As we continue our learning curve on repointing mortar joints in historic masonry buildings we need to sometimes step back and re-evaluate our recommendations based on experience. One of those areas is the repointing mortar in 1/4″ layers -or lifts, usually three separate lifts in a mortar joint that has been cut out to an inch in overall depth. According to the Preservation Brief 2, “Repointing Mortar Joints in Historic Masonry Buildings”,1998 pp.10-11 states:
“Where existing mortar has been removed to a depth of greater than 1 inch, these deeper areas should be filled first, compacting the new mortar in several layers. The back of the entire joint should be filled successively by applying approximately 1/4 inch of mortar, packing it well into the back corners. This application may extend along the wall for several feet. As soon as the mortar has reached thumb-print hardness, another 1/4 inch layer of mortar – approximately the same thickness – may be applied. Several layers will need to fill the joint flush with the outer surface of the masonry. It is important to allow each layer time to harden before the next layer is applied; most of the mortar shrinkage occurs during the hardening process and layering thus minimizes overall shrinkage.”
I do not agree with the 1/4 inch layering for several reasons. First, I believe the potential for cold joints can occur between layers. Second, the possibility of a mason being able to place his thumb back into a 3/8 inch mortar joint cut one inch in depth is not practical. And, third, I do not believe the process adds any benefit for the long-term durability and performance of the new mortar in exchange for the added cost factor to repoint a building three times as compared to one. That being said, I understand the PB2 authors point to establish methods to minimize shrinkage cracks as the mortar hardens – which this process likely does – if the mortar formulation is not correct, let me explain.
Our experience has been that most shrinkage cracks occur within the first 16 hrs after placement due to three primary reasons: 1) excess water in the mortar material; 2) the incorrect aggregate gradation in the mixture – generally the sand is not coarse enough for the width of the joint – the wider the joint the courser the particles need to be, or; 3) early evaporation of water from the joint causing a flash-set to occur. While we do specify repointing in lifts if the joint depth is greater than 1-1/4 inches. In this case, place the mortar in 3/4 inch layers, but this is generally not in every location. In most projects there is the occasional deep pocket or void but this is generally not a typical condition everywhere on the building. If you are getting into areas of two inches or more it time to re-lay the brick.