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[SEAOC] Re: [SEAOC] ridge vent in wood roof diaphragm

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Rick, what I often suggest (if an engineered roof diaphragm is really needed) is to install blocking between the rafters/trusses in some of the bays at the ridge, and fasten the roof sheathing to the blocking.  Since both the sheathing and framing are discontinuous at the ridge, a means must be provided to transfer the shear along the ridge from one side of the diaphragm to the other.  This can be accomplished by a formed sheet metal strip, stapled to the sheathing on both sides of the ridge, at least in the higher shear zones near the ends of the diaphragm. APA has a technical note that provides design shear capacity for sheet metal strips stapled to plywood sheathing. The blocking/shear strips can be calculated to transfer the shear forces, but can be discontinuous so that vent holes or slots can be placed in some of the roof framing spaces, or can be located in the sheathing away from the ridge blocking if the ridge vent design provides a ventilation path.  Because the shear forces are transferred along a shorter discontinuous length, the forces are higher than if continuous blocking/shear strips are used, so more nails than the minimum schedule for roof sheathing may be required (e.g., 3 or 4 in. oc).

At the eaves, every other piece of blocking between trusses/rafters can have screened "bird hole" vents (three 1.5-2.0 in. dia. holes in each 22.5 in. block).  The intermediate blocking pieces can be solid, and the roof sheathing nailed to the solid blocking to transfer shear forces to the walls.  Toenails or better yet shear connectors (Simpson A35 or similar) can be used for the shear transfer from the roof blocking to the walls.  The wall double top plate acts as a continuous diaphragm chord.

SBCCI Standard 10-95 has recommended construction details for these connections for high wind regions.

John Rose/APA, Tacoma, WA

>On pitched (gabled) wood roofs using plywood sheathing over either 
>prefabricated wood trusses or stick-built rafter framing there quite 
>often is a continuous ridge vent, which requires a continuous opening 
>in the diaphragm a couple of inches wide along the ridgeline of the 
>1. Does anyone have advice on how forces in the diaphragm are handled at 
>this opening?
>In conjunction with the ridge vent (for air flow out), there often are 
>continuous soffit vents (for air flow in) to provide the code-required 
>ventilation of the attic space.  Blocking in the spaces between the 
>trusses for tranfer of diaphragm forces to the top plate unfortunately 
>blocks this air flow.
>2. Does anyone have advice on how these two conflicting requirements 
>(structural force transfer vs. architectural ventilation requirements) 
>can be handled? 
>Rick Burch
>Columbia, SC
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