Mold on Roof Sheathing

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Old 04-25-17, 08:43 AM
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Mold on Roof Sheathing

Iím buying a new home in southeast Alaska that has developed some mold issues; and Iím looking for suggestions from you fine folks on how to take care of the problem. Iíve read many of the previous posts on the subject but did not run across circumstances quite the same as I have; so here goes.

The home is new(er); being built by the current owner/builder over the last 2 years. He has a good reputation, has been in construction the last 40 years, and seems to be a very honest person. He has been doing this a long time and says he has not seen a similar situation, although most of his work has been in eastern Oregon.

The mold is growing on the underside of the roof sheathing and has been spreading over the last 8 months; which is when we first started looking at the house. Photos are shown in following posts.

The house has a metal roof with 5/8Ē CDX roof sheathing. There is no ridge vent. There are dual 12-inch vents on 2 of the 3 gable ends and eave venting (1-1/2Ē Ė 2Ē dia holes) in every other truss space (24Ē oc). The builder did not put in a vent on the 3rd gable fearing leakage because of the exposure to driving wind and rain. My first impression was there is a fair bit of venting for the size of the house; but thatís just my layman view. A schematic of the house is included below.

There is no leakage in the roof and the dryer and bathroom are vented to the outside at one of the gables; so there is no obvious sign of where warm, moist air would be coming from. However, the garage floor is completely unfinished and there is no insulation in the ceiling. This space opens up to the rest of the attic space and is a likely culprit for where moisture is generated on the interior of the house.

One thing that really surprised me is that there even appears to be some mold growing on underside of the plywood on the covered exterior walk-round deck. Itís definitely not as prevalent as inside the house; but there nonetheless. I donít think that was there when we first looked at the house. See the photos below.

Once I purchase the house I plan to immediately install a slab in the garage which will hopefully take care of any interior moisture. Iím hoping this is the only main source of moisture. After that the thought would be to apply some type of mold disinfectant. Iíve been seeing a lot of posts recommending Foster 40-80. Finally, Iíll insulate and finish the garage ceiling.

Any observations on suspected sources of the problem and my proposed fixes would be greatly appreciated.
 
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Old 04-25-17, 08:49 AM
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Interior mold on the underside of the roof sheathing inside the garage. There is no floor in the garage, which may be the source for moisture on the interior of the house. You can see the attic space above the living space in the far right of the photo. The mold appears to be worst above the living area; possible because of warmer air?Name:  Mold in Garage.jpg
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Old 04-25-17, 08:54 AM
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Mold beginning to grow on underside of roof sheathing under exterior walk around deck. It is growing on the east and west sides of the house. Name:  East Exposure.jpg
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Old 04-25-17, 08:58 AM
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Curiously no mold on exterior roof sheathing on south-facing porch. At least, not yet.
 
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Old 04-25-17, 09:01 AM
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As we know mold requires moisture.

You stated that the house is 1200 sq ft and only has 2 12" round gable vents.

You need 288 sq in (MIN) of free vent area and you currently only have about 225.

On top of that they are not ridge vents so air flow is probably not perfect.

Not sure if there is anything unique about your area but it appears your ventilation is not sufficient!
 
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