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Plywood Roof - Continuous Venting

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emart0429

emart0429

Structural
Mar 22, 2007
7
I'm working on a renovation project for an existing wood framed building with plywood sheathed, hipped roofs throughout. The architect wants to improve the attic ventilation by having the roofing contractor install continuous "shingle over" style ventilation just inboard of the exterior walls. These vents require cutting a continuous 3/4" strip out of the roof sheathing. The image below is the vent manufacturer's standard detail.

Attic_Intake_Vent_br4kru.png


This seems like a horrible idea to me and would result in a complete loss of roof diaphragm strength for the existing building. The architect's question at this point is "how much can I cut out around the perimeter?" To which my quick response would be no more than 10% of the perimeter length of the diaphragm (following the IEBC's 10% increase in lateral forces methodology).

Anybody have any input on whether this is as horrible an idea as I think it is?

Thanks,

Erik
 
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Is it a blocked diaphragm or unblocked? If unblocked, it will still work about as well as it did before, though the diaphragm stiffness will likely be reduced.

In an unblocked diaphragm, each rafter is part of the diaphragm's load path by passing the edge shear from panel to panel through weak axis bending of the rafter over a moment arm of about 1" to 1.5". This will increase that moment arm to 1.625" to 2.125".

If it's a blocked diaphragm, then you're right...this will ruin it.
 
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It's currently an unblocked diaphragm, so I suppose you're probably correct. Not all that much worse than the standard 1/8" gap the contractor is supposed to leave between all the sheathing edges to allow the sheets to swell if they get wet, etc. BUT, at the same time, if this was done as part of new construction I would typically require the contractor to block around all unsupported edges of the plywood where the panels are less than 24" in width if for no other reason than to keep from ending up with a 12" wide strip of plywood that someone could easily step right through while working on the roof. So, I suppose even if I'm not going to worry about the diaphragm action of the roof I should still worry about the gravity load issues this cut out could create.

Thanks for the feedback!
 
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No problem, and you're right about the gravity load transfer...though the vent product may solve most of that. It's usually differential movement between plywood panels causing damage to the roofing materials. I'd look into their specs and see what they say.
 
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If the gap is at the ridge, I would not worry about it. The diaphragm shear at the ridge will be zero, or near zero, since it is at midspan of the diaphragm.

DaveAtkins
 
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DaveAtkins said:
If the gap is at the ridge, I would not worry about it. The diaphragm shear at the ridge will be zero, or near zero, since it is at midspan of the diaphragm.
Click to expand...

A ridge may be at midspan of the diaphragm in one direction. What about the orthogonal direction?

Also from the OP's post it sounds like the vent is at the edge of the roof.

 
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Thanks for the feedback everyone.

I told the architect I wasn't a huge fan and they are looking into alternatives. The slot would be along all perimeter edges of the roof and not at the ridge. The ridge already has a large steeple or cupola style vent, so no extra work needed there.
 
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What's the point of a vent in the sheathing down low? Don't you want that one in the soffit to get proper air flow?
 
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Yes, ideally there would be vents in the soffit. But, this roof is constructed w/ no rafter or truss tails extending beyond the exterior face of the walls (no overhang on the roof). So, the architect was hoping we could use the slots down low on the roof to provide the venting that would typically be thru the vented blocking between trusses/joists or thru the soffit of the roof overhang. At this point I think the architect is leaning toward adding a small overhang as needed to allow for soffit vents to be installed. We'll see where this goes from here.
 
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