Ceiling requirements at commercial hood

[Robert]

Greetings. This is a head burner for me. I'm remodeling a restaurant that involves moving a Type 1 hood to a new location (with an 8' gyp. bd. ceilng instead of the existing 10' gyp. bd. ceiling). The hood will need to penetrate the ceiling about 6" because of the height & lower ceiling. Does the ceiling and penetration need to be rated? The shaft will be rated. This is in CA. I do see in CMC507.2.4 that reductions in clearances allow 3" to limited combustible materials, and 0" to non-combustible materials. The hood will be supported by threaded rod going to roof framing (not the ceiling framing that it is penetrating). Thank you for your thoughts. Also, I have the UL listing number but I'm getting goose eggs on the UL site.

 

[classicT]

Is the existing ceiling part of a rated horizontal assembly?

 

[steveray]

Alot of questions that need to be answered there...least of which being what will be the installed height of the hood lip?

 

[Robert]

The ceiling is not part of a rated assembly. The hood lip is below the ceiling line but the top of hood is above the ceiling. I'm going to box in the hood above the ceiling for a 1 hour enclosure (since it's in a concealed space there) but am confused about the side clearances of the hood where it penetrates the ceiling. In CMC 507.2.4...The "limited combustable" part of the code....does gyp. bd. comply? If so I can keep the ceiling 3" away from the sides and use a metal spacer to close the gap.

 

[Yankee Chronicler]

Unless California is different from the IMC, hood clearances don't require a rated assembly, they just address clearance from combustible materials. What's the framing for the roof or floor above? Wood, or steel?

 

[Yankee Chronicler]

The ceiling is not part of a rated assembly. The hood lip is below the ceiling line but the top of hood is above the ceiling. I'm going to box in the hood above the ceiling for a 1 hour enclosure (since it's in a concealed space there) but am confused about the side clearances of the hood where it penetrates the ceiling. In CMC 507.2.4...The "limited combustable" part of the code....does gyp. bd. comply? If so I can keep the ceiling 3" away from the sides and use a metal spacer to close the gap.

Gypsum wallboard is non-combustible. Limited-combustible is fire-retardant treated plywood and similar materials. What matters is what's behind the gypsum board.

 

[Robert]

Unless California is different from the IMC, hood clearances don't require a rated assembly, they just address clearance from combustible materials. What's the framing for the roof or floor above? Wood, or steel?

Yes agreed. But since the hood penerates the ceiling up into the attic (concealed space....wood framing) I need a 1 hour enclosure of it.

 

[Yankee Chronicler]

Yes agreed. But since the hood penerates the ceiling up into the attic (concealed space....wood framing) I need a 1 hour enclosure of it.

Why? If the roof-ceiling assembly isn't rated, why do you need a rated enclosure around the hood to penetrate a NON-rated ceiling?

IMC 2021, section 506.3.11, exception:

Exception: A duct enclosure shall not be required for
a grease duct that penetrates only a nonfire-resistance rated
roof/ceiling assembly.

But you still need to provide the required clearance from combustible materials, and that applies even if you put a 1-hour rated enclosure around it. The rated enclosure will have to be made of non-combustible framing if it's less than the required sistance from the hood.

 

[Robert]

Why? If the roof-ceiling assembly isn't rated, why do you need a rated enclosure around the hood to penetrate a NON-rated ceiling?

IMC 2021, section 506.3.11, exception:

Exception: A duct enclosure shall not be required for
a grease duct that penetrates only a nonfire-resistance rated
roof/ceiling assembly.

But you still need to provide the required clearance from combustible materials, and that applies even if you put a 1-hour rated enclosure around it. The rated enclosure will have to be made of non-combustible framing if it's less than the required sistance from the hood.

That's great information. It's a 1920's Type 3 building, 2 stories, 7000 sq. ft. I don't know if it's considered 1 hour construction or not....that's what is making this difficult. I don't even think they had 1 hour construction back then....but if built today, it would have to be. The AHJ said to not worry about what it would be if built today...just that it is type 3 with no 1 hour requirement. So if I assume it's not 1 hour construction, then my grease duct can go straight up through the attic and roof framing with no enclosure or fire wrap....just keeping the distances to combustibles like you say. I see a provision (NFPA7.7.1.2) that states buildings more than one story requires a rated shaft (this building is 2 stories)....and a similar requirement in CMC 510.7. but each of these requirements seem to be referring to a rated ceiling (as you suggested) ....which I'm making an assumption I don't have. And just to add to the confusion, the original drawings for the UL listed hood (back in 2001) required a 1 hour shaft (by the hood manufacturer....not the AHJ) and quoted the 1 hour shaft requirement for buildings less than 4 stories. Perhaps they just assume this with their hoods if they don't know the building rating. We are relocating this hood to a different part of the space. I appreciate your help on this.

 

[Yankee Chronicler]

Just to be clear -- you keep referring to a 1-hour rated ceiling, but rated ceilings are actually about as rare as pink unicorns. What most people refer to as a "rated ceiling" is actually a rated floor-ceiling assembly or a rated roof-ceiling assembly.