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A caution on Type “C” drywall

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.