Avoid Cracked Plaster by Using Stucco Control Joints
Since stucco plaster is so thin, it must be divided into sections during installation to reduce cracking due to volume changes over time. Stucco control joints are used to help aid in preventing cracking in the stucco overtime. Stucco joints can also separate different textures, colors, or materials. Depending on the type of movement expected, the joint must be able to move in one direction or two by using some combination of materials, or by prefabricating the joints at the job site.
One Piece Joints
Stucco joints are commonly available as one piece or two piece joints. A one-piece joint is typically classified as a control joint. These kinds of joints normally handle the contraction of the mortar mix and minor expansion and contraction. Shaped like a "V" or "M," it is usually fabricated with expanded flanges to increase bonding characteristic.
The "M" type provides movement control but also gives the appearance of a narrow reveal. The "V" type joint is typically used when the area is larger than 144 square feet. A one-piece joint is limited in the amount of movement it can handle, and it can only move in one plane. Vertical joints should be attached to a stud where necessary.
"J" control joints are manufactured with expanded flanges so that the crack is less likely to occur or be exposed. The design of "J" joints locks the stucco to the edge of the joints and reduces separation at the joint's edge. The plaster must force stucco under the upside down "J" shape for this bead to perform optimally. They are normally manufactured with tape over the groove for easier and faster cleaning.
Two Piece Joints
A two-piece expansion joint can slide in different planes. These are usually located through wall expansions, using a female expansion over a flexible membrane which prevents leakage, something quite normal in these beads. They can also be formed by using back-to-back casing beads mounted over a flexible membrane. The benefit of using this method is that the joint is filled with backer rod and silicone caulking, which works better than the flexible standard membrane. A two-piece joint manages some larger movements in the range from 1/4 inch to just under 1/2 inch.
There are also three piece joints, rarely used, that can be installed at outside corners and soffit vent expansions. Stucco control and expansion joints are available in roll-formed galvanized steel, zinc alloys, and polyvinyl chloride (PVC).
Stucco Control Joints: Joint Spacing
The use and spacing of stucco control joints depend on several factors: type of materials, the orientation of the building, and the type of surface. Stucco can be applied over concrete and masonry surfaces and will only require joints over concrete or where there is a change in material. If stucco is applied using metal lath, joint spacing should follow Portland Cement Plaster/Stucco Manual, EB049, and is based on ASTM C1063 – Standard Specification for the Installation of Lathing and Furring to Receive Interior and Exterior Portland-Cement Based Plaster requirements.
Generally, joint spacing should meet these criteria:
- Joint spacing should not be greater than 18 feet.
- No panel should exceed 144 sq. ft. on vertical applications.
- No panel should exceed 100 sq. ft. over curved or angular sections.
- No length-to-width ratio should exceed 2 ½ to 1 in any given panel.