Repointing: Less is not always more

July 17, 2020

slaton patterson [1]FAILURES
Deborah Slaton, David S. Patterson, AIA, and Timothy Penich, AIA

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.

A thin layer of mortar was installed over existing mortar, which resulted in cracking and debonding of the new mortar. Photos © Timothy Penich and Deborah Slaton, WJE[2]
A thin layer of mortar was installed over existing mortar, which resulted in cracking and debonding of the new mortar.
Photos © Timothy Penich and Deborah Slaton, WJE

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.

Bond line failures were observed at many locations. Also note the chipping along the edges of the joint, which likely occurred during preparation for repointing.[3]
Bond line failures were observed at many locations. Also note the chipping along the edges of the joint, which likely occurred during preparation for repointing.

Some suggestions for proper repointing to avoid premature failure include:

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 dslaton@wje.com[4].

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 dpatterson@wje.com[5].

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 tpenich@wje.com[6].

Endnotes:
  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/01/Tim.jpg
  2. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/07/Photo-1-comp.jpg
  3. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/07/Photo-2-comp.jpg
  4. dslaton@wje.com: mailto:dslaton@wje.com
  5. dpatterson@wje.com: mailto:dpatterson@wje.com
  6. tpenich@wje.com: mailto:tpenich@wje.com

Source URL: https://www.constructionspecifier.com/repointing-less-is-not-always-more/