QUESTION: What are the industry recommendations on recesses in masonry veneer?


Recessing of masonry courses adds architectural interest to a building façade.  However, there are three aspects of recessing that should be considered.  Firstly, recessing creates a “ledge” on the outside face, at the bottom of the recess.  Just as with raked and struck mortar joints, this “ledge” increases the risk of water infiltration at the mortar/unit interface.  International Masonry Institute (IMI) recommends installing a mortar wash at this “ledge” (IMI Detail 01.030.1001) at the same time that the units are placed on the mortar bed.  However, if the recessed course is at or below eye level, this mortar wash will be visible, which will impact aesthetics.

The second aspect to consider is the impact on the cavity behind the veneer, where the recessed course creates a projection into the drainage space.  This projection must not decrease the air space behind the veneer to less than the one-inch minimum that is required by Code.  Also, consideration should be given to the impact of that projection on the ability of water that has penetrated the veneer and entered the cavity to flow freely downward to the flashing and weep holes.  Similar to the mortar wash on the outside face, IMI recommends a mortar wash on the inside face of the veneer at the top of the recessed course.  Unless the cavity is wide enough, however, this mortar wash could be difficult to accomplish and may contribute to mortar droppings in the cavity.

The third aspect to consider is the reduction in flexural strength, or resistance to cracking.  When the veneer consists of solid units (net cross-sectional area of 75% or more per ASTM), which are laid with a full mortar bed, the reduction in strength at a slightly recessed course is small and insignificant.  However, when hollow units (net cross-sectional area less than 75% per ASTM) are constructed with recessed courses, strength reduction can be significant because these units are typically laid with face shell mortar only and there is little to no face shell overlap at the recessed course.  Consequently, hollow units should not be used when recessed courses are desired, unless they are solidly filled with mortar or grout.  When the magnitude of recess equals or exceeds the depth to the core holes in solid units, extra care is required during construction to mitigate the potential for water entry into those core holes.

The magnitude of recessing in veneer is governed by the corbel provisions of the MSJC Code, Section 1.12.2.  The maximum offset per course must not exceed one-half the nominal unit height nor one-third the nominal unit thickness.  If successive courses are each recessed, the total offset should not exceed one-half the veneer wythe thickness.  Whether one course or multiple courses are recessed, the back, or inside, surface of the veneer must remain within one-inch of plane.

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