New shingle warranty only valid if roof has proper through ventilation. Do I?

[JDBS32]

Installing a new roof and want to assure that I have proper through ventilation for my new shingle warranty.

Spent lots of time reading
http://www.inspectionnews.net/home_inspection/attic-areas-home-inspection-and-commercial-inspection/869-ridge-vent-size.html
on ridge vent size. Tried substituting my own numbers, but not sure what the numbers are saying.

Attic space is 38'x30' = 1,140' sq. I rounded this to 1,200' sq for ease of math.

New ridge vent will be Certinteed (either 9" or 12", not sure which the roofer uses), 31' long.
Existing soffits are 2" wide x 35' long * 2 sides of the house.
I think the slot in my ridge sheathing is 3/4" on either side for a total of 1.5" along the 31'.

The shingle manufacturer states: Forconventional roofs, the ratio of unobstructed vents to the totalinsulated ceiling area must be atleast1/300 sq. ft. Vents should be distributed as follows: 55% at thebase of the roof (soffits) for air inflow and 45% at the ridge forair outflow.

Doing the math:

A 9" ridge vent has 16” squared/liner foot => 16 * 31’ = 496” squared for whole ridge vent = 496/144 = 3.444’ squared for whole ridge vent.

A 12" ridge vent has 18” squared/liner foot => 18 * 31’ = 558” squared for whole ridge vent = 558/144 = 3.875’ squared for whole ridge vent.

Formy soffits: 2” wide * 35’ long = 2” * 12”/foot * 35’ = 840’squared/side of house = 1,680” squared for both sides of house. 1,680” / 144 = 11.67’squared for the soffits. NOTE: I don't know how to account for the actual slot area; these were installed a few decades ago. I expect the actual ventilation area is less than the 11.67' squared.

This gives me a ratio of 23 / 67 for thr 9" ridge vent, and 25 / 75 for the 12" ridge vent. Certainly not the 45 / 55 ratio noted above.

So am I at least 1/300 sq ft as per the shingle manufacturer guidelines? Is the ventillastion ratios I have better than the 45 / 55 ratio?

And if I do not meet the 1/300' squared rule, then what needs to be done?

thanks,
signed confused

[Jim Luttrall]

There is a Net Free Area for each vent provided by the vent manufacturer. Use that number divided into your total required vent area to arrive at how many vents are required. Then keep in mind, you want at least 50% of the ventilation at the lower area (soffit) vents with the remainder at the upper roof. In the case of continuous ridge or soffit venting, there is a NFA per linear foot provided by the vent manufacturer.

[Welmoed Sisson]

Are you installing them yourself? Many shingle manufacturers won't honor warranties unless the roofing is installed by a licensed roofer approved by the manufacturer.

[JDBS32]

manufacturers won't honor warranties unless the roofing is installed by a licensed roofer approved by the manufacturer.

Good thought - I've checked with the shingle mfg. not an issue as long as I have a reputable roofer doing the install. In my case, the roofer has a BBB rating of A+.