We are working on an existing building and need to know if the exterior 8" masonry walls are 2 hr fire rated in order to make a determination as to the Building Type. The exterior walls are covered in direct applied stucco and the interior walls are covered with gypsum drywall. Can we be reasonably sure that the walls, with stucco and drywall, meet the requirements for 2 hours?
The only way to be 100% sure is to remove 6 block intact from the structure and send them to the lab to be tested to determine that they meet the requirements of C90 and what the equivalent thickness of the units are in accordance with C140. you would also need to verify that the material used in manufacturing the units is limestone.
All that being said, you can make some conservative assumptions and be relatively certain that the walls meet 2 hours using the provisions of Chapter 7 of the 2017 Florida Building Code 6th Ed., Building.
Since 99.9% of all block manufactured in Florida use limestone as the main aggregate you can safely assume that the block in the building are limestone based, thus requiring a 4" equivalent thickness in accordance with Table 722.3.2.
The least equivalent thickness for any block regularly produced in South Florida is 3.6 inches. An example of this would be the CEMEX T007. Thus 3.6 would be a conservative assumption for the block used in a building in SW Florida. Interpolation of the table is allowed so that 3.6" gives you 100 minutes. Thus, you are .4" and 20 minutes short of your required 2 hr rating.
Section 722.214.171.124 gives you directions on how to calculate the fire rating value of finishes applied to the non fire-exposed side of the wall and Section 7126.96.36.199 gives you directions on how to calculate the value of finishes on the fire-exposed side. These sections refer you to Table 7188.8.131.52(1) for the non fire-exposed side and Table 7184.108.40.206(2) for the fire-exposed side. If there is doubt from which side the code is requiring the rating the best thing is to calculate from both side and take the least value.
Conservative assumptions on the wallboard and stucco would be 1/2" wallboard and 3/8" (.375") single coat stucco (the stucco would not be to code but would be conservative for the purpose of calculating a conservative value of the fire rating).
Running the calc with the fire side assumed as the exterior you get:
FIRE RESISTANCE RATINGS OF CONCRETE MASONRY ASSEMBLIES
Don Beers, PE, GC
Jerry Painter, FASTM