Row House Attic Fire Separation

[kiwijbob]

So to help settle an argument I'm sharing the below, a developer is pushing this detail for cost reasons clearly. I have multiple problems with it as follows:

1. Not a ULC test assembly, but using table D 2.3.4 -A as the centre truss is load bearing it would appear that 5/8" type X would be acceptable.
2. Unlikely to meet the STC 50 requirement but I find that AHJs never seem to have an issue with this in an Attic space.
3. For lot separation I prefer to bring 2 non load bearing trusses up to the u/s of the roof sheathing insulated & fire rated.
4. I note that the city of Coortenay in BC does it similar to the way the developer does - https://www.courtenay.ca/assets/Departments/Development~Services/2019 Party Wall Fire Separation.pdf

Any thoughts?

 

[tmurray]

Continuity of the fire separation is broken at the interface between the wall and trusses. 9.10.9.2.(1)
Table D 2.3.4 -A is for light wood framed walls, this is not a wall. It is a truss.
Attic spaces should be reviewed for ASTC (9.11.1.4) not STC.

The major issue with using trusses is the differences in thermal expansion between wood and metal. In fire conditions the high thermal expansion of the metal causes the truss plates to pop off, resulting in structural failure of the truss in a fire condition. This results in the fire rated assembly having a much lower performance than otherwise expected.

I heard all the same arguments when I started fighting this fight 10 years ago.

Also, the designer is required to tell you the source of the rating in Division C. It's not your job to guess.

 

[kiwijbob]

Continuity of the fire separation is broken at the interface between the wall and trusses. 9.10.9.2.(1)
Table D 2.3.4 -A is for light wood framed walls, this is not a wall. It is a truss.
Attic spaces should be reviewed for ASTC (9.11.1.4) not STC.

The major issue with using trusses is the differences in thermal expansion between wood and metal. In fire conditions the high thermal expansion of the metal causes the truss plates to pop off, resulting in structural failure of the truss in a fire condition. This results in the fire rated assembly having a much lower performance than otherwise expected.

I heard all the same arguments when I started fighting this fight 10 years ago.

Also, the designer is required to tell you the source of the rating in Division C. It's not your job to guess.

Good points, although the partition itself does appear to meet UL U338 depending on load bearing or non-load bearing.

 

[tmurray]

U338is a UL tested assembly that has been ported into the ULC system. The designer needs to take into account the load restricting factor of this design (usually an engineer does this).

Secondly, the design uses wood studs, not a wood truss system.

The tested assembly would use wood studs fastened in accordance with the requirements of the code, not truss connector plates. You might want to consider if you need input from a qualified expert on if the attachment change will produce an effect on the rating.

 

[Plumb-bob]

I just went through Appendix D quite thoroughly, and I do not believe you can create a vertical fire separation in an attic space using a fire rated gypsum sheathed truss for Pt9 construction. Unless you can find a UL listing that is specific to this assembly.
The hand-out from Courtenay is interesting.
Anecdotally, I have been in a townhouse after a fire and the fire separation was carried into the attic with the same detail you show, and the separation held under fire conditions.

 

[kiwijbob]

I just went through Appendix D quite thoroughly, and I do not believe you can create a vertical fire separation in an attic space using a fire rated gypsum sheathed truss for Pt9 construction. Unless you can find a UL listing that is specific to this assembly.
The hand-out from Courtenay is interesting.
Anecdotally, I have been in a townhouse after a fire and the fire separation was carried into the attic with the same detail you show, and the separation held under fire conditions.

The intent is not to rate the truss but instead to rate an attic partition. Fine from a fire separation standpoint but I still don't see it meeting the STC requirements though, particularly for flanking sound transmission at the the wall top plate over the party wall.