Association of the Wall and Ceiling Industry Logo

Does This Finish Make My Butt Joint Look Big?

Q: Gypsum board has tapered edges running the length of the panel. The ends of the panel are not tapered, and occasionally it’s inevitable that butt joints will occur where two panels meet end-to-end. While taping and finishing the tapered joints leaves a fairly uniform surface, taping and finishing butt joints will produce raised areas that are very difficult—if not impossible—to hide. How does one deal with the shadows created by the butt joints on gypsum board ceilings subjected to direct natural light? Can the whole ceiling be skim coated or the joints feathered out? How about some kind of finish that hides the shadows?

A: Once the job is done, it can indeed be very challenging to obscure a butt joint created by the ends of two gypsum board panels. Feathering the joints out and using an ultra-flat paint may help, but shadows cast by direct natural light may still persist, and the only fix may be breaking up the natural light using a combination of diffusion materials and objects that mute the shadows created by the butt joints or cast additional shadows that obscure them. Using darker colors, especially ultra flat black, may hide the shadows, but even flat paints often have an angular sheen when subjected to severe lighting conditions, so the shadows may yet persist. And convincing a decorator that flat black looks as good as shell white may be more difficult than just re-doing the ceiling.

Applying enough joint compound to level out the surface and obscure joint shadows may be theoretically possible, but it violates the intent of a skim coat as described in ASTM C840, Standard Specification for Application and Finishing of Gypsum Board and GA-214, Recommended Levels of Gypsum Board Finish, where the following passages weigh in on the topic: “A ‘skim coat’ is essentially a ‘film’ of joint compound and is not a readily measurable thickness.” “A ‘skim coat’ will not approximate a plastered surface.” “Once the skim coat dries, the gypsum board paper may show through and the treated joints, filled voids, and spotted fasteners will likely be visible.” In other words, a skim coat is intended to even out the profile and porosity of the surface, not level out surface irregularities with the magnitude of a taped butt joint.

One way to avoid bulging butt joints is to recess the joint into the wall cavity while installing the board to create a hollow area that can be taped, filled and built up to level with the board’s surface. There are specialty products available specifically for this purpose. The Gypsum Construction Handbook describes a procedure that calls for using joint compound to adhesively attach the back sides of the butt ends to a piece of wallboard placed behind the joint. A wood strip and another piece of board are temporarily placed over the joint to force the joint edges into the wall cavity. Once the compound is dry, the extra pieces are removed to reveal a butt joint that is recessed and ready for finishing. One precaution: This approach cannot be used in fire-rated designs.

Q: When performing the bond strength test for spray applied fireproofing as described in Appendix B of Technical Manual 12-A, is the epoxy applied to just the face of the wooden disk, or to the face and the sides? And how does this affect the resulting bond strength calculation?

A: The alternate method mentioned above is for testing SFRM bond strength exceeding 1,146 pounds per square foot. This method is different from the standard cohesion/adhesion test described in the main body of the manual used for standard SFRM, which is the same procedure described in ASTM E736, Standard Test Method for Cohesion/Adhesion of Sprayed Fire-Resistive Materials Applied to Structural Members. The alternate method requires that a dried/cured sample of the material have a 1 3/8-inch diameter hole be drilled at least 1/4-inch deep into it. A wooden disk, also 1 3/8 inches in diameter and 1 inch thick is coated on its face and placed into the hole until the epoxy sets up. The wooden disk is fitted in the middle with an eye bolt. When ready, the sample is placed in a machine that tests its tensile strength by pulling away at the eye bolt and measures the amount of force necessary to pull the disk away from the sample, presumably with some of the SFRM embedded in the epoxy.

It took me several conversations and emails with the manufacturers’ technical people before I finally got a full explanation that addressed the concern that if the epoxy contacts more than the face of the disk, which it most certainly will no matter how carefully applied, how will that not alter the calculation of the bond strength because the calculation includes the area of the wooden disk. According one manufacturer’s technical rep, only the area of the face of the disk where it contacts the SFRM is calculated because the area on the edge of the disk that may or may not have epoxy spilling over is unknown. Also unknown is the contact with the edge of the disk and the side of the hole, so accurately calculating the area of contact on the edge of the disk with the side of the hole is impossible. Consequently, only the area of the face of the disk is used in the calculation.

Lee G. Jones is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

Browse Similar Articles

You May Also Like

As we start a new year with the 2024 International Building Code now available, we also start a new code development cycle. Every three years, the International Code Council starts

I often get calls about specific fire rating details: Do I have to install this backer piece behind my control joint? Do butt joints have to be staggered? Is it