California ADU and Fire Assemblies for Wall and Floor/Ceiling

[FunHouse]

Hi all - I'm a residential designer (*not a licensed architect) working on a project in Southern California. The project is converting the ground floor of an existing two-story structure into an ADU (Accessory Dwelling Unit). Currently there are 4 garage bays on the ground floor with a rental unit above them which is accessed by an exterior staircase. The owner would like to convert one of the downstairs garage bays into an ADU. The ground floor is a slab foundation w/ standard wood frame construction built in 1939. We submitted plans and the city has responded by saying they want to see a 1-hour fire rated floor ceiling assembly and 1-hour wall assembly for the supporting walls. The problem is the existing construction of the upstairs floor is nominal lumber, 2x8 wood joists 16" O.C. and the owner is hoping not to have to tear into the upstairs unit to modify anything. Everything I can find as far as floor-ceiling assemblies seem to call for 2x10 or 2x12 joists. I found one detail from National Gypsum (PDF attached) where it looks like we could leave the upstairs floor alone however it's using 2x10 also. Below is a diagram for reference and the exact comments from the plan checker. Curious if anyone know of any details similar to the one attached by any other manufacturers that show an option for a 2x8 floor joist? Ultimately I am trying to save the homeowner on costs and hoping the fire rating can be achieved from modifying the new downstairs and not the unit upstairs. Thank you in advance for any help with this much appreciated.



[ Comment 00047 | Sheet A1.6 ]
Dwelling units shall be separated from each other by FLOOR ASSEMBLIES having not less than a 1-hour fire-resistance rating tested in accordance with ASTM E119 or UL 263.
Fire-resistance-rated floor/ceiling assemblies shall extend to and be tight against exterior wall. CRC R302.3
Provide one-hour fire resistance rated floor assembly. Specify the approval listing agency for the floor assembly along with the approval number and a detailed description of the assembly per the approval.

[ Comment 00048 | Sheet A1.6 ]
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. Provide detail for the supporting wall or other supporting members to be at least 1-hour fire rated. CRC R302.3.1

[ Comment 00049 | Sheet A1.6 ]
Floor-ceiling assemblies separating dwelling units from each other shall have a sound transmission class of not less than 50 where tested in accordance with ASTM E90, and shall have an impact insulation class rating of not less than 50 where tested in accordance with ASTM E492.
Specify the approval listing agency for the Floor-ceiling assemblies separating dwelling units along with the approval number and a detailed description of the assembly per the approval that provides the required STC and IIC rating. SDMC Section 149.0341, CBC 1206.2, 1206.3

 

[Yikes]

Welcome to the forum!

CBC Table 721.1(3) item #21 does not specify the depth of the dimensions of the floor joists at all, and only requires them to be spaced no greater than 24" o/c apart. With no minimums, that could work for 2x8s at 16" o/c. Floor thickness is min. 1/2 of plywood.
You put (2) layers of 5/8" type X gyp board on the underside of the joists to get the one hour rating. This adds up to 1 1/4" of gyp board thickness.

This does not solve your sound rating, but it does solve the fire rating.




This does not

 

[FunHouse]

Welcome to the forum!

CBC Table 721.1(3) item #21 does not specify the depth of the dimensions of the floor joists at all, and only requires them to be spaced no greater than 24" o/c apart. With no minimums, that could work for 2x8s at 16" o/c. Floor thickness is min. 1/2 of plywood.
You put (2) layers of 5/8" type X gyp board on the underside of the joists to get the one hour rating. This adds up to 1 1/4" of gyp board thickness.

This does not solve your sound rating, but it does solve the fire rating.


View attachment 11529
View attachment 11528
This does not

Hi Yikes - Thank you so much for that. I was told by a colleague to reference this manual for assemblies that could work: GA-600-2018 Fire Resistance and Sound Control Design Manual. Do you happen to know of a way to reference this without having tp purchase it for $70? I have to include the sound part...thank you!

