IBC section 420 vs 508.4 vs 711

Regarding the separation for R-2 and M, which code would I follow to determine the horizontal separation rating for a Mercantile on the ground floor and R-2 units above?

IBC 420 states that walls separating differing occupancies associated with R shall be in the same manner as the walls between the units themselves and references section 711. Chp. 7 states that the horizontal assemblies shall be 1 hr non-sprinkler and 1/2 sprinkler.

IBC table 508.4 references section 420 but also provides a separation rating higher than what is prescribed in section 711. (I know confusing)

Which takes precedence? The stricter requirement or the footnote?

Hello @gfilcidor -

Thank you so much for posting such a clear “code thread” for how you were finding your fire rating separation. You are correct in your process, the only thing that I think you are missing is in regards to chapter 711. Check 711.2.4.1 here it states that;

711.2.4.1 SEPARATING MIXED OCCPANCIES-
Where the horizontal assembly separates mixed occupancies, the assembly shall have a fire-resistance rating of not less than that required by Section 508.4 based on the occupancies being separated.

This means that the rating on the chart 508.4 is the minimum allowed hours of fire separation between mixed occupancies. If there is another portion of the code that says it should be a higher rating (for example based on construction type) then follow the most stringent option.

Let me know if you need me to elaborate further.

-Elise

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