The practice of adding daubs of sealant over the fastener used to attach lath would seem like a good idea. After all, it does provide some extra insurance against leaks, right? It certainly would not cause any harm—or could it? 

Most actions are related to cause and effect. History is full of examples: schools used to spend time teaching valuable life lessons. We used to learn about unintended consequences; this was the cause and effect for actions, and why they need to be well thought out. Placing daubs of sealant over the fasteners used to attach a lath for plaster is a good example.

This practice has become popular with many building envelope consultants. Some believe it is critical for a successful cement plaster cladding but it’s not. Placing sealant over fastener heads is not required by code, ASTM or other recognized standards. Also, testing has verified that fasteners properly selected and used to attach lath into the framing members are proven to seal against moisture intrusion. 

Shine a Light

There is some rightful concern over fasteners that missed the framing member. Some call these “shiners.” Independent testing has shown that even shiners do not leak during proper water testing of completed walls. This flies in the face of some that claim to have witnessed water dripping off the end of the protruding fastener that missed the framing member. However, there is more to this story that should be discussed.

The only way to actually witness water dripping from the fastener tip that missed the framing members is to remove the interior finish (drywall), as well as the insulation. The second required item is to add negative pressure during the water test. Negative pressure essentially creates a vacuum to draw or suck water in through even the smallest of openings. If the drywall (taped and sealed) is in place, the negative pressure becomes a moot point. Testing for water leaks only after removing the drywall is like testing the brakes on your vehicle only after you remove the brake pads. Failure is virtually guaranteed. 

The next comment is that AAMA protocols allow for the removal of the drywall. This test is for windows and penetration flashing, not the field of the wall. The other argument is for ASTM field testing per E2128. This standard also allows the testing agency to incrementally increase negative design pressures far beyond being code compliant. Why? Because this ASTM is to find and trace down a “known” leak on a problematic wall assembly. It was not intended to create leaks. 

Why Not Go the Extra Mile? 

Could there be consequences by simply adding these daubs? Consider that the membrane of cement plaster functions best when at a nominal uniform thickness. Placing daubs of sealant along fastener heads will make the cement plaster thinner along the line of fasteners. Plus, they typically align with the framing. We know stresses placed on cement plaster can come from the framing.  Would making the cement membrane thinner at these locations induce more cracking? This can be equated to a chocolate bar with score lines; apply pressure and where does it tend to crack? 

But are all daubs bad? No, they should be reserved for visually obvious openings in the vertically oriented water-resistant barrier. The product should be a one-component moisture-curing elastomeric polymer sealant compliant with ASTM C920 or AAMA 714-19 and compatible with the selected WRB. Should the owner, designer, consultant, contractor or inspector mandate the installer apply daubs of sealant arbitrarily over fasteners as routine practice, they should be notified. This written notification should include the increase of hairline cracking of cement along the framing members due to reduced plaster thickness. An SMA contractor can use the SMA Tech Times bulletin to strengthen their position, then follow written directions for installation. The directing party will inherit some responsibility should the stucco wall exhibit excessive cracking. 

This following story hit home with a recent job issue regarding water leaks through a stucco parapet with no metal cap. In reviewing construction pictures, a contractor went to extraordinary steps on the walls, including flashing tape over the paper aligned with studs and sealing every fastener. Along the parapet top, they also used sealant, attempting to seal the fastener penetrations used to attach the lath and cornerbeads. It did not work. The horizontal up-facing surface cracked and then let water pool and enter the wall. What would have been better? Focus more on the parapet, not with sealant but wrap the top of the parapet with lath and only attach on the walls and wire tie the corners, with no penetrations on the top. Success is certainly more likely.