Expansion/Control Joints -- the Most Controversial Metal Lath Accessory, Part 2.

Control/Expansion Joint Installation and Problems Arising from Improper Installation

The first rules of control joint (one-piece) installation after proper placement is to cut the lath behind the joint and wire tie it in place. I realize that wire tying is not conducive to high production installations but it is necessary and required by ASTM C10631 ASTM and every code that references C1063. It is important to cut the lath and not the paper backing when this procedure is performed. The easiest way to accomplish this is to pop a chalk line in the proper locations and use electric shears to cut the lath.

When a joint is nailed, screwed or stapled in place it is impossible for it to function as it is designed. Some would argue that the balance of the lath area is nailed in place but my argument to that is that we are only talking about 1/32 to 1/16 inch of movement during initial shrinkage of the stucco. Either the joint will open or close to accommodate the movement or the stucco will separate from the control joint leaving a very characteristic hairline crack along the edge of the bead. This is especially true of the “M” or “V” types of one-piece joints. During pressure differentials in the building between interior and exterior pressures; large volumes of rain water can enter through these cracks if other water shield precautions aren’t put into place. This could eventually lead to catastrophic problems.

Most manufacturers recommend that all junctions of all beads be embedded in caulk. That is to say that a dollop of caulk should be placed under the junction of two beads and in some cases and additional amount of caulk should be placed over the junction of these beads for added protection. This is especially true of expansion joints since they can be expected to move to some degree after installation.

Where a junction occurs between horizontal and vertical expansion/control joints, the vertical joint must be continuous and the horizontal bead must be broken. If the horizontal joint is continuous a dam will be formed at the junction providing the perfect place for water intrusion to occur. The horizontal abutment should stand off from the horizontal at least 1/4 inch to allow the vertical section to open. Once again caulking is essential.

Where the expansion/control joint meets a corner bead the end of the joint should be beveled to match the slope of the corner bead. Where the joint meets a casing bead the end of the joint should fit neatly into the ground of the casing beads. In order to accommodate the thickness of the lath the control joint is often 1/8 inch smaller in ground size than the stucco thickness and the grounds on the casing beads. For example if the stucco thickness is 7/8 inch and the casing beads, which are usually installed before the metal lath is also 7/8 inch in thickness. However, since the control joint is installed over the lath (lath is cut and joint is wire tied to lath) a 1/8 inch thickness reduction in the control joint can be made to allow for the added thickness of the lath.

Stucco thickness is measured from the back plane of the lath. Also, C926 states that all control joints shall be clean and free of finish or stucco material. Stucco finish material is composed of portland cement stucco or in some cases acrylic finish material. These materials dry to a hard or firm consistency and will block the natural movement of the joint. It will primarily impede its ability to close.

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Choosing the Correct Joint for the Job:

I would say that based on sales data at least 90% of all control/expansion joints that are premanufactured for this purpose are one-piece joints. Since casing beads used to form a two piece joint presents an unknown entity it is possible that the one-piece percentage is slightly lower. However, it is safe to say that the vast majority of control joints used today are one-piece joints.

I will also state that in my experience many residential projects use few if any expansion joints or if they do it is usually only at floor lines on two story homes or in extremely large areas. I should say at this point that I have also witnessed some excellent residential installations that are built to meet current codes in every way. However I do see many residential stucco projects that have no joints or if the have any expansion joints, they are often installed incorrectly.

On the other hand most commercial jobs are specified with the appropriate number of control joints. However, from my experience not all are installed properly. Many are nailed in place or the lath is not cut behind them and they are nailed or screwed into place. One quick way of checking for this is to look for wire ties every six inches on center along each flange of the joint in lieu of screws, nails or staples.

Two-piece expansion joints are used where the installer or designer expects to have expansion and contraction levels that will exceed the norm. This would include floor lines in multistory buildings, areas with through wall expansion such as expansion joints in concrete block walls, or areas were the framing changes directions or types. For example a wall that is framed with studs that meets a CMU block wall. There will be a joint created at this juncture of different materials that would be expected to move at different rates. Other than these isolated occasions the project will likely require the use of one piece control joints.

The Controversy Will Continue:

If you gather an equal number of contractors, consultants, and architects in a room it is unlikely that you will get a consensus of agreement from the group on the proper installation techniques for these joints. I have spoken to contractors that have stated that in thirty years they have never cut the lath behind the joint and have never had a problem. I have also see lath that was literally torn where it should have been cut. It was torn in my opinion because the joint tried to open and was restrained by the lath.

No matter how many opinions there are on this subject the fact remains that ASTM C1063 as cited in the new International Building Code and most model building codes; requires the installation of control/expansion joints and states that the lath must be cut behind the joint and wire tied on either side of the joint. That is the law of the land until the law is changed for some logical reason that has yet to be presented to ASTM, and it must be followed.

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