Up from the Ashes: Assessing and Addressing Effects of Fire on Historic Stone Masonry

In August 2017 the Sprague Fire roaring through Glacier National Park reached the remote mountain site of Sperry Chalet, constructed in 1913. The roof and log-framed interior structure were destroyed, leaving only the four stone masonry walls.
Preserving the masonry walls would be the first step in restoring the chalet to operation. This process would begin with an investigation into the effects of conflagration on the performance of the historic stone and evolve into a rehabilitation strategy that would allow the walls to once again house a robust structure and use. This process was at all times complicated by challenges of site and schedule. Perched on a cliff, the chalet is accessible only by trail and then only in summer.
High temperatures are damaging to metals and cementitious materials such as concrete and mortar. Effects of high temperatures on stone is less well-known, although thermal differentials that occur during severe fires are known to cause damage in the form of cracking and delamination. Information critical to understanding high temperature effects are the peak temperature experienced by construction materials and the duration of the fire. These parameters combine to provide an indication of the depth to which stone and concrete materials are affected.
Fire temperature and duration was diagnosed based on site observations of temperature effects on construction materials. Deformation and melting of lead, aluminum, and glass, and color changes within concrete members, suggest a maximum sustained fire temperature of between 1,700 and 2,100º F, with a high temperature duration of 1 ½ to 2 hours.
The masonry walls consist of two interwoven leaves of locally quarried argillite, a sedimentary stone consisting mainly of lithified muds. The primary effect on the interior stone leaf, which bore the brunt of this heat, was delamination along multiple lines of fracture to a depth of one to two inches. Stone that seemed to be intact delaminated at minor contact such that stabilization efforts provoked showers of debris.
When it came to rebuilding the structure within the walls, the architectural implications of the delamination became paramount. The safety of future inhabitants suggested that loose material be removed. This painstaking process left irregular surfaces and a lack of competent material for anchorage of enclosure elements. At the top of most wall and gable conditions, too little material remained for attachment of the roof. Jambs and sills were likewise significantly impacted. Uneven delamination across multiple planes made dutchman repair unrealistic, leaving two options: retaining the stone in its deteriorated state or replacing it with stone from the original quarry. The restoration solution became a stone by stone decision, balancing strategic replacement with restraint, allowing the damaged stone to tell its story.