Wildfire-damaged historic buildings may not be as ruined as they appear, but saving them means acting fast
The damage is sometimes only centimeters deep. Three engineers explain the colors of burned masonry and other signs preservationists look for amid another destructive fire year.
In a summer of devastating wildfires that triggered Spain's first-ever national emergency for a forest fire, historic stone structures like the Nevera de Castro are facing a critical challenge. Though the outer surface appears blackened and ruined after such fires, the damage is often limited to just a few centimeters deep. The reason for this is that masonry, made of materials like stone, brick, and block, conducts heat poorly.
This means that the interior structure of these historic buildings is often left largely intact, despite the dramatic exterior appearance. For instance, the Church of São Domingos in Lisbon, which experienced a fire in 1959, showed only black soot and white calcination on its exterior, while the undamaged brick remained hidden beneath.
The key to saving these historic buildings lies in swift action. Once a masonry wall loses its roof or flooring, it becomes unbraced and vulnerable to collapse from wind alone. Moreover, the fire can start deteriorating the stone itself. When heated to approximately 1,380 degrees Fahrenheit (750 degrees Celsius), limestone decomposes into solid calcium oxide and carbon dioxide gas.
If rainwater subsequently touches this surface before it's covered, the calcium oxide can react with the water and expand by up to 20%, causing internal cracking.
However, thick walls do not fail as one might expect. In thick walls exposed to wildfires, the exposed surface heats up while the cooler interior remains relatively unchanged. This creates stresses that may cause the outer surface to crack or spall (chip or flake off), but the heat usually doesn't penetrate deeply enough to compromise the structural core.
Research on fire-exposed sandstone structures has revealed a distinct boundary between the heated, red-colored surface and the unaltered stone behind it. The damaged crust is only about an inch deep, leaving the structural core largely intact.
Thus, even after a severe wildfire, significant portions of these historic buildings may still survive, retaining their original fabric and reducing the cost and complexity of restoration.
Written by urgent.news from The Conversation's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
