We’re replacing a 6/12 hip roof on our home. Currently our attic ventilation is a nightmare so I’ve been researching the best route to go with the replacement (and boy, has this board been a wealth of info!) since I don’t want this to continue to be my headache.
For both aesthetic and functional reasons, a ridge vent seems to be the way to go for the house. We’ve interviewed 3 roofers so far (all established, legit companies) and each one has spec’d 50’ of ridge vent (total across the top and the two projecting ridges in the front), and varying opinions on soffit vents (ranging from adding 10 to adding none, sigh).
My math, using the NFA formula for 1/300 ratio I’ve seen, comes out different. Here’s what I get:
Attic floor sq. ft. = 2500, going through the math it comes out to providing 600 sq. in. NFA each in the exhaust and in the soffits. Our main ridge is 36 feet, and from reading the installation instructions it seems that stopping 12 inches from the edge is required so I have 34 feet available for venting across the top of the main ridge. Figuring a ridge vent that supplies 18 in. per lineal foot, that’s 612 sq. in. of exhaust, which is close to the recommended number of 600 I get from that calculation sheet.
Why would all three recommend adding ridge vent to the other areas (the attic is completely open, so there are no closed-off places needing special attention)? I got “more is always better” from 2 of them and “well, that’s what we usually do but if you want just the top ridge that’s fine” from the 3rd. I’m leaning toward a combination of 1. it’s more profit to sell and install 50’ rather than 34’ and 2. they just don’t know attic ventilation all that well. Or am I missing something here? I’m not a roofer. If we need 50’ I sure don’t want to cut corners, but from what I’ve read 50’ is overkill, and “more is always better” isn’t the case… especially when not adding enough soffit vents. I keep reading that too much exhaust is bad (and likely what our problem is right now with 8 box vents and a power vent).
About the soffit vents: currently we have 19 4x16s, which gives 427.5 sq. in. of intake. Even with 34’ of ridge vent, I’d need 9 additional ones to get 630 sq. in (everything I’ve read said to have more intake rather than exhaust if you weren’t 50/50). The guy that recommended 10 more soffit vents did so in conjunction with 50’ of ridge vent, so even he’s leaving me with more exhaust than intake.
The short question is: is my math right on this? 34 feet of ridge vent and 9 more 4x16s in the soffits to have good ventilation in the attic? Or is there a legit need for the 50’ and can someone explain the “why” of that to me?
Thanks for reading all that! It’s a long first post but this is all new info for me and I want to make really sure it’s done right.