First I want to say I’m thankful to have found a forum on roofing issues. My contractor, while careful, honest and thorough doesn’t seem to have the best grasp on ventilation issues. He was a referral and has done good work for friends and family. Due to the mild weather we’re going ahead with a rip-off and re-lay this week. I’m in Northeast MA.
The problem is that I have hip roofs, and from what I’ve learned properly venting them can be difficult. Here are the details:
The problem is the main house which is 28x44. If I measured correctly the slope is 7/12. Ridge line is 16ft in length (measured from inside attic.) Current exhaust venting was 5 box vents (three on rear, two on either end, none in front) mounted on 8"x11" cut holes within a foot of the ridge. I have appx (11) 16"x8" louvered soffit vents around the perimeter. Current box vents I think are probably about 50" NFA so total exhaust is around 250". Current soffit intakes I have no idea, but guesstimating effective area of 15"x7" at 50% NFA, that would be about 50" per, so maybe around 550" total.
Roofer is recommending ridge vents and uses GAF cobra which has 16.9" NFA per lineal foot when hand nailed. exhaust/intake of 50/50 requires 17.5 feet which is insufficient, but I see some say 40/60 is acceptable and this then reduces to 14ft. Since I’ll apparently lose about 1ft at each end of the ridge for blending, this is right at the limit. This is using the 1/300 rule which I think is still valid for using high/low vents even when no vapor barrier exists (House uses simply blown-in insulation, no vapor barrier.)
Will this be adequate? Should I ask for SnowKing ridge vent instead which has 18.5" NFA/lft (roofer claims these have been problematic which seems to contradict experiences here.) I’m not sure power is an option since many say they can pull air from house and aren’t effective during winter months. I will be addressing the insulation issue since we have had minor ice dams in the past although none have leaked thru. I only have the next day or two to figure out what to do in the main house section. How can I get enough venting without my roof looking like some kind of commercial building from the back and sides? Would replacing the louvered soffit vents with a continuous soffit vent help?
I also have a separate attic over a 24x44 addition which uses 4 box vents and has 2" continuous soffit venting for about 110 lineal feet. Its actually a double hip so the ridgeline forms an “L”. There’s 32ft (12 + 20) of ridge so I’m not too concerned about this one. Its properly insulated with vapor barrier included and I think it should be OK with ridge vents installed in place of the box vents. My main concern is what to do with the main house section.
I’d really appreciate any advice or experience that would help me figure out what to do here.
p.s. We chose Certainteed Landmark 30yr in Charcoal Black which should give us a very nice architectural look with a good bit of grey/black variation to give some nice additional pop to the texture.