Question about roof purling connection details for wood frame flat roof 1

[AlexWong122]

Hi All,

I am new to wood frame design.

It seems to be a common practice in my area (and I also read some threads in this forum talking about this design) that for a flat roof , people are putting a 2x4 purling on top of roof rafter for cross ventilation purpose. The roof sheathing is then put on top of the purling, acting as a diaphragm. I have a question about this design:

As a diaphragm, the roof sheathing will be nail to the framing member for the complete load path. In this case, the sheathing is nailed to the purling, and the purling is nailed to the rafter. Let's say I need a diaphragm resistance of 400 lbf/ft, and I can use 0.5" plywood sheathing with 2.5" nail with 4" o.c. to achieve this capacity. However the connection resistance between the purling and rafter seems impossible to achieve that capacity with just nails (because there is only a few contact points (let's say both rafter and purling is spaced 24" o.c.). What is the common way of doing this connection?

Thanks in advance for any inputs.

Alex

 

[XR250]

The perimeter is typically what is important in the roof diaphragm. The interior connections are there to provide stability to the panel and to get the load into the diaphragm. Could work as long as the perimeter has continuous members nailed to continuous bands etc. with the same pattern as the sheathing.

 

[jayrod12]

As XR alludes to, the bulk of the diaphragm nailing is important at the perimeter only. However you'll need to take a look at the connections at the edge of the first few panels from each side to make sure that the nailing for each panel can transfer the required shear at that point. The purlin to rafter connection doesn't really play into the diaphragm too much besides providing overall stability. I would bet that the uplift requirement of the purlin connection may be also difficult to achieve.

 

[AlexWong122]

XR and Jayrod, thanks for your reply. It makes sense to me.