I have gone back and forth over the years on whether a mech shaft going through the 2nd floor to get ducts from the roof to the first floor needs to be rated or not and here I am again except this time with a new twist.
First off, I always get hung up on CBC 708.7.3 which makes me think that this one story shaft needs to be a rated shaft since it is concealed within the construction of walls. But yet the Code does say WALL (singular) and not within the construction of wallS. I do understand that 7.1 states a shaft enclosure is NOT required in this situation but what if you install one anyway, then does 7.3 kick you into a rated shaft?
The definitions of shafts and shaft enclosures don't help
SHAFT ENCLOSURE. The walls or construction forming the boundaries of a shaft.
SHAFT. An enclosed space extending through one or more stories of a building, connecting vertical openings in successive floors, or floors and roof.
They sure sound the same to me. If I under the Code correctly, the definition of shaft enclosure should read,
FIRE RATED walls or construction forming the boundaries of a shaft.
If it read that way, I think it would be clear that this is a shaft since it is not required to be rated.
Now for the twist, there is a corporate architect who represents my client and he wants to use the shaft as a return pleum and leave the bottom open. Now the thread linked above would indicate that that is OK per 708.11.1 since this duct chase is a shaft and NOT a shaft enclosure.
What are your thoughts