Retrofitting movement control joints as a last resort

By - , Build 84

Movement control joints are necessary to ensure that the plaster finish on stucco claddings doesn’t crack when it dries. They need to be installed before plastering, but there is a remedy if you find unexpected cracks.

Figure 1: Make a clean cut in the plaster and then fill with sealant.

This is when your movement control joint should be installed — before the plaster reinforcing is installed. Mesh is fixed each side of control joint former.

Movement control joints are an essential part of stucco claddings. They ensure that the plaster finish can accommodate the inevitable shrinkage as the cement-based plaster drys, and also any building movement caused by live loads and changes in moisture level and temperature.

Control joints need to be incorporated into the design of the cladding system so they can be installed before plastering. They can then accommodate any shrinkage movement that occurs immediately after the plaster has been applied.

Shrinkage cracking in stucco can usually be seen within 7 days of applying the plaster. This is the period when the stucco hydrates or cures and the bond strength of the plaster develops. But sometimes shrinkage can occur over an extended period, such as when the plaster has been applied in winter or spring, before the heat of the next summer.

There are instances where movement control joints have been omitted and little or no cracking has occurred, more often in old houses. These cases would have required the stucco plaster to be cured very carefully so that any shrinkage was fully controlled.

If movement control joints have been omitted and cracking has occurred, it may be desirable to retrofit a control joint. The earlier the retrofit, the greater the chance of successfully controlling subsequent shrinkage movement. Even where control joints have been used, there may be some situations where more are needed. For instance, where:

• wall surfaces get warmer than expected, particularly on north and west walls

• a joint may have been incorrectly located or installed

• wall panels are irregular shapes or sizes and need to be broken up.

Minor cracking

Where shrinkage cracking has occurred in that initial hydration period there are two remedies. The surface of the stucco can be coated with either a:

• bag-rubbed, stiff cement slurry to fill the cracks (then painted)

• coating system that has the ability to bridge the shrinkage cracking.

Coatings generally fail sooner along the crack lines if cracks are painted over, since the cracks act as stress raisers for the coating.

Retrofitting control joints

For badly cracked stucco, the only safe option is to remove the cracked cladding completely and reapply new plaster incorporating movement control joints.

When retrofitting a joint, the plaster must be sufficiently hard to allow it to be cleanly cut with a diamond-tipped saw blade. If saw access is limited, a hacksaw blade may be needed to complete the saw cut to give a straight edge. Two parallel saw cuts, 10 mm apart and to the face of the scratch or first coat, need to be made and the material between the cuts removed. The saw cut should only penetrate through to the face of the first or scratch coat. It should be made over the line of the cavity batten to maintain support to the backing and reinforcing mesh. After the plaster has fully cured, the edges of the saw cut should be primed, a backing rod installed and the joint filled with sealant as shown in Figure 1.

Installing control joints after the plaster has been applied may create more problems than it solves because:

Figure 1: Make a clean cut in the plaster and then fill with sealant.

• newly applied plaster may not be hard enough to cut early enough to prevent further cracking occurring

• it is difficult to neatly cut the stucco and get a straight, evenedged cut, particularly where the cut may end under a soffit or fascia board

• the plaster may spall at the cut and create rough edges

• sealant joints will require regular maintenance

• if the plaster has already cracked subsequent movement is likely to continue at that crack rather than at a retrofit control joint.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Make a clean cut in the plaster and then fill with sealant.

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