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RE: Diaphragm and Framed Dormers

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Thanks Bill,

I just got your message after talking to the architect.  I have done a number of messy little projects for him over the years.  He understood and says they will adjust to correct the problem.  He actually went to the drawings and then called me back and ask me a couple of questions.  We’ll be able to work it out.  The shear through the remaining portions of the diaphragm were going to be too high and unworkable.



From: Bill Allen [mailto:t.w.allen(--nospam--at)]
Sent: Sunday, February 08, 2009 5:46 PM
To: seaint(--nospam--at)
Subject: RE: Diaphragm and Framed Dormers


On the projects I've worked on which had dormers, I had a difficult time justifying using the dormers as part of the diapragm.


Repeat after me "block and strap" and you will be fine.





T. William (Bill) Allen, S.E.


Consulting Structural Engineers
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Joseph R. Grill [mailto:jrgrill(--nospam--at)]
Sent: Sunday, February 08, 2009 12:16 PM
To: seaint(--nospam--at)
Subject: Diaphragm and Framed Dormers


I am working on a proposal.  Not too many of them coming through this part of the country lately.  Anyway, at a portion of the structure (this portion is about 50’ x 80’) the roof diaphragm, 8:12 pitch with the main ridge parallel to the 80’ dimension.  The main roof diaphragm is interrupted by several framed dormers in line with each other.  If the plan “footprint” of the dormers are not included in the roof diaphragm there isn’t much roof diaphragm left between the dormers for shear transfer to the  exterior shear walls below.  The pitch of the dormer roofs are also 8:12.  The ridge of the dormers are perpendicular to the 8’ dimension of the portion of the building in question.  Can the dormer roof sheathing be considered a part of the roof diaphragm for transferring shear loads to the walls when the lateral load is parallel to the 80’ dimension.  I have run across this before, but not to this extent.  I have always had enough sheathing between dormers to transfer the diaphragm loads to the shear walls.


The office could really use this project at this time, so I am trying to really look at the time that may be involved in the design.  If I can use the dormer sheathing for diaphragm capacity when the lateral load is parallel to the 80’ direction (perpendicular to the dormer ridge) then the design will be greatly simplified.


Thanks for the help.

Joe Grill