Roof Diaphram w/ sheathing on roof and Ceiling

[TimmyH76]

I am designing a roof diaphram that is right at the limit of the allowable shear per foot. Is it common to in a situation like this to put sheathing on the roof and the ceiling effectively doubling the load allowable shear in the diaphram? Thank you.

 

[structuralaggie]

Did you block the roof diaphram?

 

[minorchord2000]

You can also use subdiaphrams. A great reference for this is Desgin of Wood Structures by Donald Breyer, etal, 4th edition chapter 15.

 

[TimmyH76]

Yes it is blocked, the problem is the owner has a line on 7/16" material that he "has to use". So it's one of those things were the client comes to you and says I have six boxes of toothpics and some post cards can you make it work so that I can build a mansion that won't fall down , but this is all you can use. Thanks.

 

[structuralaggie]

It seems analogous to using a double sided shear wall. The weekest side capacity times two for total capacity.

 

[minorchord2000]

I would not rely on the ceiling as a diaphragm. The ceiling may get removed by some future owner. If you have a problem with the high boundary shear force, double up the roof sheathing and fasten as close as the code will allow. Couple this with the subdiaphragm theory and you should be good to go.