Attic Party Walls - 1 Hour Rating with Single Layer 5/8" Type-X?

[Steve French]

My second thread today! Hope no one minds...

Anyways, looking at party walls for Part 9 buildings - semi's & townhouses - where a 1-hour FRR assembly is required between the dwelling units in order to subdivide, but the code doesn't appear to make any allowance for drywall application directly to roof trusses that would allow for more than 45-minutes beyond the very last row of the Fire & Sound Resistance Tables where it notes 2 layers 15.9mm Type-X will give a 1-hour rated based on the membrane alone where supporting members are spaced appropriately.

Spoke with a few other folks and everyone seems content to accept 1 layer 5/8" Type-X (one layer each side of the truss obviously) as equivalent to an hour once you roughly factor in the ceiling drywall within the unit (1/2" regular) and the blown-in insulation overtop that. Is this common practice across Canada?

Thanks Folks!

 

[tmurray]

We were seeing this here in NB about 10 years ago and we all started questioning it. Based on further research, we (the province) do not feel that sheathing a truss in sheetrock acheives the desired rating. The Truss Plate Institute of Canada indicated there were significant concerns related to early structural failure of trusses as the differential thermal expansion between the lumber and metal truss plate tends to cause the truss to structurally fail well before the desired time.

Kind of a "but this is the way we've always done it" situation.

 

[Steve French]

Appreciate the response!

Doesn't seem to be a lot of motivation to question the status quo here... I take it that in NB the standard for wood-framed construction is to construct a wall in the attic to the underside of roof sheathing? Trying to get a firestop spec for that underside of roof joint has also been impossible so far.