Nail Pops & Efflorescence
Lee G. Jones / June 2015
Q: We applied drywall over wood framing in the late fall. After running the heat all winter, we’re seeing nail pops in several places. What’s going on? Is the drywall shrinking? How can we avoid this?
A: The Gypsum Association’s document, GA-222-14, Repairing Screw or Nail Pops, offers the following common causes for nail pops: Lumber shrinkage occurring after gypsum panel product application; loose attachment of the gypsum panel product to the framing; improperly aligned, twisted, bent, warped or bowed framing; incorrect fastener length, or improper fastening.
The bulletin explains that the combination of green lumber (which may contain up to 19 percent moisture) and nails longer than needed can easily lead to the emergence of nail pops. Simply put, lumber with a moisture content hovering around 19 percent is permissible and often used in new construction, but the wood’s moisture content will invariably drop down to something like 10 percent once the heat has run through the cold months. This causes the wood to shrink sufficiently to effectively create a gap between the back of the gypsum board in the face of the framing. If longer fasteners are used, that gap can be double what it would be with fasteners of the proper length.
And yes, the gypsum board can indeed shrink due to the loss of moisture and exposure to heat, but the change in thickness can only be measured by thousandths of an inch and will not result in nail pops.
GA-222 offers the following for fixing this problem: “Fastener pops are repaired by reattaching the gypsum panel product to the framing by driving a new screw or nail of the proper length into the same framing member about 1 1/2 in. (40 mm) from the popped fastener. Apply sufficient pressure to the gypsum panel to bring the back surface into firm contact with the framing member to which it is being reattached. Remove the fastener or reseat the popped fastener so that it is just below the face of the gypsum panel. Remove any loose material before treating with joint com-pound and redecorating.”
P.S.: One of the beneficial properties of cold-formed steel framing is that it doesn’t shrink.
Q: Lately, we’ve seen a lot of efflorescence on portland cement stucco. Why is this likely to happen, and how can it be avoided?
A: This was a topic of discussion at the April meeting of AWCI’s Portland Cement Plaster/Gypsum Plaster/Metal Lath Committee. Several of the seasoned plasterers in the room quickly opined that properly consolidating the brown coat is essential to avoiding efflorescence.
Efflorescence is a commonly occurring reaction between the calcium hydroxide that is integral in portland cement and carbon dioxide in the atmosphere (although there are plenty of other salts in a stucco mix that can produce the same result). When the moisture in the stucco mix delivers the calcium hydroxide to the surface, it can combine with carbon dioxide and sit there as a salty deposit, and seems most prone to do so in cool weather conditions when the moisture can remain on the surface for extended periods. There are four elements that must be present for this to occur: the salts in the cement, moisture, some physical force to draw the salt-laden moisture to the surface, and a passage for the salt-laden moisture to pass through to the surface. The first three are all but impossible to avoid, but the last element can be ameliorated if not eliminated by properly consolidating the brown coat, which is the final step in applying the brown coat and is done by hand with a hard float.
The PCA Portland Cement (Stucco) Manual stresses the importance of this step: “Floating of the brown coat is the most important part of plastering. Floating must be done only after the plaster has lost sufficient moisture so that the surface sheen has disappeared and before the plaster has become so rigid that it cannot be moved under the float. This interval is critical, since the degree of consolidation that occurs during floating influences the shrinkage-cracking characteristics of the plaster.”
Several of the same seasoned plasterers and a few others who have weighed in on the topic insist that the floating of the brown coat must be done with a hard float (wood or hard plastic) to ensure proper consolidation; the foam rubber floats do not properly consolidate the material. So, if the brown coat is properly consolidated using a hard float, the passages through which salt-laden moisture can pass to the surface are reduced, which reduces the potential for efflorescence deposits on the stucco surface.
Lee G. Jones is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.