By Elizabeth A. Hnatiw and Christopher M. DeRosa
As holds the principle of bioethics, so should the approach to treating historic stone: “First, do no harm.” If the degree of deterioration does little to detract from the architectural character of the building or to call into question its structural stability or performance, no repair may be needed.
Too often, application of a “protective” waterproof coating, aimed at preventing hypothetical moisture-related deterioration, has the unintended effect of sealing moisture inside the masonry surface and leading, in many cases, to real and irreversible damage. Stone masonry buildings of traditional construction were designed to “breathe;” to be permeable to air and moisture, allowing water to leave the wall assembly through the natural process of evaporation. Introducing impervious coatings and vapor barriers disrupts the balance of moisture across the building envelope, which can result in dire consequences for the stone, at both a macro- and micro-structural level.
As a shortcut to address deteriorated mortar joints, a thin layer of mortar is sometimes added without first removing the existing mortar to the appropriate depth. While quicker and cheaper than full repointing, face-pointing is inadvisable, as it does not provide suitable stability and tends to crumble out of the joint.
Introduction of pointing mortars of incorrect composition may restrain the natural expansion and contraction of stone masonry subjected to moisture absorption and drastic temperature swing cycles, leading to cracking and spalling.
Faulty patching mortars and ill-conceived pinning methods may have similar detrimental consequences. Even stone cleaning, which sounds innocuous enough, is fraught with hazards, as using abrasive or chemically incompatible cleaning methods can abrade or otherwise damage the stone surface.
Elizabeth A. Campbell, AIA LEED AP BD+C is Project Architect with Hoffmann Architects, Inc., an architecture and engineering firm specializing in the rehabilitation of building exteriors. She develops historic stone restoration strategies that combine material science and preservation. Christopher M. DeRosa, AIA, PE, LEED Green Associate, Project Architect with Hoffmann Architects, is experienced in designing stone cleaning and repair programs for traditional masonry buildings.