Is this mold on roof sheathing

This appears to be mold on the roof sheathing in the attic. It is only on the back side of the house and is darker towards the bottom. The added layer of insulation in the attic floor is blocking the soffit vents. My question is how to approach this in my report.

From the pic it looks like mold due to the lack of air flow.
Attic insulation incorrectly installed and blocking soffit vents on North side of home. Appears, due to this incorrect installation, mold is forming on the underside of the roof sheathing.
(If your state doesn’t allow you to take a sample to send to a lab) Recommend testing by a certified Mold Assessor.


Thank you Robert. That is the direction I was heading. I appreciate your input.

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I would say “microbial growth” instead of mold and do your same recommendation. Per one of the InterNachi lessons, you should avoid using the term “mold” unless you are testing for mold


Correct Ryan. I always use “mold like substance. without testing I can not determine if the substance is mold or not”


May I ask how you came to that conclusion just by look at one photo?


Blocked soffit intake vents are ineffective in providing good under-roof or attic ventilation causing wet and possibly moldy roof sheathing on those roof sections where no venting is provided.
Proper attic ventilation in your home is important for many reasons. During the warmer months, heat builds up in your attic during the day and results in higher energy costs for cooling the home. Also, moisture within the home may move into the attic and if this moisture is not properly or effectively removed, it can condense and cause insulation and construction materials to deteriorate. Thus, temperature and moisture control are the major reasons for providing attic ventilation.
Attic ventilation is critical to the performance of the roof and attic structure.
A properly ventilated attic will help building materials last longer and help protect the home from costly damage.


Good statement Marc. Thank you for your input.

I am with Roy, I wouldn’t be the troubleshooter or reporting any diagnosis… I’d always let the expert do that. I would report observation and recommend correction of venting as well as anything else the “expert” deems necessary. If a contractor that comes after you finds others issues contributing to the “mold” issue, you’re on the hook because you stated it was the venting!

Roy, mold don’t grow where air blows.

Simply use words “I suspect that it is due to lack of ventilation”

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Additionally ventilation, by cooling the attic, should extend the life expectancy of your roof covering.


Posted by Martin for the owner Lizbeth K.
Without fresh air circulation (blocked bird vents) this could very well be a microbial substance usually Aspergillus. Seen this many times in attics. Most common is a bathroom exhaust vent not exiting the roofline. (Broken or kicked over). Definitely call or recommend a Mold inspection company to test before calling in a company to remediate.

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I’ve seen a lot of homes this summer with cooked roofs and mildew growth on the trusses and sheathing. Really surprising that so many insulation contractors will blow or roll the attic full and block off the soffits. Only about 1 in 20 this past month has been properly ventilated.

Sometimes the attic will be OK & the problem got created from the exterior.
Here’s one from 3 weeks ago. The home is only 4 years old.
Some dum-dum sprayed the soffit vent screens closed.




I would try to ascertain where the moisture was coming form.
Why is it moist in the first place…

Kurt, I had a similar issue few months ago, see photo

I observed this from attic hatch, I did not went further because I did not had the proper PPE to enter the attic, I disclaim inspection of the attic due to the health concern.

I am a licensed mold assessor - this is what I wrote:
Apparent mold growth observed on the underside of roof decking. Mold assessment/sampling is not within the scope of home inspection as per NYS home inspection Standards of Practice. Recommend further evaluation/assessment by a Licensed Mold Assessor. I don’t use the word microbial (like mold, bacteria is also microbial).

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If that is mold that place is shot…
It looks like smoke/fire damage.
But you being an assessor it must be mold.

Sounds good.
Thank you

Appears to be that but I would refer to that condition as “microbial growth” unless you have tested it to confirm that it is actual “mold”.
Just trying to limit your liability risk.