# Plywood allowed on non-combustible interior walls, with drywall finish in Type IIB



## Morphdesigns (Jan 9, 2014)

Have a Building classified as a Type IIB building, using steel studs for interior walls.  Contractor wants to put full sheets of OSB sheathing against the studs in the bathroom and behind the cabinets in the kitchen/snack area, as use it for "blocking", for attachment of the cabinets, grab bars, mirrors, and wall hung sinks.  He then will put 5/8" gyp. bd over the top of that for the finish.

Is that allowed?  I cannot see anything definite under the IBC 603 exceptions that clearly defines that or not.

Thanks


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## steveray (Jan 9, 2014)

OSB will not work for blocking IMO.....does not hold a screw that well (grab bars and cabinets)...Again..IMO....FRT plywood would certainly be fine, the IBC does not put a limit on the amount of combustible blocking that I am aware of...but you might have to look at NFPA 13 for concealed combustible spaces.....


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## Morphdesigns (Jan 9, 2014)

Building in not sprinklered.

Should have mentioned that in original post.


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## mtlogcabin (Jan 9, 2014)

Welcome to the forum

What the contractor wants to do is not "blocking" as understood in the construction trades

If he wants to fully sheath the wall to provide attachment points for various items then he could use exception 25. Be aware that fire-retardant would requires specific fasteners and should not come in direct contact with standard steel studs as it could cause deterioration of the metal studs.

2304.9.5 Fasteners and connectors in contact with preservative-treated and fire-retardant-treated wood.

Fasteners and connectors in contact with preservative-treated and fire-retardant-treated wood shall be in accordance with Sections 2304.9.5.1 through 2304.9.5.4. The coating weights for zinc-coated fasteners shall be in accordance with ASTM A 153.

Exception: Plain carbon steel fasteners in SBX/DOT and zinc borate preservative-treated wood in an interior, dry environment shall be permitted.


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## cda (Jan 9, 2014)

Welcome!!!

How did you find us


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## Morphdesigns (Jan 9, 2014)

Somebody I work with was doing code research and found this forum.  He has used it multiple times with success with getting answers from his posts and finding answers already posted.  He forwarded me the link and today was the first time I needed to post a question.


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## mark handler (Jan 9, 2014)

Proper support in metal stud walls


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## cda (Jan 9, 2014)

Morphdesigns said:
			
		

> Somebody I work with was doing code research and found this forum.  He has used it multiple times with success with getting answers from his posts and finding answers already posted.  He forwarded me the link and today was the first time I needed to post a question.


Well welcome again

Ask for code sections and helps to state what building code used and edition


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## steveray (Jan 9, 2014)

I understand why he wants to use sheet goods...I did a university locker room in another life and by the time everything was blocked (mirrors, pt and tp dispensers, soap, partitions, grab bars, coat hooks, sinks, etc.....) it wouls have been 4X faster to sheath the whole wall...Maybe the same gage sheet metal as the blocking Mark shows, but in sheet form?

MT...My book doesn't go up to 25....I will have to look at that one....


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## High Desert (Jan 9, 2014)

Cobustilble blocking is permitted for handrails, millwork, cabinets and door frames per 603.1 Item 11. If they want to fully sheet the wall, they need to do what mtlogcabin said...used FRT plywood


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## mark handler (Jan 9, 2014)

Metal BACKING PLATE ADDED SUPPORT WHERE SHELVES, CABINETS, FIXTURES OR HANDRAILS ARE INSTALLED

http://www.clarkdietrich.com/products/backing-systems/backing-plate


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## steveray (Jan 9, 2014)

High Desert said:
			
		

> Cobustilble blocking is permitted for handrails, millwork, cabinets and door frames per 603.1 Item 11. If they want to fully sheet the wall, they need to do what mtlogcabin said...used FRT plywood


603 allows combustible blocking......The question is,how much? I see no limit expressed or implied...."14. Blocking such as..." Gives examples...it is IMO, not intended to ba a catch all list.....


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## High Desert (Jan 10, 2014)

Didn't say it was a catch all list. Just said it was allowed in noncombustible construction. Also added that if they want to sheet the wall fully, it would have to be FRT plywood. The OP said they wanted to use the plywood to attach cabinets, grab bars , mirrors, sinks, etc. The proper material for that in section 603 is blocking, not sheathing the entire wall with plywood.


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## steveray (Jan 10, 2014)

I am just trying to make clear that this code requirement is not.....what would the difference be between a wall fully blocked or a wall fully sheathed and laminated with drywall? (besides the extension rings for you sparkies    )....FRTW is also combustible (even though much less so)...and can only be used where allowed by 603.1 1 or anywhere combustible wood is allowed


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## Builder Bob (Jan 10, 2014)

Not an engineer, but thought I would try to answer your question - the blocking has a smaller surface area and a higher thermal mass than plywood.......the plywood is a smaller thermal mass thus absorbs heat at a faster rate and would allow the plywood through pryolysis to degrade, generate ignitable vapors, and ignite faster and quicker than a 2x4 blocking material.

Also, blocking is not continuous thus has built in fire stops... plywood doesn't.

Pyrolysis is usually the first chemical reaction that occurs in the burning of many solid organic fuels, like wood, cloth, and paper, and also of some kinds of plastic. In a wood fire, the visible flames are not due to combustion of the wood itself, but rather of the gases released by its pyrolysis, whereas the flame-less burning of a solid, called smouldering, is the combustion of the solid residue (char or charcoal) left behind by pyrolysis. Thus, the pyrolysis of common materials like wood, plastic, and clothing is extremely important for fire safety and firefighting.

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen (or any halogen). It involves the simultaneous change of chemical composition and physical phase, and is irreversible.

The face of the plywood would be ex[posed on one side- either the internal stud cavity or to the room exposure.......type II construction limits the combustible products located within to make the building as fire resistant as possible without adding structural fire protection to the mix - (rule of thumb - type II-A does have limited structural protection within the design of the building)


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## steveray (Jan 10, 2014)

BB......  That is wicked scientific, and I do appreciate you taking the time to write......but what is to say that the blocking cannot be plywood? And where is the code requirement  that it cannot run the entire height of a stud bay and each bay in the wall? I am sure as soon as they can find a way to make it work or market it there will be PVC blocking


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## steveray (Jan 10, 2014)

How much is too much?


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## peach (Jan 11, 2014)

603 actually allows FRTW for blocking in type I and II, recognizing that it's a combustible material. I can't really find that non fire treated wood products are allowed at all.


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## peach (Jan 11, 2014)

Code seems to be silent on the amount of combustibles... unless you go to 717.5 which throws us back to 603 .. judgment call, I guess.


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## Builder Bob (Jan 14, 2014)

steve ray, the blocking used in your picture I would agree with. the intent of the code is to "limit" combustibles in a type I or type II building. I would agree that the blocking in your picture is ok as long as this is for cabinetry and/or grab bars. I also noticed that the level of blocking varies from room to room.


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## steveray (Jan 14, 2014)

We usually "ask" for FRTW...if they want to sheath a wet wall for blocking purposes, I do not have an issue with that. If you ever had that kind of fire inside the wall I am sure the water pipe would let loose and self supress....


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