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Meshing with Metal Lath

I have a question in regard to the use of Diamond Mesh Metal Lath. Why is metal lath being used on exterior vertical wall over building paper? Each fastener used to secure metal lath to the framing members (wood or metal studs) cuts through the building paper creating potential water 6 inches on center.

A. In answering this question I consulted two sources. First was The Metal Lath Handbook by Gary Maylon and the second was a conversation with Tom Donnelly of Donnelly Stucco in Minnesota. Several things have to be in place for this application to be successful. First is the application of building paper. Maylon says that two layers of building paper are required. In fact, several building codes require two layers. The flashings around penetrations in the exterior wall are very important and may not be part of the contract of the stucco applicator. The building paper, flashings and the foundation weep screed all work together to form a water resistant barrier behind the stucco.

In Maylon’s book he advocates using self-furring metal lath. Self-furring lath provides a gap between the lath and the building paper that forms a cavity to allow moisture that finds it way into the system to escape into the atmosphere and evaporate.

ASTM C1063 states, with the lath installed with the long dimension horizontal that the fasteners be placed at 6 inches on center directly into the framing. The fasteners used must engage a specified area of the lath, meanings the use of fasteners with oversized heads. When these fasteners are seated tightly against the metal lath, they also compress the building paper between the lath and the substrate. This compression of the two layers building paper along with the oversized heads of the fasteners forms a seal and prevents entrance of moisture behind the paper. As an old waterproofing contractor once told me, you’ll never stop the flow of water because it will always follow the course of least resistance. In the case in point, with a water resistance barrier, flashing, building paper and foundation weep screed properly installed, moisture that happens to reach the back side of the lath will escape down the cavity.

I am sure you have a concern about moisture intrusion into the system and, considering some of things that happened in the past years, it is a valid concern. As I tell many people who call about water intrusion, the exterior system used in most cases is irrelevant. Water damage is water damage, no matter what. However, as Maylon states in his book: “Portland cement stucco systems have the advantage of being moisture vapor permeable, which allows moisture to escape the system. This remains true if none of the system components compromise or impede the movement of moisture vapor through the cavity wall.”

He also writes, “This moisture comes primarily from two sources. If a structure is occupied by people, moisture is created in the form of moisture vapor in larger volumes than might be imagined. Secondly, if proper construction techniques are not followed to avoid moisture intrusion from the exterior, moisture can accumulate within the wall cavity. Moisture vapor can and does pass through a breathable stucco system to escape into the atmosphere. With water resistant paper, drainage enhancing accessories and proper flashing water should never gain access to the wall cavity.”

In summary, the attachment of metal lath while penetrating the building paper is only one of several elements when improperly installed can cause problems downstream. In reality, in a properly constructed assembly only water vapor, not what we call water, as in running water, occurs in the cavity.

About the Author

Donald E. Smith, CCS, is AWCI’s director of technical services.

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