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

Spent lots of time reading entry 8 on this url

ridge vent size

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