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Plywood over metal decking?

1

1ndog

Registered User
Joined
Feb 23, 2024
Messages
8
Location
Washington DC
Hello,

I have a tilt up concrete project that is going for construction in Virginia. It has utility rooms in the space with a hard lid that is not exposed to the exterior. The room is only 10 feet tall or so and the building has a roof of 30-40 ft. I know the utility rooms have to be 1 hour rated including the lid. What is the fire rating of a metal deck with a plywood on top. Ive only seen this done for mezzanines where there is not fire rating...

I cant seem to find anything online making me think that this is not a 1 hour rated assembly.

Thanks
 
1ndog said:
Hello,

I have a tilt up concrete project that is going for construction in Virginia. It has utility rooms in the space with a hard lid that is not exposed to the exterior. The room is only 10 feet tall or so and the building has a roof of 30-40 ft. I know the utility rooms have to be 1 hour rated including the lid. What is the fire rating of a metal deck with a plywood on top. Ive only seen this done for mezzanines where there is not fire rating...

I cant seem to find anything online making me think that this is not a 1 hour rated assembly.

Thanks
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What type of construction is this? Type I or II would not allow for the use of plywood per Sec. 603.
 
Also, metal decking generally has no fire-resistance rating. You'll need to pick an assembly - likely something with gypsum board, concrete, etc.
 
1ndog said:
It is Type IIB Fully Sprinklered Unprotected B
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Plywood is not permitted in Type II-B. Does not meet an exception in Sec. 603.1.

2021 IBC 602.2 Types I and II​

Types I and II construction are those types of construction in which the building elements specified in Table 601 are of noncombustible materials, except as permitted in Section 603 and elsewhere in this code.
 
Right that is what I thought, Just couldnt find the exact section. What would be an appropriate assembly that will meet a 1 hour rating? Other than concrete over metal decking. We still have metal decking but is there some cheaper option that can achieve the same rating?
 
classicT said:
Plywood is not permitted in Type II-B. Does not meet an exception in Sec. 603.1.

2021 IBC 602.2 Types I and II​

Types I and II construction are those types of construction in which the building elements specified in Table 601 are of noncombustible materials, except as permitted in Section 603 and elsewhere in this code.
Click to expand...
I see it says fire retardant wood(Im thinking of fire retardant plywood) can be permitted in roof construction. I guess because this is technically not a roof that it doesnt count right?
 
1ndog said:
Right that is what I thought, Just couldnt find the exact section. What would be an appropriate assembly that will meet a 1 hour rating? Other than concrete over metal decking. We still have metal decking but is there some cheaper option that can achieve the same rating?
Click to expand...
There are thousands of options.

What is supporting the metal decking? Joists?
 
1ndog said:
I see it says fire retardant wood(Im thinking of fire retardant plywood) can be permitted in roof construction. I guess because this is technically not a roof that it doesnt count right?
Click to expand...
Correct, not a roof.
 
1ndog said:
Right that is what I thought, Just couldnt find the exact section. What would be an appropriate assembly that will meet a 1 hour rating? Other than concrete over metal decking. We still have metal decking but is there some cheaper option that can achieve the same rating?
Click to expand...

You have to look through the U.L. rated assemblies to find one that you can use.
 
classicT said:
Type I or II would not allow for the use of plywood per Sec. 603.
Click to expand...
Unless it's flooring, correct? I ask because plywood is used as a finished flooring on catwalks in theatres frequently, and ply in metal deck - the structural floor - seems to be economical. 410.2.2 may be why this has not been flagged.
 
bill1952 said:
Unless it's flooring, correct? I ask because plywood is used as a finished flooring on catwalks in theatres frequently, and ply in metal deck - the structural floor - seems to be economical. 410.2.2 may be why this has not been flagged.
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Right that is what im being told, apparently non compustible plywood is permitted in type IIB construction. But I cannot find where it states that in the code!
 
1ndog said:
Right that is what im being told, apparently non compustible plywood is permitted in type IIB construction. But I cannot find where it states that in the code!
Click to expand...

There is no such thing as "non-combustible" plywood. Fire-retardant treated plywood is classified as "limited combustible," not "non-combustible."

Whether or not FRT plywood could be used as a finished floor doesn't help the OP. The OP needs a 1-hour rated floor-ceiling assembly.
 
Yankee Chronicler said:
There is no such thing as "non-combustible" plywood. Fire-retardant treated plywood is classified as "limited combustible," not "non-combustible."

Whether or not FRT plywood could be used as a finished floor doesn't help the OP. The OP needs a 1-hour rated floor-ceiling assembly.
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haha i think I am OP. Yes, that is what I was told. I understand there is no such thing as non combustible plywood. I am also playing devil's advocate to see if there is any validity to the contractors' statements.
 
Then go to the USG Structo-Crete web page, scroll down to the bottom, and click on the link at lower left for info on rated floor assemblies. You will need to register and they send you the info. I don't need it, so I don't care to register, but my guess is that's where to find your answer.
 
Youd will likely be best served if you consult with an Architect.
 
Reading IBC section 805.1, my understanding is that wood flooring (subfloor or finished floor) is allowed when installed over a noncombustible structural floor system. If the wood is not adhered directly to a non-combustible substrate, the voids beneath the wood have to be fireblocked into areas not larger than 100 square feet. Depending on the size of the utility rooms, attaching plywood directly over fluted M deck could probably satisfy this criterion, as long as the M deck is capable of supporting the floor load without the plywood -- otherwise the plywood becomes part of the structural system, which is not allowed.

However:

I cant seem to find anything online making me think that this is not a 1 hour rated assembly.
Click to expand...

That's not how it works. NOTHING is assumed to be a fire-resistance rated assembly unless and until you find a test by an accredited testing agency that demonstrates that the assembly meets the requirements for a fire-resistance rating of [__time__]. If you can find a 1-hour listed assembly that has plywood over M deck, you should be good to go. I don't think you're going to find one. You might find one using the Structo-Crete panels, but the problem is for floor-ceiling assemblies, the test fire is in the chamber underneath the sample floor. That's going to expose the M deck directly to the test fire, so the 3/4-inch of Structo-Crete on the top may not be enough to get the 1-hour rating.

But the links on the Structo-Crete page suggest that there are some rated assemblies, so I would recommend signing up to have USG send you the information.
 
I've recommended and have seen structocrete used on several projects, but not as part of a rated assembly. Used for seating risers in places of assembly, less expensive and lighter weight than concrete on decking or foam. It is structural gyp board. Can't be left exposed to foot traffic - needs vinyl, carpet, wood, tile, something. I had immediate response from reps listed on web site - like I contacted late Friday afternoon and he responded on Saturday. Very good rep work. Might be more/faster help than downloading data.
 
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