#Metal lath and plaster walls how to

A more “wet” scratch-coat can slump off the wire before it can set. Because of the stiffness created in the intersecting welds, the wires stay uniform during the scratch-coat process that provides embedment.Īpplication issues that do arise are seen when wet plaster mix is hand trowel applied during the scratch-coat. The welding improves stiffness of the lath which enhances the wire’s rigidity during installation and plastering. Welded Wire Lath: Welded wire lath features an open-wire design like woven wire, but the manufacturing process incorporates fuse-welding at every crosswire intersection. The red area is self-furred, but this is often inconsistent. These inconsistencies can result in areas where embedment may not occur. Additionally, over- or under-fastening woven wire lath can restrict its ability to respond appropriately to the pressure of the trowel when applying the scratch-coat. The self-furring inconsistency reduces the woven wire’s ability to remain in the center of the scratch-coat. The contractor must then create proper furring depth using a special furring nail or furring wad. Some producers of this type lack the self-furring feature in the wire and don't provide the 1/4 inch self-furring needed on the wall. While woven wire has an open design and provides for easier keying and embedment of the scratch-coat, this style faces different challenges like maintaining uniformity in furring distance on the wall. Woven wire is made of 17 to 20 gauge wire that is wound into a hexagon with each adjoining shape woven together. Woven Wire Lath: For woven wire lath, ASTM C1032: Standard Specification for Woven Wire Plaster Base defines the gauge of wire, the required galvanization levels, and the opening size.

#Metal lath and plaster walls how to

While it is possible to achieve acceptable embedment with diamond mesh lath, the applicator must know precisely how to mix plaster to achieve a stiffness ideal for penetrating the diamond surface. However, this stiff mix increases the difficulty of pushing the plaster through the diamonds, thereby reducing the level of embedment. Plastering contractors often run their plaster “stiff” (i.e., less water in the stucco mixture or use of fibers) to get better surface contact with the diamonds and to assure adhesion. Paper-backed lath-where a WRB is glued directly to the diamond mesh-increases the challenge of properly embedding the lath by amplifying these difficulties because they can impede the plaster from spreading through the diamonds to get to the backside of the metal lath for embedment.īecause of this variability in diamond opening size, pressing wet plaster through the lath can be challenging. Expanded metal lath that is over-fastened to the substrate will restrict the passing of plaster through the diamond-shaped openings in the lath, negatively impacting embedment, especially at the edge joints and laps. These openings range in size but are approximately 3/8 inch by 1/2 inch in length and width.ĭiamond mesh lath manufacturers engineer their sheet products to achieve these specified weights. Lath Types and Their Embedment Performanceĭiamond Mesh Lath: ASTM C847: Standard Specification of Metal Lath defines sheet lath by weight, expressed in lbs./square yard, and the size of the diamond openings is not standardized.

This type of corrosion, resulting from lack of embedment, can stay on the wall for many years and then finally, “give away” from a storm or wind event. The result is lath corrosion and potential stucco failure. If not dried out, oxidation begins and the 3 mils of zinc coating protecting the expanded metal lath begins to sacrifice, eventually eroding the zinc to expose the carbon steel. As wind-driven rain and vapor drives force moisture through the plaster to the weather barrier, vapor condenses adjacent to the bare lath. If the plaster scratch-coat sits on top of the diamond lath, the back side of lath is minimally covered and exposed. The better the lath embedment, the less likely it will oxidize and eventually rust under the stucco cladding. If the moisture persists and plaster cannot dry out, its presence could result in a premature failure of the lath. In some cases, incidental moisture that may enter the plaster matrix can cause rusting and corrosion of the metal lath material if plaster is not allowed to dry out. Stress-cracking often seen in corners can be caused by poor embedment, or omission of control joints placed for stucco membrane movement.Ĭorrosion is another risk resulting from improper stucco embedment.