Does the backer rod have to be larger or smaller than the joint that is being created?
Backer rods are important to prevent 3-point bonding at the crack in the back of a joint. Also, they control the amount of caulking that a joint requires.
For expansion joints the backer rod should be just large enough to fit snugly into the control joint (which should be 3/8" for CMU walls). There should be about 1/2" of depth from the backer rod to the surface of the masonry wall. The backer rod should be snug enough to stay in place during caulking of the joint.
Jerry Painter, FASTM
Don Beers, PE, GC
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