Rated attic space in commercial building?

[palikona]

I'm working on a single story, 4,000 sf oral surgery building that will be sprinklered and Type VB.
There will be suspended ACT ceilings below the prefab trusses of the gable roof. The plan is to put drywall on the underside of the bottom chord of the trusses to create a return plenum. See my attached concept section drawing.
Does the drywall at the bottom of the trusses need to be rated, therefore creating a rated attic?
BTW, the attic is not accessible, and I am planning on draft stopping there, per 718.4.
Thank you for the help.

 

[ICE]

Forgot the section drawing.

 

[steveray]

Probably not....how many people will be under anesthesia at the same time? And you are sprinklering the attic correct?

 

[palikona]

Forgot the section drawing.

See attachment.

 

[palikona]

Probably not....how many people will be under anesthesia at the same time? And you are sprinklering the attic correct?

Possibly 4, so we do need the automatic system. I'd think the attic would have to be sprinklered, right?

 

[ICE]

BTW, the attic is not accessible

If the drawing is an accurate representation, there will need to be access to the attic. You should ask the truss company about adding the weight of drywall to the bottom chord.

Might need sprinklers in the plenum area.

 

[palikona]

If the drawing is an accurate representation, there will need to be access to the attic. You should ask the truss company about adding the weight of drywall to the bottom chord.

Might need sprinklers in the plenum area.

I will provide for access into the attic and ask about that added weight. Do you think the drywall needs to be rated, therefore creating a rated attic? And where would I determine if the plenum needs to be sprinklered separated?
Thank you!

 

[ICE]

I will provide for access into the attic and ask about that added weight. Do you think the drywall needs to be rated, therefore creating a rated attic? And where would I determine if the plenum needs to be sprinklered separated?
Thank you!

I do not think that the drywall would need to be rated... mostly because i don't think that drywall is rated... an assembly is rated. I would use type-X just because I could. The plenum area is a concealed space so I would presume that it shall have sprinklers... note that earlier I said "might" need... but I am not the person to ask about any of this.

Give it a few hours and the experts will chime in.

 

[steveray]

In an NFPA 13 building...MOST concealed combustible spaces get sprinklers....Except small stud cavities and chases....

 

[ICE]

In an NFPA 13 building...MOST concealed combustible spaces get sprinklers....Except small stud cavities and chases....

Might... most... wait for the real help.

 

[steveray]

Might... most... wait for the real help.

Help?....Well here you go then, you don't have to draftstop the attic when you install an NFPA 13 system:

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 (279 m2). 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.

 

[ICE]

903.3.1.1 NFPA 13 sprinkler systems. Where the provisions of this code require that a building or portion thereof be equipped throughout with an automatic sprinkler system in accordance with this section, sprinklers shall be installed throughout in accordance with NFPA 13 as amended in Chapter 35 except as provided in Sections 903.3.1.1.1 and 903.3.1.1.2.

Has it been determined that a sprinkler system is required by the code? If the code does not require a sprinkler system, can you put a few here and there but not everywhere? And it’s been said that there’s no such thing as a stupid question.