All Things Gypsum: Joints and Gaps in Gypsum Systems
Gypsum board systems can either be non-rated or rated for fire, sound, or shear resistance. The joint tolerances in gypsum board systems rated for fire, sound, or shear resistance need to be tighter than the joint tolerances in non-rated systems. In total, higher levels of expected performance require higher levels of precision in construction.
However, whether you are installing a fire-rated, sound-attenuating, or nonrated system, joints between boards or panels must comply with the basic parameters established in either GA-216, Application and Finishing of Gypsum Panel Products, or ASTM C 840, Standard Specification for Application and Finishing of Gypsum Board. These documents define the fundamental criteria for application of interior gypsum board and gypsum panel products and in so doing establish the following basic rules for joints between panels:
Panels are to be abutted so that they contact each other, but they are not to be forced together.
Joints on the opposite sides of a partition must not occur on the same stud.
In a single-layer system, panels must be installed so that end joints don’t line up. End joints, i.e. where the non-tapered ends of panels abut, should be staggered between alternate courses and end joints should not line up with joints on the opposite side of the wall. You don’t want a checker-board pattern; you want a staggered pattern.
In a multi-layer system, panels must be installed so that end joints on the same side of the partition are staggered between alternate layers. Also, base layer joints should not line up on opposite sides of a stud.
To sum up: Don’t line up end joints on one side of a wall; don’t allow joints to occur on both faces of the same stud; and don’t force panels together so that you stress the panel edges. If you move beyond the basic rules and work with a fire or sound test, make sure that you stagger the joints in accordance with the requirements of the test. If you are building a shear wall or diaphragm ceiling, back-block the joints (i.e. install blocking behind the joints in the panels) as described in the tested system.
WHAT TO DO?
Unfortunately, everyone has a project where an inspector gets overaggressive about a joint application or an application situation where a joint gets a bit out of kilter. What then? What do you do to either: 1) convince someone that the panels have been installed correctly and that the joints are not a problem (and that you do know what you are doing when it comes to panel application), or 2) fix a minor problem? Let’s briefly look at both situations, beginning with the second issue first.
If you have an unrated application and you have gaps between the joints the simplest solution is to fix them using one of the following methods described in both GA-216 and ASTM C 840:
If the gaps are very narrow-less than 1/8 inch in width-pre-fill them with all-purpose or taping compound and then finish the panels using normal application techniques.
If the gaps are somewhat larger-between 1/8 inch and 1/4 inch-pre-fill them with setting-type compound and then finish the panels using normal application techniques.
In both instances, the type of compound used to pre-fill gaps in the joints must be compatible with the compounds being used to tape and finish the wall or ceiling.
Gaps larger than 1/4 inch are unacceptable and need to be completely repaired. Gaps that wide clearly violate the mandate requiring panel edges to be butted together and are indicative of a poor quality installation.
Fixing gaps in a rated application is a different issue, since in a rated application, panel edges must be installed so that they are in “moderate contact” with one another. Moderate contact is a concept that has long been incorporated into building codes (most likely the first reference was written into the Uniform Building Code); however, it has never really been well defined by the codes.
Simply defined, it means that the panel edges are essentially touching each other. Given the permissible manufacturing tolerances for gypsum panel products, it is possible for the panel edges to be in moderate contact yet not touch along the entire length of the joint. Therefore, in systems rated for fire, sound, or shear, small gaps spaced sporadically along the joint are acceptable as long as the panel edges are essentially touching each other along the majority of their common length. Larger gaps, such as those described previously for unrated applications, are not acceptable with a rated installation; higher expectations require greater precision in application.
Only one area of tolerance for a gap in a rated partition easily comes to mind and that is where Underwriters Laboratories does permit a gap around metallic outlet or switch boxes of up to 1/8 inch in a 2 hour or less fire-resistive rated system. What they do not allow, however, is for that concept to be expanded beyond its stated intent. Nothing in the UL Fire Resistance Directory allows the “outlet gap” concept, for example, to be interpreted as permitting 1/8-inch gaps between individual panels, nor does it imply that the total calculated area of the permissible gap around the outlet can be used to calculate the allowable area of a small hole in a rated partition.
Using the concept of “moderate contact” allows some level of precision to be established for the higher performance needs of systems rated for fire, sound, or shear; however, it probably is a bit tight for a non-rated system. Although good construction practices dictate that moderate contact should continue to be the goal in both rated and non-rated systems, it has long been recognized that tolerances are needed for gypsum board joints in non-rated systems in today’s construction environment. Stud dimensions have tolerances, wall heights have tolerances, and even the gears inside a screw gun are manufactured to be within some “plus or minus” tolerance. Therefore, it stands to reason that gypsum board joints in non-rated systems also should have some specified maximum tolerance.
What to do about that grumpy inspector or owner’s rep? Well, essentially, use the information provided herein to prove that the application you have provided is acceptable. If you have gaps in non-rated construction and they are within the tolerances described above, fix them as recommended. GA-216 is referenced by the International Building Code, and the repair language is in Section 4 of the document, so, in theory, an inspector working to the IBC should not have a disagreement with the remedy. If you are installing rated systems, note that it is permissible to have very small, intermittent gaps between panels as described previously. Always finish all face layer joints in a rated system to the minimum level required by the fire or sound test, and be especially diligent in making sure that all systems are sealed tight. W&C