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This small building has a footprint of 10' x 12' with a gable roof (sideways, so the gables are 10' apart). Slope is 4:12, so the peak is only 2' above the walls. My ridge isn't vented, so I'll be using gable venting.

The building is cabin-like, so the ceiling will have drywall, a vapor barrier, and insulation. There will be plenty of water vapor to deal with, and the interior will have its own ventilation. Our climate is very mild (snow dams and summer cooling are rarely a problem), but humidity is high most of the year.

I know it's common to vent at eaves/soffits, but they're easier to construct if I skip venting there. Since my roof is only 10' long, I think the middle of the attic will still get enough airflow with only gable-end vents. Is that correct?

I've read that the basic rule for sizing screened ventilation is attic area / 150, but area / 100 if it's louvered, too. If I only do gable-end venting, then I put 1/2 of that at each end, correct?

Any other tips?

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Don't overthink it Jay. A nice 9 to 12 inch screened/louvered gable end vent on each side will be great. –  shirlock homes Oct 22 '12 at 9:44
    
I will second the advice of shirlock homes. The only point for condensation might be where the insulation prevents air-flow at the ceiling joist to exterior wall connection. Upsizing the rafter material would permit enough space for air-flow everywhere. –  Paul Dec 18 '12 at 13:39
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2 Answers

Venting should be 1/300 of insulated ceiling area, distributed uniformly on opposite sides of the building. (1/150 if slope is less than 1 in 6)

So whether you go with your 1/150 number mentioned in your question, or 1/300: using any vent larger than 8" x 8" at each gable end you are already going well above and beyond either requirement, so like sherlock homes mentioned in his comment, don't overthink it.

If you use a 12" round or octagon louvered vent at each end you will be safely above 1/300, 1/150, and 1/100.

"The recommended ventilation ratio to provide for vented attic assemblies when an air barrier is present, is the 1:300 ratio (as specified by most building codes). This is based principally on good historical experience and simple psychrometric analysis (Handegord & Giroux, 1984)." - Building Science Digest 102 (2006-10-26) - Understanding Attic Ventilation

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Your size looks comparable to my shed, which is 11'x9', also a simple gable roof with a ridge beam. What I did when I laid the roof plywood panels was keep the top edge of panels on both sides about 3" away from the ridge beam so when I set shingles, I used a ridge vent. I also have soffits/facia and have simple 4 (2 on each side) 4" soffit vents (pointing down). I don't know if it's too late for you to install the ridge vent but I think it is the ultimate solution against ceiling mildew and other forms of condensation. Unlike you, I don't have drywall inside -- it is just pretty much a tool shed on steroids (has water and power).

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He said that he doesn't want soffit vents. A tool shed with water and power sounds mighty awesome though. –  decker Feb 13 '13 at 23:34
    
thanks... it is pretty friggin cool, i can tell you that. i got a ton of shelves inside, room for a road bike, and (will build) a racks for my kayaks on one side. sorry, this is kindof O.T. but i'm passionate about my craft :) –  amphibient Feb 14 '13 at 1:24
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