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Re: ridge vent size
Let's do some math.
house is 40 feet long by 30 feet deep, or 1,200 sq ft. This is the 'attic area' which includes a 2 foot overhang, not the 'house area'.
Required ventilation is 1:300 if 50% or more is near the ridge, 1:150 if soffit vents only.
1,200 sq ft / 300 = 4 sq ft / 2 = 2 sq ft. If *at least* 2 sq ft of net free vent area is at the ridge, then we can use the 1:300, otherwise we need to use the 1:150 rule.
Okie dokie, the ridge is 40 feet long, or 480 inches long less 21 x 1.5 = 31.5 inches for a total open area of 448.5 inches.
The ridge vent slot in the sheathing is 3/4 inch to each side of the ridge board, or 1-1/2 inches wide total. 1.5 X 448.5 = 672.75 sq inches.
672.75 sq inches / 144 = 4.67 sq ft That makes it doable, now, though, what is the net free vent area of the ridge vent itself?
This will be given in sq inches per linear foot.
The net free vent area of the ridge openings are 672.75 sq inches / 40 = 16.8 sq inches per linear foot. Thus, as long as the given sq inches of ridge vent are less than 16.8 inches per linear foot, that 3/4 inch cut out to each side of the ridge is going to be adequate.
Anyone want to check my math and make I did it right? Thanks.
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