Net free vent area + rafter blocking hole size
I’m calculating net free vent area for a cathedral ceiling and wanted to run my calculations by the experts here. I believe the IRC calls for a minimum of 1/150 net free ventilation.
Given 13′ long 2×8 rafters at 16″ OC, each bay has an area of 13’*(14.5″/12)= 15.71 sq ft.
15.71 sq ft / 150 = 0.10472. Multiply by 144 to get sq inches and we have 15.1 sq in net free area (nfa) required per bay.
According to Building Science (https://buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf), 60% of venting should be at the eaves and 40% at the ridge to create positive pressure. This gives 9 sq in per bay at the eaves and 6 sq in per bay at the ridge.
3″ circle vents with louvers and screens only offer 2.74 sq in nfa each. If I bump the size up to 4″, the nfa jumps to 4.8 sq in. Will two 4″ holes in my blocking be too large? Will they compromise the integrity of the blocking?
If I use 3″ open screen vents, I can get 4 sq in nfa per vent, but would the open vents cause issues with wind-driven rain? I’ll have a roof overhang of 1′.
Vents for reference:
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Replies
With split venting it reduced to 1/300
Also, if you can, use v slots in top of block as this offers venting at top of bay.
I like the v slot idea, but I'm not sure how to put a bug screen over the slot without it looking tacky.
usually you apply to the backside
That seems like it would invite critters to build nests in the slots.
shedworkshop,
With open rafter tails I cut the blocking short and attach apiece of perorate flashing to the top. Similar to this: https://www.menzies-metal.com/metal-flashings/perforated-l-channel-rain-screen-high-back/
Malcolm, that was my original plan, but then for some reason I got it in my head that it wouldn't be as structurally sound if the blocking didn't extend up to the osb (rotation maybe?). The perforated flashing seems a whole lot simpler though. I'll do some more research.
I have not found them too, I don’t think they are particularly more inviting than the rest of the exterior nooks and crannies. I have found the occasional wasp nest in there but have found more on other aspect of the soffit area. Malcolm’s approach seems good also, though maybe a little more costly.
Freyr_design,
We are lucky here. Because of the rain-screen requirements lumberyards carry perforated stock, and we can get any profile made up quite cheaply.
As a rule of thumb I try and only use insect screen where it can be easily accessed for replacement, as my experience has been it often gets damaged over time.
shedworkshop,
Full height blocking fastened on all faces is definitely stronger, but in practice the blocking only gets a couple of toe-nails into the top-plate below, and (much like solid blocking on floors) I'd bet only one in ten roofs have it nailed to the sheathing above. The main resistance comes from the shape of the blocking itself, and the diaphragm created by the sheathing. Unless it's called out as part of the engineering, I wouldn't have any reservations about using shorter blocking.
Reply #8:
I haven’t found that to be the case with metal bug screen, especially protected on the back side of blocking. How would you replace a L shaped perforated screen? Seems like you would need to take the block out either way. But there is certainly the possibility it gets damaged, at which point you could find a solution for the front of the block.
Also, I work a lot in CA and they do call out EN at those blocks often.
Malcolm, freyr_design, I think I'll either go with four of the 3" open vents per bay (2 top and 2 bottom) or DIY it with metal bug screen as mentioned. Thanks!
Freyr_design,
You are right - insect screens behind blocks are a lot less susceptible to damage than those used closer to grade. I'm just not sure it makes sense to have any part of what is an otherwise robust exterior envelope protected by something that fragile. That said, people frequently use it successfully.
"How would you replace a L shaped perforated screen?"
That's the primary reason to use it. Being much stronger, it is unlikely to ever need replacing.