Wildfire-damaged historic buildings may not be as ruined as they appear, but saving them means acting fast

In a summer of wildfires severe enough to trigger Spain's first-ever national emergency for a forest fire, a historic stone structure called the Nevera de Castro stands in a landscape burned bare in the Castellón region. Its walls are blackened by smoke and heat. A local news outlet noted that "a technical inspection will be necessary to know its real state."

That one sentence names a problem that plays out after every major fire.

When a wildfire reaches historic masonry buildings – stone, brick and block structures that have stood for centuries – the first photographs tell a simple story: Everything is black and looks ruined. But masonry conducts heat so poorly that the damage is often only centimeters deep.

Timber roofs, wood floors and interior framing in historic buildings are extremely vulnerable to fire and can let fire jump building to building, but the load-bearing masonry shell often survives. Preserving even a part of the original structure can dramatically reduce reconstruction costs and retain the historic fabric that gives these places their identity.

But to save these buildings, people have to act quickly.

A masonry wall that has lost its timber roof or floors is now unbraced. Wind alone might bring it down. The stone itself can also start to deteriorate. When limestone is heated to about 1,380 degrees Fahrenheit (750 degrees Celsius), the calcium carbonate, which makes up most of the rock, decomposes, leaving solid calcium oxide and carbon dioxide gas. If rain reaches that surface before it is covered, the calcium oxide can react with the water and expand by as much as 20%, enough to crack the stone from within.

Some types of masonry can start to disintegrate days after a fire if no one has weatherproofed it after the fire.

Why thick walls don't fail as you might expect

Masonry conducts heat poorly. In a thick wall exposed to a wildfire, the exposed surface heats up while the cooler interior remains relatively unchanged.

These thermal gradients generate stresses that may crack or spall – chip or flake off – the outer surface, but the heat often does not penetrate deeply enough to compromise the structural core. Research on fire-exposed sandstone structures found a sharp boundary between the heated, red-colored surface and the unaltered stone behind it, with the damaged crust only about an inch (2 to 3 centimeters) deep.

At the Monasterio de San Jerónimo de Guisando, a 14th-century monastery in Ávila, Spain, the whole hillside burned in July 2026, but the monastery reported that the church wall itself acted as a firebreak, stopping the fire's advance.

The 650-year-old wall did exactly what a thick, noncombustible wall should do.

Clay brick is especially resilient because it was already fired at 1,650 to 1,800 F (900 to 1,000 C) when it was made. A 2025 study tested structural clay units reheated to 1,800 F and found no statistically significant loss of compressive strength – its ability to hold weight.

The weak link is the mortar between the bricks, not the bricks themselves. Industry fire-rating tables show that a solid brick wall only about 150 millimeters thick can withstand four hours of standard fire exposure. Many historic masonry walls are two to four times that thick.

How engineers evaluate the damage

As architectural engineers, including the director of the Heritage 3D Lab at Penn State, we help communities rescue historic buildings after fires.

Evaluating a burned building starts with color. When concrete or stone turns pink or red, it signals that the stone has been exposed to temperatures above roughly 570 F (300 C) and iron in the material is effectively rusting in the heat. That is the threshold where it begin to lose significant strength. Gray or white means the stone got even hotter, water is cooking out and the material is turning to ash.

But color is only a surface clue.

The critical question is how deep the damage goes. Fire damage in a thick wall is almost always one-sided. When researchers tested concrete masonry walls with fire on one face, those walls retained roughly 46% of their compressive strength, enough that they can be stabilized and repaired. When the same walls had fire on both faces, their compressive strength dropped to 14% to 27% of the original.

Engineers can estimate the depth of the damage using methods such as petrographic analysis – cutting thin sections from the wall and examining the microcracking under a microscope.

After the Notre Dame Cathedral fire in Paris in 2019, researchers drilled cores through the limestone vaults and mapped exactly where the heat stopped. The fire was catastrophic, but the heat stopped short of the full depth, even in vaults – the arched stone ceilings of the cathedral – only about 5 to 6 inches (12 to 15 centimeters) thick.

Unlike the prolonged heat associated with many structure fires, a wildfire front can pass a building within minutes. Even when flames are intense, the relatively short duration of exposure gives heat little time to penetrate thick masonry walls, assuming interior materials don't catch fire and continue burning from within.

The outer surface may crack or spall as it expands under rapid heating, while the interior remains comparatively cool, preserving the mechanical properties of the masonry.

The clock is ticking for recovery

The clock is already ticking by the time an assessment can be conducted to determine whether a building can be saved.

The first step to saving it is shoring it up – propping up weakened arches, bracing freestanding walls and weatherproofing exposed surfaces to avoid more damage.

The governing principle in the International Council on Monuments and Sites guidelines for structural restoration of architectural heritage is minimum intervention – do only what is needed to guarantee the safety and durability with the least harm to the historical or cultural value of the property.

Fire-damaged mortar joints are repointed. Spalled or calcined stone faces are cut back to reach sound material and then patched or replaced with stones from the same quarry, where possible.

After the 1997 fire at Turin, Italy's Cappella della Sindone, the Chapel of the Holy Shroud, restorers replaced 1,400 marble elements and consolidated 4,050 more. They reopened the original quarry at Frabosa to get stone that would match the 17th-century building materials.

These are not fast processes. The chapel took 21 years to restore. But the window that determines whether restoration will even be possible is measured in weeks and relies on shoring up the masonry and weatherproofing it.

When demolition is (and is not) the answer

Not every fire-damaged building can be saved.

When flames penetrate the interior of a building and attack its walls from both faces, the fire causes far greater damage to the masonry. Some types of masonry block can disintegrate days after a fire if minerals in the blocks get wet and expand.

But demolition should be a careful decision based on proper assessments. And right now, no country has a postfire assessment protocol for historic buildings comparable to what exists for assessments after earthquakes.

Wildfires determine what burns. People determine what is lost. Until those protocols are developed, the default will often be to treat a standing wall as debris rather than a valuable building block for the future.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Rebecca Napolitano, Penn State; Giorgia Giardina, Delft University of Technology, and Tiago Miguel Ferreira, Universidade de Lisboa

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Giorgia Giardina receives funding from NWO and EU.

Rebecca Napolitano and Tiago Miguel Ferreira do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.