Black/grey mold spots on attic gable vent blockers

I had a new roof installed 15 months ago. Because I switched to a ridge vent, I blocked my 2 attic gable vents with plywood nailed up over the vents from the inside. The 2 pieces of plywood are now covered in spots of black/grey mold. Any ideas on what could be causing this?

At first I thought it was a general attic moisture/vent problem. But there are no other signs of mold or moisture on the roof sheathing or trusses, or any wood in the attic. Only the plywood blocking the vents. Both bathrooms and the kitchen fans are vented up through roof vents and were replaced when I replaced the roof. My roofer swears I have enough soffit vents for adequate ventilation. Should I drill out some more soffit vents, anyway?

Then I thought maybe the plywood was already contaminated somehow. When I replace the plywood, should I install insulation on the inside and housewrap on the outside? What about painting the plywood with a good outdoor primer and paint?


sounds to me that for some reason, the gables are still acting as an intake. Is the plywood tight or can you feel drafts? Also, plywood is not an exterior product. You should have painted it in the first place. I would think that after this much time the plywood is soaking water and is twisted and destroyed. Another possibility, if the moisture isn’t from within, are the gables leaking from wind driven rain still somehow? There has to be moisture causing the mold, but not necessarily from within in my opinion. I may have missed some details of your post too, but is the mold centered on the plywood or near the edges? To me, that seems to be the key. Follow patterns in the mold and you will find the problem.

WHen i close off a gabel vent i use aluminum that is caulked to the wood then i use a piece of plywood. Sounds like it is leaking but i might be wrong i would have to honestly look at the home to see if there is enough soffit vents.

Thanks for the info. Wind-driven rain, thin indoor plywood and plywood not air-tight – it’s sounding like it’s one or more of those. I’ll replace the plywood with something better and see if it comes back.

Make sure that you seal it with a quailty caulk of some sort to prevent this from happening again. Good luck with your project and have a great day!

I heard a new potential cause of the problem: condensation…

“In winter, the temp in the attic will generally be warmer than the air outside. The plywood blocking the gable vent will be in contact with both, and will therefore have a relatively cold surface exposed to the air inside the roof. This will cause condensation to occur on the inner surface, in the same way that a glass window will experience condensation when the air outside is colder than inside.” So the recommendation is to change the plywood to a double-skinned panel with insulation inside.

It also can never hurt to add more insualtion to the attic and add more soffit vents.

Thoughts on this? Thanks.


There is no insulation on the plywood beside this plywood?

Nope. I just nailed up a sheet of 1/4" plywood over the gable vent.