 

[MtnArch]

There are older versions of the Gypsum Association's manual available in PDF format if you search around, but if you will be delving into multi-family (including ADU's like this) it's well worth the purchase price.

 

[ICE]

The fire and STC rating of walls that separate a garage conversion to an ADU and the existing dwelling can present issues with what's on the other side of the common wall. Quite often there is a detail on the plans that didn't reckon on there being a kitchen, laundry or bathroom on the existing dwelling side.

 

[mark handler]

Sister 2x10's onto the 2x8s. then build per details

 

[Robert]

I know you didn't ask about this but did they require you to do anything with the existing slab floor? Usually insulation and vapor retarders are required for dwellings. Can be done, but tricky.

 

[FunHouse]

I know you didn't ask about this but did they require you to do anything with the existing slab floor? Usually insulation and vapor retarders are required for dwellings. Can be done, but tricky.

No they didn't, in fact they seem pretty relaxed about it when I spoke with them. They said as long as I show the added fire and sound layers to the ceilings and walls that should suffice (in California there is a real push to get ADU's through permitting as quickly (but safely) as possible since we're having such a severe housing shortage/crisis here).

 

[FunHouse]

There are older versions of the Gypsum Association's manual available in PDF format if you search around, but if you will be delving into multi-family (including ADU's like this) it's well worth the purchase price.

Good point thank you.

 

[ICE]

in California there is a real push to get ADU's through permitting

The push includes doing away with a plan check prior to issuing a permit. Inspectors are supposed to do a plan check in the field. The solar industry has achieved the same thing. In both cases there will be no plan check.

This was done to speed things up...so they say. The truth is that the government does not care how long it takes to get a permit. I was done because there is just so much of it and they don't want to work that hard.

did they require you to do anything with the existing slab floor?

The floor gets a coating that should seal out water. Most garage floors slope towards the door but there is no requirement for a level floor ... the slope can remain.

 

[ICE]

since we're having such a severe housing shortage/crisis here

The housing crunch affects the lowest rung of the economic ladder. ADUs are seldom rented to people on that rung as the rent is beyond their reach. California politicians have scant regard for people with limited resources.

While I am not so well informed as to know the reasoning behind allowing ADU's to proliferate, I suspect it has to do with the already out of control, illegal dwelling problem. If the problem can't be corrected, reclassify it as an ADU.

A probable unintended consequence is the habit of over occupying all available housing. A 1200 sq.ft. ADU can and will, be divided.… into how many dwellings becomes a matter of ingenuity.

Perhaps my shortsightedness misses the fact that it’s an intended consequence, for it seems odd that I see this and the politicians don’t.

 

[FunHouse]

The housing crunch affects the lowest rung of the economic ladder. ADUs are seldom rented to people on that rung as the rent is beyond their reach. California politicians have scant regard for people with limited resources.

While I am not so well informed as to know the reasoning behind allowing ADU's to proliferate, I suspect it has to do with the already out of control, illegal dwelling problem. If the problem can't be corrected, reclassify it as an ADU.

A probable unintended consequence is the habit of over occupying all available housing. A 1200 sq.ft. ADU can and will, be divided.… into how many dwellings becomes a matter of ingenuity.

Perhaps my shortsightedness misses the fact that it’s an intended consequence, for it seems odd that I see this and the politicians don’t.

You raise a lot of good points. I have been approached by numerous clients who already have existing conversions who want to make them legit/legal ADU's now. Mostly because they see how it can often raise property values by having two units vs one on their property. Basically any property that has a garage (detached or attached) is allowed to convert that garage into a living unit now under the ADU law. What's insane is how there is zero requirement for property line setbacks anymore. I think the law was genuinely implemented to try and help with the housing shortage, but developers have seized on it and are gobbling up all the housing. And sadly you're right California has become so expensive that even these ADU's will likely just turn into very expensive rentals.

 

[ICE]

I have been approached by numerous clients who already have existing conversions who want to make them legit/legal ADU's now.

I suppose that many think it is just a matter of paperwork when it is worse than starting from scratch.