Demising wall construction in existing attic space

[palikona]

I have a tricky demising wall question:
I have a 1 story tenant improvement project that is an existing commercial building (9,000sf total) with a triangular attic full of wood trusses and bracing. There is an existing layer of drywall on the existing bottom chord of the trusses.
The client's space will be 3,000sf and we obviously need to add a rated demising wall, as the adjacent space will be a different tenant. Same use though (B). The existing building is Type V with no sprinkler system.
Construction has started and it has become clear that building the upper part of the rated demising wall is really difficult, with all of the bracing in between the trusses above. I've asked the AHJ about options and he said the only other option would be a sprinkler system to be installed for the whole building, which is cost prohibitive for my client.
My question is: are there any other options that I am not thinking of that would work to properly subdivide this building into two tenant spaces? Thanks for any help!!

 

[ICE]

If I understand this correctly, there is a 9000 square foot attic space with no draft stops. Wide open, as in I could throw a cat from one end to the other. There is a proposed 3000 square foot (B) tenant space with a “rated” attic separation with an exception that allows a sprinkler system in lieu of the attic separation.

 

[Paul Sweet]

Rated tenant demising walls are only required in covered or open mall buildings. (IBC 402.4.2.1) These are defines in IBC Chapter 2 as:

"COVERED MALL BUILDING. A single building enclosing a number of tenants and occupants, such as retail stores, drinking and dining establishments, entertainment and amusement facilities, passenger transportation terminals, offices and other similar uses wherein two or more tenants have a main entrance into one or more malls. Anchor buildings shall not be considered as a part of the covered mall building. The term “covered mall building” shall include open mall buildings as defined below.

Mall. A roofed or covered common pedestrian area within a covered mall building that serves as access for two or more tenants and not to exceed three levels that are open to each other. The term “mall” shall include open malls as defined below.

Open mall. An unroofed common pedestrian way serving a number of tenants not exceeding three levels. Circulation at levels above grade shall be permitted to include open exterior balconies leading to exits discharging at grade.

Open mall building. Several structures housing a number of tenants, such as retail stores, drinking and dining establishments, entertainment and amusement facilities, offices, and other similar uses, wherein two or more tenants have a main entrance into one or more open malls. Anchor buildings are not considered as a part of the open mall building."

 

[ICE]

718.4 Draftstopping in attics. Draftstopping shall be installed to subdivide attic spaces where required by Section 708.4.2. In other than Group R, draftstopping shall be installed to subdivide combustible attic spaces and combustible concealed roof spaces such that any horizontal area does not exceed 3,000 square feet. Ventilation of concealed roof spaces shall be maintained in accordance with Section 1202.2.1.
Exception: Buildings equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1

718.3.1 Draftstopping materials. Draftstopping materials shall be not less than 1/2-inch gypsum board, 3/8-inch wood structural panel, 3/8-inch particleboard, 1-inch nominal lumber, cement fiber-board, batts or blankets of mineral wool or glass fiber, or other approved materials adequately supported. The integrity of draftstops shall be maintained.

 

[palikona]

If I understand this correctly, there is a 9000 square foot attic space with no draft stops. Wide open, as in I could throw a cat from one end to the other. There is a proposed 3000 square foot (B) tenant space with a “rated” attic separation with an exception that allows a sprinkler system in lieu of the attic separation.

That's correct. Wide open. The new partition has to go to the sloping roof sheathing, correct? This is for a dental office in our space and likely another medical or retail business in the other space.

 

[Mr. Inspector]

What method in the IEBC are you using? If it is Level 1 or 2 I don't think it needs to be done.

 

[RLGA]

Since a Type V-B, nonsprinklered building has an allowable area of 9,000 sq. ft., a fire-resistance-rated demising wall is not needed unless another occupancy group with a more restrictive allowable area moves into one of the tenant spaces.

 

[Yankee Chronicler]

The client's space will be 3,000sf and we obviously need to add a rated demising wall, as the adjacent space will be a different tenant. Same use though (B). The existing building is Type V with no sprinkler system.

As others have already commented, although you state that you "obviously" need a rated demising wall, it doesn't appear all that obvious at all. Let's get back to basics: Why do you think the demising wall needs to be rated? If you think it needs to be rated -- what rating do you think is needed, and why?

Also, what code are you using? If your jurisdiction uses the IEBC, which compliance method are you using?

 

[ICE]

has to go to the sloping roof sheathing, correct?

Continue talking with the local AHJ.
I have copied an outdated Los Angeles County California Building Code which requires a draftstop (not a rated firestop) that does go all the way to the sheathing. Now that is not to say that a rated firestop would be a violation.

 

[Paul Sweet]

It's still in the 2021 IBC. I didn't read past the first sentence when I replied. The IBC doesn't require a draftstop to line up with a demising wall.