Mold just on north side of house roof plywood

New guy here, found mold just on the north side of house in attic the south side is perfectly fine. It has clear soffit vents about every 6’ and ridge venting across entire ridge. On the shingles there is quite a bit of moss growing entire north side of roof. To me it seemed like the air flow was adequate and didn’t see any moisture on the rafters. Is this happening because of the moss buildup or not enough air flow through the attic or something I’m missing? Any ideas or thoughts would be appreciated, thanks

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Lack of sunlight. Not at all uncommon. Definitely want to call it out for cleaning/monitoring/cleaning/monitoring. And then more monitoring and cleaning as needed

We see this a lot in our climate (PNW).

The moss holds moisture and creates a cool area on the north side, in addition the north section of the roof gets less sun exposure.

This causes a condensing effect on the underside of the sheathing and eventually can lead to mold or mildew growing on the sheathing.

I see it all the time, the condensing effect is just my guess.


Welcome to our forum, Donald!..Enjoy! :smile:

Thank you for the quick response

Any pictures of this “mold” in the attic?

Any chance I could tell the client a new roof (and keeping it clear of moss) and better ventilation would fix the problem?

Sure, if you want to pay for the real fix if that doesn’t work. :flushed:


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You’d probably be right in that assumption. Additional ventilation could prevent it altogether.
Moss is the lesser of the two concerns (unless it’s A LOT)

As a side note I’d be careful with making too many recommendations for the remedy, it’s helpful but leaves you a bit more liable.

I’d tell the buyer your opinion of possible fixes, but in report recommend a licensed/qualified professional review and make repairs/recommendations as needed.


I hear you, I always want to help them out but need to remember my place

Thanks Patrick, that dose help me out.