All Things Gypsum: Finishing Joints in Fire-Rated Construction, Part 1
Somewhere there has to be an old Vaudeville joke where the stand-up asks the straight man: “So, how do you confuse a drywall contractor?”
After much fumbling by the straight man, the comic finally relents and says “You put him in a room with fire-rated walls and ask him to finish the joints.”
In 1923, no doubt a riot of rotten tomatoes followed that whopper; however, in today’s drywall finishing market, many contractors often seem to feel as if the big hook that yanked the rube off the stage after a bad performance lurks right around the next corner at the next project. And generally, it is not a contractor who is at fault when a problem ensues; it often seems to be an uninformed inspector or prime contractor who throws the unnecessary barrage of decayed vegetables at the applicator.
FOLLOW THE TEST PROTOCOLSo how do you avoid bearing the brunt of a bad joke on the jobsite? Well the simple solution when finishing the joints on a fire-rated system-and by system in the context of this article we mean a wall, ceiling, roof, column, beam, or any other fire-rated system that has a gypsum board membrane that functions as a part of the fire-resistant system-is to do exactly as the fire test indicates. Fire-rated systems are generally tested with the joints finished to a prescribed degree, and to fully comply with the fire test, you have to finish the joints accordingly.
But there are some slight twists and turns to that staid principle; therefore, examining a specific system proves instructive.
A basic, illustrative system is WP 3605 as shown in GA-600-06, Fire Resistance Design Manual. WP 3605 is used as the foundation of many one-hour wood frame wall system tests and is reinforced by one of the most basic Underwriters Laboratories (UL) listings, UL Design U305. The quick description of the system: a layer of 5/8-inch-thick type X gypsum board on either side of 2x4 wood studs.
In GA-600, the system is supported by three specific fire tests and two listing descriptions. Picking one of the fire tests, R1319-4-6, at random-because it was first in the filing cabinet drawer-and reviewing the descriptive language reveals the following joint finishing language: “All joints … were sealed with joint tape and cement. Cement only was applied over the nail heads.” Note that “cement” in this instance refers to joint treatment material, not the cement used to make mortar or concrete.
It’s a simple and straightforward description. Cover the joints with joint tape and joint treatment and spot the nail heads and you have complied with the test. And it doesn’t have to be pretty-all the language requires is that you cover the joints with tape and compound-unless you intend to add additional coats of compound or a skim coat. The other two fire tests contain similar language that is intended to achieve the same level of finish.
But, say you don’t have the fire test handy and you need to prove that you have adequately finished the wall to comply with the fire test. In this instance, the language contained in General Explanatory Note #6 in GA-600 gets you to the same point as the original fire test: “Unless otherwise specified, the face layers of all systems…shall have joints taped (minimum Level 1 as specified in GA-214, Recommended Levels of Gypsum Board Finish) and fastener heads treated.”
THE LEVEL 1 MINIMUMThe keys to that phrase? The parts of the language that require “minimum Level 1 finish” and “fastener heads treated.” A Level 1 finish is described in GA-214 as: “(a)ll joints … shall have tape set in joint compound…. Tool marks and ridges are acceptable.” This level is differentiated from Level 2, where the tape covering the joints has to be “embedded in joint compound and wiped with a joint knife.” All Level 1 requires for the joints is that the tape and compound cover the joint; the compound does not necessarily have to completely cover the tape as is described in Level 2.
Where confusion occurs is that the ancillary descriptive language for Level 1 indicates that “fastener heads need not be covered with joint compound.” This occasionally gets misinterpreted to mean that the fasteners in a fire-rated system built using a test taken from GA-600, don’t have to be finished. Not true. The specific language in question refers to the general use of a Level 1 finish and does not address its use in a fire-rated system. The language in GA-600 supercedes that in GA-214 and clearly does require the fasteners to be finished with a coat of joint treatment.
Then why isn’t GA-600 written simply to refer to Level 2 in GA-214 since Level 2 requires fasteners to be finished? Quite simply, a Level 2 finish requires embedment of the tape in joint treatment, a step that is not necessary based on standard fire test practice. Fire tests are typically run to approximate the joint finishing requirements as described in Level 1; to require embedment of the tape would be overkill.
Interestingly, UL creates some confusion when you begin to pick apart the specific listing for UL U305. Here’s why.
Note 11 in its General Information for Fire Resistance Ratings in the UL Online Certifications Directory says:
“Unless otherwise specified in the specific design all gypsum board systems except those with predecorated or metal covered surfaces have joints taped and joints and fastener heads covered with one coat of joint compound (fire taped).”
Seems straightforward and identical to the philosophy in both GA-600 and the original fire test; however, there is a hitch and it occurs when you read the specific language in the listing for U 305:
“Joints and Nail-Heads-Exposed or covered with fiber tape and joint compound, except where required for specific edge configuration. For tapered, rounded-edge gypsum board, joints covered with joint compound or fiber tape and joint compound. Nail heads exposed or covered with joint compound.”
EXPOSED: HOW AND WHEN?Clearly, via the use of the word “exposed,” the language does allow for some instances where the joints and the nail heads do not have to be finished. But where and when?
The best way to answer that is to consult with the manufacturer of the gypsum panel products being used in the installed construction, since many manufacturers and most tested systems, notwithstanding the language in the U 305 listing, do require the joints to be finished. Given the variety of possible edge configurations and the wide assortment of products contained in the U 305 listing, it is prudent to rely on the basic fire test or the answer provided by the manufacturer for the correct information.
And this language is not unique to the U 305 listing. Other UL systems listed on-line do contain similar language, and assuming that the word “exposed” gets you off the hook for having to finish the joints and fasteners could cause you grief during an inspection.
So the listings and test reports are reasonably consistent. You do have to finish the joints, and you do have to spot the fastener heads in nearly every situation. When can you get away with not doing that? That’s Chapter 2 of the story, and we will address it in a future column when we review and discuss specific building code language. W&C