Flame Barrier for Bonus room over garage.

[Naison]

Hello all, long time reader and value the experience shared on this forum. Figured I'd make my first post about an issue I am uncovering in my project to see what others have done. If i missed a past thread on this please let me know but can't seem to find anything directly applicable. 

Background: 
I am in the final stages of designing a 3 bay timber frame garage which will have a bonus room and bathroom above. 

After briefly reading the IBC, my understanding is the garage ceiling is the separation between uses (U to R-3) and must have a 1 hour fire resistance rating (FRR). Typically this is accomplished in stick framing with 5/8" Type X gypsum over the 16OC joists. 

However in my case, I cannot see covering the red oak timber floor system with sheetrock, so looking for what others have done to meet the code. I believe the intent of the code is to protect the structure and not prevent spread of fire, which prevents my original idea of putting the 5/8 gypsum into the floor system.

Potential solution:
So far I have uncovered TR-10 published by the AWC which provides some guidance that FRR can be designed into the frame based on the structural loads of all the timbers and an assumed char rate of the wood. Is this really the approach or have others come up with clever compliance approaches.


Any advice will be appreciated. 

[Old Greenhorn]

I am not a construction guy, but wanted to throw in another thought for you from a firefighter's perspective. The fire resistance rating is designed to allow time for occupants to exit the structure and fire suppression to take place as safely as possible in the initial stages of a fire. Now having said that, stick construction burns much faster than timber because there is more exposed surface area plus the strength of the individual members is much less in stick than timber. What this means is, uncovered or unprotected, your timber construction already has a much longer fire resistance time as compared to your stick build. In other words, you may not need to cover the timbers and can still meet the code. Now the decking material and all the other components also play a part in this, so be sure before you proceed. All I am saying is: stick and timber are not rated the same so keep that in mind. Structural firefighting is a science and one needs to understand a good deal about construction methods before commencing an attack. I would rather enter a burning timber built building any day over one made with cement block and steel roof trusses. You need to wait and see how the real construction code experts weigh in here, but I wanted to put up that thought for you.
Good Luck,
Tom
P.S. Welcome to the forum! Fill out some of your profile info so we know about where you are and can get you better answers.

[D L Bahler]

You do have fire separation in the garage to prevent fire spread into living areas. It's not just there for the structure, it's there for the occupants too. 

A timber frame should have more than a 1 hour fire rating if designed properly. 
If your floor is set into the frame and not on top of it, you can incorporate drywall into it, but will need some kind of gasket where the assembly meets the timber framing elements. A fire-rated caulk, for example. 

[Don P]

Basically what you and Old Greenhorn are describing is type IV construction in the IBC, heavy timber, no concealed spaces. If you go into older repurposed restaurants, stores, etc, noncombustible walls and look up to see open timber ceilings with ducting and piping visible, you are in a type IV building, heavy timber with nowhere for the fire to hide. Go to section 602.4 and start reading, basically if posts are 8x8 and larger and joists are 6x10 and larger they can be exposed. If you can tolerate 5/8 drywall that can be the ceiling, otherwise the subfloor is 3" thick with a nominal 1" thick finish floor laid diagonal or perpendicular over that. That is describing a commercial building but the timber elements more than pass the fire rating required so I think it should pass plan review easily. There may be wiggle room below that and I haven't read TR-10 in awhile.

The real bottom line is to sit down with the building official and hash it out ahead of time.

[Naison]

Excellent. That is the section I need. 

The plan is just as you say Don P, but before I brought an issue in front of the inspector I wanted to be as educated as possible about alternatives. 

I now have alternatives per the code and perhaps a hybrid approach with exposed posts/beams and 5/8" hidden behind a T&G ceiling to avoid the 4" floor. 

Thank you for the direction!