Robert Grupe / October 2016
Q: I have had inspectors challenge me on installed glass mat gypsum sheathing. Can you provide me with tips on allowable gaps and repair procedures?
A: Incidental damage to gypsum sheathing, in general, cannot be ignored. This is true for the more traditional paper-faced panels as well as sheathing that have what are called “facers” made up of a glass mat. This is especially significant for compliance with stringent energy codes that rely on the continuity of air- and water-resistive barriers. Also, glass mat sheathing, again similar to its paper-faced counterpart, is a stress skin panel. Damage to the glass mat can, at some point, impact the ability of the panel to transfer wind load back to the framing. We know from interior fire resistive gypsum panels that damage to the panel can have a negative effect on fire performance. The same is true for any type of gypsum sheathing that is a component in a fire-resistive assembly.
The proper repair depends on the type and extent of the damage, the project performance requirements and what type of cladding will be applied over the sheathing. A formal response to any remediation issue should come from the manufacturer of the panel in question. Let’s discuss several types of damage and offer basic guidelines on repair. The first field condition under consideration is not damage to the panel, but the limits on gaps in panel joints between adjacent glass mat gypsum products. More important is how to maintain the integrity of the overall sheathing membrane, which is required for a continuous air- and water-resistive barrier. The second item to be covered will be the incidental “dings” and gouges that may occur. Since the presence of the glass mat material provides some of the structural capacity of the total panel, what should be done if the facer is damaged or has delaminated? The final condition will be how to handle a situation in which the gypsum core has been significantly damaged.
One manufacturer, USG Corporation, follows the guidelines established in ASTM Standards on allowable gaps. The ASTM Standard for sheathing, ASTM C1280, is silent on allowable gaps. However, gaps are discussed in ASTM C840, Standard Specification for Application and Finishing of Gypsum Board. The Gypsum Association has a similar document, “GA 216, Application and Finishing of Gypsum Panel Products.” From that document the following is offered, along with methods on how to fill the ensuing gap:
“4.6.10 Where gaps occur at gypsum panel joints, they shall not be greater than 1/4" (6mm) and shall be prefilled with joint compound as specified in 188.8.131.52 and 184.108.40.206.
“220.127.116.11 Gaps not greater than 1/8" (3 mm) shall be prefilled with either a drying-type or setting-type joint compound.
“18.104.22.168 Gaps greater than 1/8" (3 mm) shall be prefilled with setting-type joint compound.”
That tells us that gaps over a quarter of an inch are not acceptable. Neither drying-type nor setting-type compounds are suitable for exterior applications, so alternate materials must be used. GA has suggested that the gap be filled with an appropriate sealant. Henry Company states that if a self-adhering air-/water-resistive membrane is used, then no further joint reinforcement is required. They further state that for liquid applied air barrier systems, when a gap is less than 1/4 inch, simply fill the gap with the same liquid air barrier and cover the joint with glass tape. Their application guide specification stipulates “Alternatively, joints not exceeding 1/8 inch can be sealed with yellow open weave glass fabric centered over joint followed by a 1/8 inch thick trowel application of air/vapor barrier membrane. Allow to dry prior to application of primary air/vapor barrier membrane.”
In addition to fluid-applied membrane air barrier sheathing preparation, for gaps less than a quarter of an inch Henry Company specifies “fill joint between sheathing with approved joint treatment sealant ensuring contact with all edges of sheathing board. Strike flush any excess sealant over joint layer to form a continuous layer over the joint.” This is in keeping with the Gypsum Association recommendations. It is important to note that any sealant that is used must be compatible with the other adjacent building materials.
Surface blemishes, those dings and gouges mentioned earlier, don’t need to be repaired. The one exception is where a gouge runs parallel to the framing for several inches. In that case, contact the panel’s manufacturer for written recommendations.
Facer delamination may require remediation. If the sheathing is part of a bonded finish system, repairs should be considered. The reason for this is bonded finish systems will not adhere to the exposed gypsum core.
The final topic concerns damage to the gypsum core, or significant delamination of the facer. That area may need to be replaced in its entirety, meaning the entire portion of the glass mat sheathing should be removed. A simple guiding point to follow in this case is, “if in doubt, tear it out.”
For an 8-by-8-inch replacement panel, follow the guidelines set forth in “GA-225-2015, Repair of Fire-Rated Gypsum Panel Product Systems.”
As mentioned before, contacting the specific manufacturer of the panel in question and obtaining their written recommendations is highly recommended. Also, if possible, get the approval of the inspection agency before you start remediation.
Robert Grupe is AWCI’s director of technical services.