dormer venting question

Started by dug, December 01, 2010, 02:27:09 PM

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dug

I need to cut holes in my dormer header to vent it.



The header is a double 2 by 12 and the vents I cut elsewhere on the ends of the rafter bays were two 2 & 1/2 in. holes. Will this weaken the beam significantly? I was thinking of drilling multiple 1 or 1/2 in. holes to equal the area, would that be better, or would a bunch of holes in a line weaken it even more?



Redoverfarm

Dug on my house (3 dormers) I just drilled 3/4-1" holes (2 per bay) and called it done.  Where you would place them would be close to the roof sheeting ( where the air baffles go).  Less chance for failure in that area compared to the bottom of the cord although I wouldn't expect much as the area is relatively small.


dug

That's what I think I'll do. Looking forward to "calling it done" and moving on to insulation!

ScottA

Dug I drill holes all the time while plumbing. The inspectors will let us drill anywhere so long as there is +2" of wood above and below the hole. The hole can't exceed 1/3 of the beam size.

Don_P

Technically from that same section of code you are allowed to drill or notch the edge up to 1/6th depth on the end thirds, but not in the midle third. As a practical point the unhangered ends are probably going to let go before the beam fails.


dug

QuoteAs a practical point the unhangered ends are probably going to let go before the beam fails.

I was wondering if someone would call me out on that. When I was framing it that seemed like a weak point but I didn't see that hangers were called for in anything I read, plus I did not have any on hand. I have some heavy L brackets I could nail or bolt in, which I think I will do before I stuff the insulation in.

Don_P

Well you knew I would  ;D
And that doesn't mean they were necessary for the span just pointing out the weak link in the chain is probably not the vent hole.