What are masonry movement joints?

ANSWER:

Movement joints are constructed within masonry to accommodate predicted volume changes of the masonry materials as well as relative movement between masonry and adjacent materials.  Movement joints are finished with sealant to prevent water entry while the joint changes dimension.  Masonry movement joints may be described as one of three types:  expansion joints, contraction or control joints, and isolation joints.

Expansion joints (EJs) are used to accommodate volume expansion of clay masonry.  According to the Brick Industry Association (BIA), clay masonry undergoes irreversible moisture volume expansion over time, with the majority of the size change occurring during the first year after manufacture[1].  As the clay masonry increases in size, or volume, the movement (expansion) joint decreases in width.  The sealant in the movement (expansion) joint needs to be capable of continuing to function as intended, by remaining intact and adhered to the joint sides, while being squeezed.

Control joints (CJs) are used to accommodate volume shrinkage of concrete masonry, including cast stone.  According to the National Concrete Masonry Association (NCMA), concrete masonry undergoes irreversible volume shrinkage over time[2].  Most of the size change occurs during the first year after manufacture.  As the concrete masonry decreases in size, or volume, the movement (control) joint increases in width.  The sealant in the movement (control) joint needs to be capable of continuing to function as intended, by remaining adhered to the joint sides without internal tearing, while being stretched.

Isolation joints (IJs) are designed and constructed to permit differential movement between masonry and adjacent materials, such as windows, doors, and non-masonry façade cladding materials.  The differential movement may be parallel or perpendicular to the length of the movement joint. Therefore, the sealant in the joint may be squeezed smaller or stretched wider or stretched diagonally, or may be subjected to a combination of these movements.  The sealant in the movement (isolation) joint needs to be able to continue to function without tearing or losing adhesion to the joint sides while accommodating these various distortions.

Masonry movement joints are separate and distinct from building expansion joints.  Masonry movement joints are designed and constructed within the masonry wythe only, but building expansion joints extend through the entire building structure and finishes.

References:

[1] BIA Technical Note 18 “Volume Changes – Analysis and Effects of Movement”, October 2006
[2] NCMA TEK 10-2C, Control Joints for Concrete Masonry Walls – Empirical Method, 2010 and NCMA TEK Note 10- 10-4, Crack Control for Concrete Brick and Other Concrete Masonry Veneers, 2001

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