Seattle Home Inspector's Blog


Why is my roof sheathing black?

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.

Black roof sheathing

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.

Hole in B-vent pipe 

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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Comment balloon 15 commentsCharles Buell • February 24 2012 06:09AM


I love your posts!  I always have something to look at in my own house afterward, or at least put on the honey do list :-)

Posted by Kristin Johnston - REALTOR®, Giving Back With Each Home Sold! (RE/MAX Realty Center ) over 8 years ago

Why indeed? It also makes you wonder if the home owner never went into their own attic space.  You would think they would have noticed that!  Or at least the person servicing their unit would!

Posted by Kathryn Maguire, Serving Chesapeake, Norfolk, VA Beach ( (757) 560-0881) over 8 years ago

Charles: Given the amount of black on this wood sheathing, will this roof need replacing, or will the repairs be enough?

Posted by Anne M. Costello (Weidel Realtors) over 8 years ago

Looks similar to the  roof sheathing from my last inspection. There we numerous and obvious (to me) reasons for the condition. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 8 years ago

Charles -- it is interesting that the furnace people did not check the piping when they were replacing the furnace.  One would guess they are not fully the professionals they may advertise themselves to be.

Posted by Steven Cook (No Longer Processing Mortgages.) over 8 years ago

Kristin, thanks---always love to have to stop by

Kathryn, I would say that most home owners rarely go in their attics and pretty much never go in their crawl spaces.

Anne, while some areas may need replacement, most can be cleaned and sealed

Jim, there are usually plenty of culprits for sure

Steven, and I see this over and over again---it surprises me too that this isn't considered part of what they are responsible for---seems like easy money to me.


Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 8 years ago

Charles, I learn stuff from you I never find anywhere else.  Black roof indeed! 

Posted by Marge Piwowarski, Phoenix AZ Horse Property, LLC (Phoenix AZ Horse Property) over 8 years ago

Marge, if only you lived in a wetter clime :)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 8 years ago

Charlie, I have seen this like you eluded to, from other sources. Sometimes it is not just one thing but many. Also adding to the load is the occupants not using ventilation when they have it.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 8 years ago

If they just paint the sheathing and rafters black, all that gets covered up.  Problem eliminated!

You're welcome.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 8 years ago

Last week the upstairs apartment of the duplex I was inspecting was so humid the condensation was occuring 6" up from the bottom of the windows.

No kitchen exhaust, no bathroom fan exhaust, laundry dryer vented indoors, heat turned way up and the residents were dressed, long sleves, sweaters & jackets.

They had 2 humidifiers going! It was a nice day out too, above freezing.

I say, if you want to live in the tropics...go there!

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 8 years ago

Don, now nobody would do that would they? :)

Jay, thats the ticket

Robert, nice---sometimes this just isn't rocket science :)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 8 years ago

Wow, what a mess.  I'm pretty sure it's standard procedure for HVAC contractors to not check any of the venting in the attic, ever.  I don't think it's ever been done.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Reuben, I would say that is true of my area as well

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 8 years ago

I think I found what was causing my roof to turn black and fixed it. I put to much insulation in my ceiling covering my vents. Now should I replace the OSB that has turned black or will it die

Posted by marshall over 2 years ago