When repointing mortar joints in an existing structure, successful long-term performance depends on a number of factors. A lack of understanding of substrate materials and conditions, insufficient joint preparation, less than adequate skill or care during mortar installation and curing, and an overly limited repointing scope can lead to premature failure of a masonry joint repointing program.
In the first example shown—a 1920s brick and limestone-clad, mid-rise office building—mortar was not properly removed from the joints between limestone units prior to installation of new mortar. The thin layer of mortar applied over the existing deteriorated mortar cracked and debonded within a few years of application. Bond line failures between the cement-rich repointing mortar and the adjacent stone led to moisture infiltration to the building interior.
In the second example shown—a recently constructed cast stone-clad building that was repointed to address defects in the original pointing mortar—widespread bond line failures were observed along mortar joints. These failures may be related to insufficient prewetting of the joints and possibly to the presence of a water-repellant treatment applied to the wall prior to repointing that inhibited bond. Additionally, the stone adjacent the joint was chipped, likely during preparation for repointing. Also, the mortar had spalled at various locations, possibly due to an inappropriate mix design, poor installation techniques, or inadequate curing, presenting other avenues for water to enter the wall assembly.
Some suggestions for proper repointing to avoid premature failure include:
- carefully consider the extent of repointing required, as ‘spot pointing’ to address localized areas may be insufficient to solve existing problems, and may also not be cost-effective (designating areas for repointing, often defined by architectural features or changes in plane, can be more effective);
- understand the nature of the substrate(s) and any prior treatments or existing conditions that may affect repointing;
- specify a repointing mortar mix that is compatible with the masonry substrate;
- specify and sufficiently detail proper preparation of the joint and placement of the pointing mortar, including removal techniques that do not damage adjacent material to an appropriate depth (typically to a depth of 2 ½ times the width of the joint or until sound mortar is encountered), cleaning of the joint to remove mortar debris and dust, pre-hydrating the joint, and installing new mortar in 6-mm (¼-in.) well compacted lifts (avoid thin surface applications of mortar over existing mortar, as these are likely to be ineffective and fail over time);
- specify proper curing for the new mortar, including temperature and moisture controls as appropriate; and
- consider the use of non-staining sealant in skyward-facing joints and at interfaces between different materials as appropriate.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at firstname.lastname@example.org.
David S. Patterson, AIA, is an architect and senior principal with WJE’s office in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at email@example.com.
Timothy Penich, AIA, is an architect and senior associate with WJE in Northbrook, Illinois. He specializes in historic preservation. Penich can be contacted via e-mail at firstname.lastname@example.org.