Amount of Ridge Vent


#1

We are having our roof replaced and I would like to correct an apparent deficiency in the venting of the current roof. The roof has a ridge along the entire width of the house with gables at each end and a lower ridge above the garage that is perpendicular to the main ridge, but at half the vertical height. The space above the garage is only connected to the main attic by the space between the rafters of the main roof that intersects with the garage roof. There are soffit vents for both the garage and main sections and box vents only on the main section. We plan to replace the box vents with ridge vents, but I am not sure if we should use ridge vents on the garage ridge. Below are my calculations. Thanks in advance for your advice.

Entire attic area including eaves is 2450 sqft.

9 box vents, 50 sqin each, 3.1 sqft total

The soffit vents are cut-outs with bug screens.
43 2.5"x13.5" soffits for the main area, 10.1 sqft
12 2.5"x13.5" soffits for the garage, 2.8 sqft
Using a 70% open area correction factor:
main 7.1 sqft, garage 2.0 sqft, (9.1 sqft total)

Using the 1/150 rule, total open area 16.3 sqft
For 50:50 balance, 8.1 sqft each
total soffit is 9.1 sqft - so okay
total exhaust is 3.1 sqft - too low

For 60:40, soffit - 9.8 sqft, exhaust - 6.5 sqft
total soffit is 9.1 sqft - little low
total exhaust is 3.1 sqft - too low

The main ridge is 58’.
18 sqin/ft vent gives 7.2 sqft exhaust
which falls between the 50:50 and 60:40 values
Adding the 21’ garage ridge gives 8.9 sqft exhaust
which seems a bit high

Separating the main and garage roofs (even though they are somewhat connected):

Main:
soffit - 7.1 sqft
ridge - 7.2 sqft
This gives close to a 50:50 balance

Garage:
soffit - 2.0 sqft
ridge - 2.6 sqft
This gives a 57:43 ratio in the opposite direction

It seems that if the main and garage attic are considered to be connected, ridge vents should only be installed on the main ridge. But if the connection is not significant enough, then ridge vents should also be installed on the garage ridge to allow proper venting there.


#2

sorry jimshep.
i dont think you need our help.
we may need yours here.

gweedo