Metal Lath Accessories Needed or Not: Casing Beads

The Metal Lath Newsletter
Volume 03, Issue 01, September, 2003

Bead Usage: Casing Beads or Plaster Stops  

A common misconception in the lath and plaster industry is that if a window, particularly a plastic or aluminum window has a return lip on the outside edge, the edge of the window can serve as the casing bead for stucco applications. Nothing can be farther from the truth. In my most recent trips to jobs in Florida and Minnesota, I viewed jobs that exhibited this problem. On some of the Florida jobs the contractors tried this stucco application on plastic (PVC) windows (note: there was an absence of casing beads, no window flashing, and the poor application of building paper on the job in question). Some of these contractors tried to remedy water intrusion problems by applying multiple layers of caulk on several occasions to try to stem the tide of water intrusion. Some of the structures that the author visited were less than a year old and well on their way to water damage and mildew problems.

Why did this catastrophe occur and how could it have been avoided? The stucco was applied directly into the casing shaped edge of the vinyl window instead of a casing bead as required by the International Building Code, and ASTM C1063, the metal lath installation specification. Anyone familiar with the action of stucco during the curing or hydration process knows that the volume of water in the stucco mix hydrates from the total stucco application resulting in a certain amount of shrinkage. This shrinkage should be expected and dealt with by use of the proper accessories. Combine this shrinkage with the fact that portland cement stucco and metal lath have a very different coefficient of expansion than that of vinyl or aluminum windows and one has to know that a crack will occur at this juncture. These cracks will change size with the expansion and contraction of the window as it heats and cools. Caulking does little good because; although it will adhere well to the window it does not adhere very well to the stucco edge.

To perform this application properly the window should have been flashed prior to the installation of the window unit. Most window manufacturers recommend flashing before and after window installation and warn that the installation flange or fin should not be considered to be part of the flashing.

The window opening is then covered with a peel and stick type flashing starting at the sill and working to the top. If the wall is covered first with water resistant paper or one of the geotextile products they can be used to flash or wrap the window opening. Some experts recommend the use of a fabricated steel or vinyl sill pan to be preinstalled at the bottom of the window opening prior to the insertion of the window unit. The window is then installed.

Following this installation a flashing that sticks only on the top edge is applied to the bottom edge of the window and a drip edge, usually "Z" shaped is placed over the window. Next the side flashing is installed followed by the top flashing. A peel and stick flashing is recommended for the sides and top. The reader should go to for additional detailed information on flashing applications and products available.

Once the window is flashed properly the casing bead can be installed next to the window. It is recommended that the window have an "L" shaped flange rather than one that has a return leg on top that would form a "C" shape.

In this way the casing can be installed approximately 3/8 inch from the upright leg of the window providing the perfect sized channel for the application of a backer rod and caulk. Why is backer rod important? The answer is simple; the round shape of the backer rod provides the perfect surface to receive caulk. As the caulking installers fill the channel and use their finger to smooth the caulk, they impart a slightly concave top surface. It is the feeling of some experts in the caulking industry that an hour glass shaped caulking application is considered ideal because it provides maximum surface for the two bonding edges and a thinner section in the middle of the span to facilitate elongation of the caulk. It is important to choose a caulking material that will adhere well and maintain its flexibility and durability for the longest possible period. The author usually recommends a silicone caulk but the reader should research this area further before making that determination.

Once the casing bead is in place the lather can then lap the metal lath over the expand flange of the casing bead to assure that the two products are joined together to form a solid bond when the stucco is applied.

On these jobs the contractor actually used a casing bead but only at the bottom of the framed wall where he was required to use a foundation weep screed.

Had this contractor followed the above procedure during he initial construction he wouldn't now be faced with devastating leakage problems at every opening in the stucco on this structure.

"The author offers no warrantees, implied, stated, or expressed regarding the information found in this article including techniques, construction methods, drawings or materials identified in this article. To the best of his knowledge the information within is correct and up to date as of its publication date. The author is not responsible for typographical errors. This article is protected by all copyright laws and shall not be photocopied, stored in any electronic format or distributed without express written consent of the author. All rights reserved by Metal Lath Consulting Co, LLC."




A-1 Note repairs around and below all openings caused by failure to use casing beads.

A-2 Note that contractor used casing bead for repair but failed to leave 3/8" gap to allow room for backer rod and caulk.

A-3 Rot and mold resulting from water leaks from leaking windows located above.

A-4 Contractor filled original window edge with caulk to try and stem the flow of water however this caulking didn't last.

A-5 Contractor flashed the outside of the window flange on the second go round. It will help to some degree.

Coming in the next issue: Accessories: Control Joints

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