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

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Rick,

Take a look at section 1505.3 of the 1994 UBC which covers the requirements
for required ventilation area at the eaves in the attic space.  In custom
homes this can be a problem since you may only be able to place the vents in
two of the walls because of gables at the other two ends of the roof or
because of stepped changes in roof elevations adjacent to interior rooms.
 You may want to make the vent opening in the block larger (5"x12" in a 2x8
block and cover with a screen) and require that only every third block can be
a vent space.  This may require that the roof sheathing boundary nailing will
have to be spaced closer together since you have fewer blocks for the shear
transfer.  You might have four different nail spacings: boundary, continuous
panel joints, edge and field.  Also make sure to provide shear transfer at
blocking not directly above the shear wall since the top plate is going to
have to act as a drag.  The framers often seem to forget A35's at blocks not
directly above the shear walls.  Ideally you may want to try and get some of
the vents placed furthur up the roof towards the ridge as allowed by the code
to reduce the number of vent spaces.   On one job, when I got the permit set
from the architect, I found that he had placed a vent opening (3"x12") in
every other block (rafters are at 16" on center) to satisfy the code
requirements.

Also, if you are using a raised foundation, you may also want to take a look
at where the vent holes are located.  Most likely they are right under the
shear walls, especially if you have lots of doors or full height windows
along the perimeter walls.  Depending on the number of vent openings, you may
no longer have the shear transfer capacity that you thought you had to the
foundation

Michael Cochran
Brian L. Cochran Associates
Los Angeles 

------------SEPERATOR------
In a message dated 96-09-11 16:00:39 EDT, you write:

<< ubj:	[SEAOC] ridge vent in wood roof diaphragm
 Date:	96-09-11 16:00:39 EDT
 From:	rburchpe(--nospam--at)worldnet.att.net (Rick Burch)
 Reply-to:	seaoc(--nospam--at)power.net
 To:	seaoc(--nospam--at)power.net (seaoc(--nospam--at)power.net)
 
 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 
 roof.
 
 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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