In the last while, it has been challenging for many contractors to obtain building materials. Unfortunately, that's led to an issue where contractors who are required to install fire-rated drywall have faced shortages of the "Type X" fire-rated drywall that is normally used in such construction.
In the past few months, our commercial inspector has encountered issues where contractors have substituted "Type C" fire-rated drywall. Our office has heard several second-hand stories of hardware stores telling contractors that Type C drywall -usually available in a 1/2" thickness - is "just as good," or "better than" 3/4" Type X drywall, and that it's perfectly OK to substitute one for the other.
That could not be further from the truth: in fact, using Type C drywall instead of Type X drywall could be a very expensive mistake.
Type C drywall is a newer fire-rated drywall. Currently, the National Building Code does not recognize Type C drywall the way it acknowledges Type X - which has been around for decades. NBC 2015 has literally hundreds of recognized assemblies with Type X drywall, as well as a long-established formula for assigning fire-ratings to various generic wall, floors, and steel columns using Type X drywall - but not a one with Type C.
Consequently, the only way our office can accept Type C drywall in a fire separation is if either if it is replacing a Type X drywall of equivalent thickness, or it has undergone a rigorous fire-exposure test. The problem is that very few assemblies with Type C drywall have been subjected to these tests, which means our office can only accept a handful of very specific systems as complaint. And of the systems that have been tested, we are only able to accept just those specific assemblies tested - with no alterations or substitutions allowed. In other words, for a Certainteed Type C wallboard used in a system we can accept, we would also have to see Certainteed tape and Certainteed drywall mud, since that's the brands of material used in the tested assembly: no substitutions are allowed, whatsoever.
While Type C can be used in some dimensional-lumber systems (Georgia Pacific, USG, and Certainteed, can all rate 1/2" Type C drywall as meeting or exceeding a 45-minute rating on 2x10 joists set 16" OC instead of 5/8" Type X, for example) this is the rare exception at the moment. For example, the number of fire-rated floor assemblies tested with engineered joists is minimal, and in most cases, the systems require 5/8" Type C or double layers of 1/2" type C.
To put it another way, if our office observes 1/2" Type C on the underside of an engineered joist, because "the hardware guy said it was just as good as 5/8" Type X," the contractor will likely either have to add a second layer of 1/2" Type C drywall (or 5/8" Type C) or add a second layer of the very 5/8" Type X they were supposed to use in the first place. This is not a mistake anyone - including us - wants to see happen.
The long and the short of all of this? Call us before you use Type C drywall in any assembly: it could save you a lot of time, and more importantly, save you a bucketload of wasted money.