Use Cedar Shake Roof as a continuous diaphragm?

[JSBischoff79]

Hi guys. Here's the problem:

I'm working on a large custom single family home on the east coast and it requires a full IBC lateral design. The owner is looking to use a traditional cedar shake roof, which uses 1x4's across the rafters to support the shakes instead of using plywood/osb roof sheathing. Normally, you would count on the roof sheathing as your diaphragm to transfer the lateral loads to the gable end shearwalls. However, in this case, I don't know that I can call the roof a continuous diaphragm without the plywood. Also, there are no published diaphragm values that I could find for a roof framed in this configuration.

Ideas?

 

[SLTA]

If you look here:

http://www.cedarbureau.org/installation/roof_manual/page03.htm#

there is a design option for continuously sheathed roofs, and still considered "traditional" shake. I'd say that this would be the best way to go for a full lateral design.

Here's the full manual:

http://www.cedarbureau.org/installation/roof-manual.htm

They also say they've tested the system using plywood sheathing but not OSB.

I hope that helps a bit...

 

[woodman88]

You could use the ceiling Gypsum Board for a diaphragm if your shear is low. See IBC table 2508.5.

Garth Dreger PE
AZ Phoenix area

 

[BikeDrD]

Check out the document ATC 7-1, Proceedings of a Workshop on Design of Horizontal Wood Diaphragms, Nov 19-20, 1979. It presents methods of determining capacities of the diaphragm you describe. It's the "Nail-Couple Method" and is very straight forward.

http://www.atcouncil.org/pdfs/atc7-1toc.pdf

I also have seen it described in the Western Woods Use Book chapter on designing for lateral forces.

 

[efsinc]

1. i think (?), suspect, the nail couple design methodology, although common years ago, is no longer accepted.
2. on the west coast, where the APA-EWS supervises most of the plywood / osb sheathing, APA-EWS will support he use of properly rated OSB as equivalent to properly rated plywood for lateral support.