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Two Family Dwelling / Dupexes (IRC R302.3)

rosegamble

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Jul 9, 2021
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5
Location
South Carolina
Hello! I was hoping to get some help with fire-rated wall requirements between duplex units. The 2018 IRC states that the requirements for duplexes are 1 hour (R302.3), but 2 hours for townhouses (R302.2). In general, the rules for duplexes over townhouses seem more lax, both in terms of rating requirement and vertical continuity (parapets, etc).

I have a simple building I'm designing with a demising wall between two residential units (duplex). Per the IRC, the 1 hour rating is required to be continuous from the foundation to the underside of the roof sheathing.

The rated wall in between is a 2x6 with a layer of Type X on each side (UL listed, from USG). The two-story structure has a flat roof, but a high crawl space below a wood-framed first floor assembly. If indeed the rating has to be continuous all the way up, I'm having trouble figuring out the detail for these conditions: (1) junction of demising wall and framed wood floor, and (2) junction of demising wall and the ceiling (in other words, how is the protection maintained during the floor assembly?). Does anyone know of any approved details that meet the intent of IRC 302.2 (i.e. are continuous)?

Thanks so much.
 

cda

Sawhorse 123
Joined
Oct 19, 2009
Messages
20,904
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Basement
Welcome

Give it a few days for great replies.

Your code must not require fire sprinklers for

Duplexes??

Townhomes??
 

rosegamble

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Jul 9, 2021
Messages
5
Location
South Carolina
Thank you! No, my state does not require sprinklers in any IRC project.

IRC 302.2 states that the "wall assembly must be continuous" so now I'm rethinking my question. Does the actual rated wall have to carry all the way through the building, even the crawl space... confusing.
 

cda

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Thank you! No, my state does not require sprinklers in any IRC project.

IRC 302.2 states that the "wall assembly must be continuous" so now I'm rethinking my question. Does the actual rated wall have to carry all the way through the building, even the crawl space... confusing.

Yes

But not sure about crawl space,,,

check this


R302.2.3 Continuity

The fire-resistance-rated wall or assembly separating townhouses shall be continuous from the foundation to the underside of the roof sheathing, deck or slab. The fire-resistance rating shall extend the full length of the wall or assembly, including wall extensions through and separating attached enclosed accessory structures.
 

rosegamble

Registered User
Joined
Jul 9, 2021
Messages
5
Location
South Carolina
That product looks interesting. Townhouses require double walls (R302.2) at the demising wall, but not duplexes. So my plan is just a simple common wall rated from both sides. Seems like a good product for townhouse demising walls though.
 

cda

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Here is yours



R302.3 Two-Family Dwellings

Dwelling units in two-family dwellings shall be separated from each other by wall and floor assemblies having not less than a 1-hour fire-resistance rating where tested in accordance with ASTM E119, UL 263 or Section 703.3 of the South Carolina Building Code. Fire-resistance-rated floor/ceiling and wall assemblies shall extend to and be tight against the exterior wall, and wall assemblies shall extend from the foundation to the underside of the roof sheathing
 

cda

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Previous discussion

 

rosegamble

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Jul 9, 2021
Messages
5
Location
South Carolina
Right. I read that one too. Didn't seem like the issue was ever fully resolved. If anyone knows of any approved details for how to continue that one-hour common wall through a crawl space and terminate at a foundation, I'd love to see! This house is small and we won't need a foundation wall below the demising wall... so I'm scratching my head.

Also, if the one-hour rated wall continues all the way up (no intersections with floor assemblies) then what happens where the floor assembly is secured to the wall? Wouldn't nails/fasteners complicate the continuous one-hour rating of the type X? I wish the IRC offered more for duplexes!
 

cda

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Right. I read that one too. Didn't seem like the issue was ever fully resolved. If anyone knows of any approved details for how to continue that one-hour common wall through a crawl space and terminate at a foundation, I'd love to see! This house is small and we won't need a foundation wall below the demising wall... so I'm scratching my head.

Also, if the one-hour rated wall continues all the way up (no intersections with floor assemblies) then what happens where the floor assembly is secured to the wall? Wouldn't nails/fasteners complicate the continuous one-hour rating of the type X? I wish the IRC offered more for duplexes!

There may be an exception

Wait for others to reply
 

cda

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R302.3.1 Supporting Construction

Where floor assemblies are required to be fire-resistance rated by Section R302.3, the supporting construction of such assemblies shall have an equal or greater fire-resistance rating.
 

Paul Sweet

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Oct 17, 2009
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Richmond, VA
I'd be nervous about gypsum board in a crawl space in the Southeast. DensGlass is meant to resist rainfall during construction, but might not withstand permanent exposure to ground moisture. FEMA wouldn't allow it if you're in a flood area. I'd go with 4" or 6" block if it isn't load bearing.
 

Joe.B

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Dec 4, 2020
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322
Location
Arcata Ca
Your state may not require sprinklers, but it may be cheaper/easier to sprinkler the project as an alternative?
 

forensics

Bronze Member
Joined
Oct 22, 2009
Messages
95
Location
The holy City CHARLESTON SC
Hi Rose Are you aware that home sprinklers can be as affordable at about $2 PSF? In SC the system can even be installed by the plumber. The multipurpose sprinklers will not need a separate water supply and can be combined with the domestic water. These systems do not require any regular annual inspections or homeowner service required. There is no backflow preventer required be cause the system complies with the code for potable water.
The sprinklers could be less costly than the foundation or rated wall assembly.
Finally the homeowner should be able to capture about 15% insurance discount.
Forensics
 
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