If you are adding an ADU (accessory dwelling unit) attached to an exisitng, main dwelling, then you may be required to provide a fire rated separation between the 2 units.
Section R302.3 from the IRC (International Residential Code), 2018 edition mandates a 1 hour fire rating between dwelling units when there are two dwelling units attached. See the full code passage below. In a sprinklered home (not common in single-family/ two-family construction) it can be reduced to half hour fire rating, and when theres an attic connecting there are some exceptions for that as well.
In general, when you add an ADU next to, below, or above another dwelling unit, then it must consider the fire separation requirement.
Floor/ceiling fire separations can be achieved in many different ways. You must meet a listed/ tested assembly. This means its an floor/ ceiling assembly of parts that has been tested to withstand a fire for the listed time.
UL (Underwriters laboratory) is one of the main authorities in provided tested fire assemblies. UL L528 was used in a project of ours recently, providing a 1 hr rating to the ceiling of an ADU going underneath and existing, raised home. The existing ceiling was 2×12 joists, 3/4″ plywood, and vInyl flooring above. So, we found this assembly, that worked with the existing construction and added elements below:
-EXISTING- 0.75″ T&G PLY SUBFLOOR
-EXISTING- 2X12 JOISTS @ 16″ O.C.
-FILL CAVITY W MINERAL WOOL, ROCKWOOLOR SIM. (~R-38)
-CLARK DIETRICH RC-1 RESILIENT CHANNEL OR SIM UNDER, 25 GA.GALV STL, 16″ O.C. PERPENDICULAR TO JOISTS
-TWO LAYERS OF 5/8″ TYPE X GYP, SCREWED TO RESILIENT CHANNELS
Light fixtures must also be rated, and rated fixtures/ electrical boxes are commonly found. HVAC can be tricky, so if you have ducts, it is best to keep them from penetrating the rated ceiling. Small penetrations can typically be allowed with fire caulk around them. Top plates of walls abutting to the ceiling are permitted per IRC 302.11.1.
Typically this floor/ceiling assembly only needs to consider fire rating from below.
Fire rating walls are often a bit more straightforward then ceilings.
UL 305 is a really common wood framed 1 hour assembly. It is basically 5/8″: drywall (UL Type ULIX) on both sides of 2×4 studs @ 16″ on center. Typically you must consider this fire rating from both sides of the wall. This can be a bit tricky if the wall is existing, meaning you may have to replace the inside of an existing wall with 5/8″ drywall, if it only has 1/2″ which is more common for non-rated residential construction.
Typically, a fire rated assembly needs to have any supporting construction also fire rated. In one example, we had a rated ceiling. The ceiling had the UL L528 applied to it. But, there were larger wood beams that extended below the rated ceiling. We could have continued to wrap the beam in this assembly (wrap it in drywall), but the client wanted to keep the beam exposed. In this case, we used intumescent paint to apply to the beam.
Intumescent paint expands when heated and provides fire protection. This was a solution that allowed us to keep the beam exposed while keeping it fire rated. Intumescent paint can be used on wood or steel, and different companies have tested assemblies for various types of medium they can be applied to.
