Exterior Fire-rated Wall Assembly?
Ready for muddy water?

If you go by solely by code,  2012 IRC Table R302.1(1) specs out a 1 hour TESTED  wall assembly with fire from both sides.  As an individual component,  IBC Specs out a 40 min rating per layer of 5/8" Type X so to have a 1 hour untested assembly you would need two layers EACH SIDE.  The only way to get the 1 hour rating is to use a ASTM or UL tested assembly.  There are hundreds of these, but not readily available to the general public.  BUT THE ABOVE INFORMATION IS VOID.  Read on. 

As much as you might like to think you are dealing with code, you are not; you are dealing with a person, the building offical.  It does not matter what the code says, it is what your local building official says.  Enter the political game played by industry professionals with the code officials on projects to minimize the impact the code official have on their projects and clients pocket books. 

There are hundreds of rated wall systems out there you SHOULD use but, here's the rub,  most building officials on homes seems to ignore that fact and will simply tell you what they will require you to have without using a spec sheet from a tested assembly.

Next is a bigger issue at hand here which is the IRC is littered with holes which require to be filled in with conversations between you and the building official.

In this case, the code says you need to both use a tested 1 hour assembly and you need to directly attach the OSB to the studs  (IRC Table R602.3(3) ).  This is confirmed by conflicting direction from two separate code officials.   Structurally, the OSB needs to go directly against the stud, but for fire rating, you need rated assembly that specs and/or allows OSB under the gypsum;  I can't remember seeing an assembly like that leaving you in a pickle.  And if you take it further, it (the IRC) does not clearly address what to do if a floor penetrates the rated partition or what to do at the ceiling.  Would they require you to rate the floor to 1 hour as well?  Or balloon frame and ledger the floor the rated assembly?  Put gypsum through the trusses up to the under side of the roof sheathing?  All of these things are a bit unheard of for the home because building officials do not go by the letter of the law as much as they let on.

Take a step back to your question though,


  is it OSB against the stud or gypsum?


In the end it is what ever your building official on your projects says it is. 

So how do you deal with this?  I would not bring to light the conflicting information otherwise you will be left with dealing with it, not them.  They could make you address both the fire rating and the structural issues with not attaching the OSB directly.

What I would do is reason with them.  Sometimes one can have a frank discussion about a matter built on the backround of code without referencing the code directly.  I would reason the OSB is structural and would not preform its function properly if mounted on top of the gypsum.  I would also play on the common misconception the 5/8" Type X gypsum is rated for 1 hour by the code as an isolated component (Even though it is not.  Ref IBC Table 722.2.1.4(2) )  Here's the next catch, to have a successful conversation usually requires a keen knowledge of the code, buildings in general, a highly developed skill with people, and a special set of skills in dealing with building officials.  True building professionals know how to do this.

If you cannot engage in this conversation correctly, you could shoot yourself in the foot by bringing to light the conflicting information; then the official might require you to do more than they initially said because of the conflict you brought up that they feel is valid and needs addressed.

So what IS the answer?  A few possible answers (No Recommendation Here)


Do what the building official says without question (Sometimes actually the best policy)
Question for understanding from the code official; read the code with them and discuss the findings, then ask what they will  require after the discussion is done (sometimes takes days and multiple meetings and may be problematic if you not done correctly).
Re-side your house without consulting you local building jurisdiction.  Know the caveats of doing so and proceed accordingly.  (FYI You should still be prudent in your efforts meaning you can still sheath it in OSB to strengthen the home and you can put gypsum on the outside and/or inside to provide fire protection)