I am repointing mortar joints in an ashlar stone wall that are about 1 inch wide. Because these joints are so wide, should the depth of the repointing exceed the 3/4-inch depth recommended by many references on repointing, including those published by the Brick Institute of America?
The depth of the mortar for repointed joints should be between 1-1/2 and 2-1/2 times the width of the joint to provide an adequate bond that will not break over time. Existing mortar should be removed completely from the surfaces of the stone before installing new mortar. The mortar should be installed in layers, the thickness of which should be one-third to one-fifth the joint depth. I recommend not exceeding 1/2-inch-thick layers for wide joints and 1/4-inch-thick layers for joints less than or equal to 3/8 inch. Subsequent layers of mortar should not be installed until the previous layer has stiffened. Because of the size of the joint, consider using a coarser sand in the mortar than would typically be used for repointing (The sand still should be well-graded and within the limits of ASTM C 144
Standard Specification for Aggregate for Masonry Mortar). This, along with prehydrating the mortar and using the softest mortar permitted for the application, will help control mortar shrinkage.