# Two Family Dwelling / Dupexes (IRC R302.3)



## rosegamble (Jul 9, 2021)

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.


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## cda (Jul 9, 2021)

Welcome 

Give it a few days for great replies.

Your code must not require fire sprinklers for

Duplexes??

Townhomes??


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## rosegamble (Jul 9, 2021)

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.


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## cda (Jul 9, 2021)

Have you seen this product



			https://literature.puertoricosupplier.com/037/MO36779.pdf


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## cda (Jul 9, 2021)

rosegamble said:


> 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.


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## rosegamble (Jul 9, 2021)

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.


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## cda (Jul 9, 2021)

If you are doing a duplex than 


*R302.2 Townhouses*

Walls separating townhouse units shall be constructed in accordance with Section R302.2.1 or R302.2.2.

Would not apply


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## cda (Jul 9, 2021)

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


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## cda (Jul 9, 2021)

Previous discussion 






						Duplex 1 hour separation through crawl space
					

In order to comply with the 1 hour separation requirement between units, wouldn't the 1 hour separation have to extend to the foundation under the building if there is a crawl space below?  Can you have a common crawl space below a non-rated floor assembly?



					www.thebuildingcodeforum.com


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## rosegamble (Jul 9, 2021)

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!


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## cda (Jul 9, 2021)

Cmu block ??? In the crawl space or poured concrete?


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## cda (Jul 9, 2021)

rosegamble said:


> 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


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## cda (Jul 9, 2021)

*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.


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## rosegamble (Jul 10, 2021)

2x12 framed first floor over a crawl space. CMU piers as the foundation.


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## ICE (Jul 10, 2021)

cda said:


> Have you seen this product
> 
> 
> 
> https://literature.puertoricosupplier.com/037/MO36779.pdf


That would be a good choice for a crawl space.  I’ll try to remember that.


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## Paul Sweet (Jul 12, 2021)

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.


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## my250r11 (Jul 13, 2021)

rosegamble said:


> This house is small and we won't need a foundation wall below the demising wall


You WILL NEED a foundation and some sort of foundation wall to achieve the separation required.


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## Joe.B (Jul 13, 2021)

Your state may not _require_ sprinklers, but it may be cheaper/easier to sprinkler the project as an alternative?


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## forensics (Aug 11, 2021)

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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## steveray (Aug 12, 2021)

If it is platform framing, the floor/ceiling assemblies would typically have to be rated as well as they "support" the walls....


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## sethturner13 (Mar 31, 2022)

rosegamble said:


> 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.
> 
> ...





rosegamble said:


> 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.
> 
> ...



Hi Rose,

Did you ever find a solution to this issue?  I am dealing with the same issue in Massachusetts right now!!


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## rosegamble (Mar 31, 2022)

No, I have yet to find a satisfactory solution! It's really an odd code situation that I don't think has been properly codified due to the fact that new duplexes are not very common. I do know the rating has to be continuous from foundation to top of building, so in the crawl space I would think a continuous wall of CMU is required between units. However, it's very odd from a construction standpoint to hold back the floor assemblies at intersections with the fire-rated wall to allow a continuous layer of Type X all the way up...


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## sethturner13 (Mar 31, 2022)

Yes it is odd.  We have built over 20 duplexes in our community and have never had an issue until a few weeks ago.  We hold the strapping back 3/4" on the 2nd floor fire wall, that way we can have continuous 5/8" drywall all the way up to the roof.  The issue for us is between the floors, the rim joist/floor joists does not have the 5/8" continuous sheetrock.  We have engineers, architects and our insulation company working on the issue.  I will keep you posted.


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## rosegamble (Mar 31, 2022)

I wonder about holding back the floor joists/rim joists 5/8" off the fire-rated wall framing, so that there are essentially 2 floors on either side of the fire-rated wall that are basically ballooned framed off that fire-rated wall. Maybe you could insert fire-rated sheathing into the 5/8" gap at the floor intersection instead of Type X? That way there is a layer of either Type X or fire-rated sheathing all the way up? Because obviously it's odd to hold the floor assemblies off the fire-rated wall for drywall since drywall happens so much later than framing. Please keep me posted - I'm so curious! When I was digging into this I did also hear about a possible solution of simply keeping it one continuous floor between units and filling in the gaps in the floor framing where there is a fire-rated wall above and below with solid wood blocking, because blocking itself could get you easily to a 1 hour rating.


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## mtlogcabin (Mar 31, 2022)

It sounds like the one hour wall is supporting the floor joist correct? If so then use Simpson




			https://www.strongtie.com/joistframingconnectors_coldformedsteelconstruction/s.dhutf_hanger/p/s.dhutf


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## rosegamble (Apr 8, 2022)

sethturner13 said:


> Yes it is odd.  We have built over 20 duplexes in our community and have never had an issue until a few weeks ago.  We hold the strapping back 3/4" on the 2nd floor fire wall, that way we can have continuous 5/8" drywall all the way up to the roof.  The issue for us is between the floors, the rim joist/floor joists does not have the 5/8" continuous sheetrock.  We have engineers, architects and our insulation company working on the issue.  I will keep you posted.


Hi Seth, just wondering if you ever got to the bottom of the tricky question!


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