picture is of a long standing moisture condition in an attic where much of the roof sheathing has turned black. There were some areas with obvious mold or mold-like fungal growth present but many areas just appeared to be stained.
Any time the inspector finds this kind of ongoing staining the prognosis for the roof is not good if repairs are not made.
Of course the 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 painted shut and blocked with insulation. Improving the soffit ventilation will be part of the solution---but this lack of soffit vents did not entirely account for why there was so much moisture that needed venting to begin with. Ideally no moisture should find its way into the attic space. Historically stopping all moisture laden air from finding its way into the attic has proven difficult---usually due to ignorance and laziness. Here is a partial list of some of the “common” pathways for moisture finding its way into the attic:
Non-airtight can lights,
Access hatches not being weather-stripped,
Missing fire stopping around wires and pipes running into the attic space,
Missing fire stopping around HVAC equipment vents,
HVAC equipment venting directly into attic,
Standing water in condensate trays,
Missing fire-stopping around chimneys,
Dryers venting into attics,
Bathroom, laundry and kitchen exhaust fans venting into attics,
Missing ceiling vapor retarders.
In this case the actual biggest culprit, because some of the other factors were also present, was failure of the b-vent from the furnace. With adequate venting most roofs will be fairly forgiving of some of the items on the list---even multiple things on the list. But having the HVAC equipment venting directly into the attic will likely have disastrous results. The reason for this is that the furnace runs the most when the roof is coldest and is most likely to condense the exhaust by-products onto the roof sheathing. Add to this inadequate ventilation and we have a recipe for “black roof.”
The large hole in the side of the outer layer of pipe is where the flue gases are entering the attic instead of going the rest of the way up the vent pipe to the exterior.
The big question here is: Why didn’t the HVAC contractor check the vent pipe when the furnace was replaced? In my experience, most b-vent pipe has a life span about the same as the furnace. Its condition should at least be checked when a furnace is replaced.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board