Q.: We're working with a specification that says control joints must be cut to a depth equal to one-third the slab thickness. All the recommendations we can find suggest a depth of one-quarter the slab thickness, with a minimum of 1 inch. Do you know of a basis for the specifier's requirement?

A.: All our references also recommend joint depths of either one-quarter or one-fifth the slab thickness. Sawing a deeper joint doesn't seem to serve any useful purpose and may reduce aggregate interlock at an undoweled joint.