Jerry's math for sizing roof ridge vents for a 1,200 square foot attic is correct. The home would require a minimum of 2 square feet of vent area based on the 1:300 ratio if at least 50% of venting is near the ridge. Jerry calculated that a 3/4 inch wide slot along the 40 foot long ridge would provide enough vent area. Manufacturers specify the proper width for their vents, which must be followed to ensure waterproof installation.

1. Sizing Roof vent for your home.
ridge vent size per sq. ft.
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.
Re: ridge vent size
The minimum and maximum width of the slot are not usually specified by code but
by the manufacturer's installation instructions. The code will let you figure out the
minimum square inches of net free vent area NFVA) that you need.

2. Sizing Roof vent for your home.
The manufacturer makes a vent that has a certain amount (in square inches) of net
free vent area per lineal foot of ridge. This sets a limit on how narrow the slot can
be, so that the vent and not the slot is the limiting factor on air flow through the
vent. The vent itself will be made to a certain width and only extends to cover the
roof a few inches on either side of the ridge. The manufacturer will specify a
maximum slot width which, if not exceeded, should ensure a watertight joint
between the vent and the roof covering.
Let's go back to that minimum slot width again and see how narrow it could be.
I've seen typical ridge vents with NFVA as low as 9 sq in per ft of ridge and as
high as about 18-20. Take that low flow vent at 9 sq in/ft. If you use that one, you
need at least 9/12 or 3/4" total width of a slot (clear of the ridge board) or 3/8" on
either side of the ridge. This is to ensure that the vent, and not the slot, is limiting
air flow. Take a higher flow vent at 18 sq ft/in. On that one you need 18/12 or 1.5
inches minimum total width (clear of the ridge board) or 3/4" on each side if the
ridge. Cutting slots wider than the minimum based on the NFVA of the vent you
are using does nothing to increase ventilation; it increases the chances that the vent
will leak.
Check on Jerry's math:
2006 IRC 806.2 has the requirements.
Quote:
§RR806.2 Minimum area. The total net free ventilating area shall not be less than
1/150 of the area of the space ventilated except that reduction of the total area
1/300 is permitted, provided at least 50 percent and not more than 80 percent of the
required ventilating area is provided by ventilators located in the upper portion of
the space to be ventilated at least 3 feet above eave or cornice vents with the
balance of the required ventilation provided by eave or cornice vents. As an
alternative, the net free cross-ventilation area may be reduced to 1/300 when a
vapor barrier having a transmission rate not exceeding 1 perm is installed on the
warm-in-winter side of the ceiling.
Let's assume a gable roof and our ventilated space (inside the attic) is 40 feet long
(at the ridge) by 30 feet wide (eave to eave) or 1200 sq ft.

3. Sizing Roof vent for your home.
Using 1/150, the required total NFVA is at least 8 sq ft. Using 1/300, it is at least 4
sq ft.
Let's assume worst case of NO soffit vents and either no vapor barrier or one with
an effective perm greater than 1. If we cut a slot along the entire length of the ridge
inside the gable end walls (assumed to be 40 feet), we would need a minimum
clear slot width (8 sq ft / 40 ft) x (12 in/ft) = 2.4 inches (total on both sides of the
ridge board). For this to work, we'd need to find a vent that provided at least 29 sq
inch NFVA per foot of ridge. (Note - some manuf say you couldn't do this
configuration with their vents, because their install instructions specify use of their
vent in conjunction with soffit vents.)
Let's assume a more typical case where we have soffit vents and they provide at
least 50 percent of the required NFVA. Now we can use 1/300 which requires 4 sq
ft total. We are going to meet that by providing 1 sq ft at each soffit and 2 sq ft at
the ridge. Our slot running the full length of the ridge (inside the gable end walls)
can be reduced to (2 sq ft / 40 ft) x (12 in/ft) = 0.6 inches (total on both sides of the
ridge board). For this to work, we'd need to find a vent that provided at least 7.2 sq
inch NFVA per foot of ridge and run it the full 40 feet.
Let's say we want to use one of those low profile ridge vents with the filter inside,
that provides NFVA of 9 sq in/foot. For this vent we would need a slot 3/4 inches
wide and (2 sq ft x 144 sq in/sq ft) divided by (9 sq in/foot) = 32 feet long. Use one
of those higher flow ridge vents (NFVA of 18 in/foot) and you need a slot 1.5
inches wide (3/4" on each side of ridge board) and only (2 sq ft x 144 sq in/sq ft)
divided by (18 sq in/ft) = 16 feet long. In theory, you could use that higher flow
vent on a 3/4 inch slot if you ran it for 32 feet. BUT code says you must follow the
manufacturer’s instructions and they will tell you how wide of a slot to cut for their
vent.