Expansion/Control Joints: The Most Controversial Metal Lath Accessory
ASTM C1063 states, "Control joints shall be formed by using a single prefabricated member or fabricated by installing casing beads back to back with a flexible barrier membrane behind the casing beads. The separation spacing shall be not less than 1/8 inch or as required by anticipated thermal exposure range (per author and to accommodate stucco shrinkage)." It goes on to state that it has to conform to the previous section that states the following: "Lath shall not be continuous through control joints but shall be stopped and tied at each side."
The specification continues: "Control (expansion and contraction) joints shall be installed in walls to delineate areas not more than 144 square feet and to delineate areas not more than 100 square feet for all horizontal applications, that is, ceilings, curves, or angle type structures. The distance between control joints shall not exceed 18 feet in either direction or a length-to-width ratio of 2 1⁄2 to 1. A control joint shall be installed where the ceiling framing or furring changes direction." Finally, it states, "A control joint shall be installed where an expansion joint occurs in the base exterior wall. Wall or partition height door frames shall be considered as control joints."
Some architects state that they don't like to use control joints because they destroy the look of the finished product and give it a chopped-up appearance. While this may be true, it is like a concrete contractor once said about a driveway: "I would have green cut grooves in your driveway but I didn't think you would want all those cracks." The response: "But I have cracks all over the place and if you had cut them, at least they would have been straight."
The same is true of stucco. Either plan crack locations using a control joint or expansion joint or take the cracks where they happen in the cementitious membrane. Structural cracks can be a huge source of water intrusion when they occur. It is far better to have a bunch of straight and symmetrical lines than to have random structural cracks.
Types of control joints and their general uses:One-piece control/expansion joints: "V", "M" or No. 15 types of this joint are generally considered to be control joints by the industry. This bead is designed to deal with normal initial stucco shrinkage during the hydrating and curing stage of the Portland cement stucco (generally exterior) or gypsum plaster (interior) coats and minor thermal expansion and contraction. These beads all have expanded flanges for extra bonding of the bead to the lath below. The other aspect of these beads that they all have in common is the sloping angular shape of the downward leg of the "M" shape. The disadvantage to this shape is that since there is no chemical bond of the stucco to this leg, if installed incorrectly a stucco separation crack can result at the juncture of the stucco to the leg of the bead.
"J", "XJ" or "Griplock J" types of control joints with expanded flanges and the narrow wing control joint with the "J" type shoulder or nose are all grouped together because of the commonality of their shoulder shape. They are also considered control joints and are one-piece joints. The advantage to this shape is that the down turning shoulder bites into the stucco and as a result the crack described in the last paragraph is less likely to occur or be exposed. The plasterer must force stucco under this upside down "J" shape in order for this bead to perform optimally. These products are also generally available with a continuous piece of tape masking over the groove making clean out of the joint less tedious and more likely to be performed as required.
Two-piece expansion joints formed with back-to-back double casing beads with a flexible membrane mounted behind the joint also moves in two planes, and when caulked utilizing a backer rod and a quality silicone caulking material, it is generally much more water resistant than the standard mechanical two piece.
Three-piece joints, though rarely used, do have a place in stucco construction and each should be approached based on their intended use and application. These would include such items as inside and outside corners, and soffit vent expansions.
These joints deal with minor movement and minor stress on the stucco membrane. The control joint was designed to deal with initial stucco shrinkage, minor thermal movement and some minor stresses imparted on the stucco membrane. How much movement are we talking about? Not more than 1/32 to 1/8 inch for shrinkage and approximately the same for all other movement. If it moves more than that due to settlement, seismic movement, wood shrinkage or any other reason, you have the wrong bead. A one-piece joint is limited in the amount of movement it can handle and it can only move in one plane.
The two- and three-piece joints are usually employed where there are larger amounts of movement as explained in the preceding paragraph. The typical two-piece can move in two planes, in and out, up and down, and it can be expected to deal with a greater degree of movement. It is not that much greater but can range from 1/4 inch to just under 1/2 inch. This is a cumulative measurement of closing 1/4 inch and opening 1/4 inch since it is usually installed in the already open position of 1/4 to 3/8 inch. Movement of the sections an additional 1/2 inch would most likely separate the two sections.
Types of materials available for control/expansion joints: Control/expansion joints are available in roll formed G60 galvanized steel and zinc alloy (99 percent pure zinc), extruded polyvinyl chloride vinyl and the two-piece joint is also available in extruded anodized aluminum.
Installation points and precautionsThe 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. Wire tying is not conducive to high-production installations but it is necessary and required by ASTM C1063, 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 rainwater 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 the two beads and in some cases, an 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 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 then the casing beads, which are usually installed before the metal lath, are 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.
The correct joint for the jobBased on sales data, at least 90 percent of all control/expansion joints that are pre-manufactured 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. 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.
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 6 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.
If one gathers 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. There are contractors who have stated that in 30 years they have never cut the lath behind the joint and have never had a problem. I have also seen 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 by ASTM, it must be followed.
Types of joints (galvanized, zinc alloy, vinyl, anodized aluminum)• One-piece joints (control joints)
• V type one-piece expanded flange
• J type one-piece expanded flange
• J type one-piece without expanded flange (AKA control joints)
• V and J type inside corner expanded flange
• Two-piece joints (expansion joints)
• Two-piece male/female #40 type expansion joint
• Double casing beads with stretchable membrane flashing
• Three piece specialty joints
• Three-piece exterior corners
• Three-piece interior corners
• Three-piece soffit vents
• Three-piece special modifications