Roof Mold and Ventilation

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Old 04-27-17, 04:39 PM
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Roof Mold and Ventilation

Im buying a home in southeast Alaska that is developing some mold issues; and Im looking for suggestions from you fine folks on how to take care of the problem. Ive 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. 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 5/8 CDX roof sheathing. There is no ridge vent. There are dual 12"x12" vents on 2 of the 3 gable ends and eave venting (1-1/2 2 dia holes) in every other truss space (24 oc). The builder did not put in a vent on the 3rd gable fearing leakage because of the exposure to driving wind and rain. 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. The garage floor is dirt and 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. Its definitely not as prevalent as inside the house; but there nonetheless. See the photos below. I looked at some of the other homes in the area and did not see similar issues on the underside of the roof.

Once I purchase the house I plan to immediately install a concrete slab in the garage which will hopefully take care of any interior moisture. Im hoping this is the only main source of moisture. After that the thought would be to apply some type of mold disinfectant. Ive been seeing a lot of posts recommending Foster 40-80. Finally, Ill insulate and finish the garage ceiling. I'm also considering installing a fan at one of the gables to increase air flow through the attic space.

Any observations on suspected sources of the problem and my proposed fixes would be greatly appreciated.Name:  20170424122932.jpg
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Old 04-27-17, 04:43 PM
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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 04-27-17, 05:59 PM
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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 is worst above the living area; possibly because of warmer air from living space?Name:  Mold in Garage.jpg
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Old 04-27-17, 06:02 PM
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Curiously no mold on exterior roof sheathing on south-facing porch. At least, not yet.Name:  South Exposure.jpg
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Old 04-28-17, 08:54 AM
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A garage must be isolated from the attic over a living space.
Did he get a permit to build this house and related inspections, they should have flagged that?
The round soffit vents are terrible for providing vent area, some as low as 10% net free area (NFA).

All home leak air into the attic and unless he took measures to seal that off there would be a lot of leakage. Those measures go way beyond normal construction.

Concrete slab with a plastic layer under it plus the code required closure between garage and house.

What you are referring to as mold under the porch roof may just be mildew. I'm not an expert on this topic but every piece of wood I leave outside turns dark.

Sounds like you are short on needed vent area. Guidance says 1 ft of NFA for every 150 ft of attic floor. Given the limited open area of the current soffit vents and only 2 smaller square vents in 2 gable ends your total is low. When I don't have the mfg NFA rating on those 12" x 12" gable vents I use 50%. We don't have the area for the house but if I use 1,500 ft then you would need 10 ft of NFA, half high and half low.

Fans should not be necessary and can pull a lot of conditioned air out of the house. Plus, they need a lot of vent area to support their exhaust flow. So first step is to get the natural ventilation working.

Where a gable vent is needed but risks infiltration of snow or rain, it can be connected to a duct and into a small tub that can be drained to somewhere it won't freeze. You have to keep your eye on it until you determine its need.

Once you close off the garage attic from the house you will need to provide venting other than the 2 gable vents for the rest of the house. Google "Venmar roof vents pictures" as an option.

I just noticed the 4 holes in every other rafter space in the garage photo. 4-2" holes would give you 12 in of vent which depending upon how they are covered and screened to keep bugs out my be much less, like of that at best.

Enough for now
Bud
 
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Old 04-28-17, 05:48 PM
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Thanks Bud. The attic space for the garage and for the living space are definitely not separated. That will be something I need to add to the list. There was no permit for house construction as there is no building department for the area. The owner is a life-long contractor; so I guess I would have expected him to know that requirement. The folks I have talked to up there speak highly of him and his building skills.

I'll also definitely look into adding more venting. I did not explain the gable vents well. There are actually two 12"X12" vents at 2 gabled ends. However that's still going to be short of what's needed.
 
 

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