R2 Unit Separations and Draftstopping

[nealderidder]

Hello All,

Im looking at a 4-story R2 building (A-2 on first floor) of type V-A in CA. Fully sprinkled NFPA 13.

Seems like last time I did multi-family I needed draftstops at every unit separation or every other one, something like that. I don't think that's required now in CA for this particular case.

I've got 1-hour rated floor and roof assemblies and 1 hour demising walls. We're planning open web joists for the roof framing, around 2' deep. We will be sprinkling that concealed roof space. The roof assembly looks something like below (just noticed we're missing roof sheathing):



Any reason to believe I need draftstopping in the "attic" where shown above (dashed blue line)?

420.2 requires my demising walls to be Fire Partitions (which can stop at the rated ceiling assembly).
Exception #1 of 708.4.2 applies.
Therefore 718.4 doesn't apply.

So any reason to think I need any draftstopping within the depth of the trusses?

Note that the wall shown above isn't my demising wall, which is a double stud sound wall.

Thanks for any input!

 

[RLGA]

I see two problems with your detail as presented (not the one you mentioned), which have nothing to do with draftstopping. However, I will address the draftstopping issue first.

If sprinklers will be provided within the attic space as required by Exception #1, then draftstopping is not required.

On to my other two problems...

1. Where is your roof sheathing? Your detail shows the insulation spanning from joist to joist.
2. Compliance with CBC Section 1206 will be difficult to achieve since you have a flanking path for sound, which is through the ceiling, over the wall, and back through the ceiling into the adjacent unit. I do not think the resilient channels behind the ceiling will provide a sufficient reduction in sound transmission equal to or better than the double-stud sound wall below.

 

[nealderidder]

Thanks Ron. The missing sheathing is just a drafting error. Good point about 1206. The ceiling doesn't have to equal the demising wall but but does need to hit 50STC (or 45 tested). We might want to at least look at some plenum sound board.