Unreinforced masonry, often abbreviated URM, is any brick, stone, or concrete block structure built without internal steel reinforcement. The walls rely entirely on the weight and bond of the masonry units and mortar to hold the building together. This construction method was standard practice for centuries, and thousands of these buildings still stand in cities across the United States. They remain functional and often beautiful, but they carry real structural vulnerabilities, especially in earthquake-prone areas.
How URM Buildings Are Constructed
Masonry is a composite material: individual units (bricks, cut stone, or concrete blocks) held together by mortar. In unreinforced masonry, there is no steel rebar running through the walls, no metal ties connecting layers, and no reinforced concrete core. The structure’s strength comes from the compressive weight of the masonry stacked on itself and the adhesive bond of the mortar between units.
The mortar itself varies significantly depending on when the building was constructed. Buildings from before roughly 1870 typically used lime mortar, a mixture of lime, sand, and water. Lime mortar is softer and more flexible than modern alternatives, and it has some useful properties: it accommodates slight structural movement without cracking, it breathes well enough to let moisture escape, and it can even self-heal small fissures when residual lime reacts with water to form new crystalline bonds across cracks.
Later URM buildings, from the late 1800s through the mid-1900s, increasingly used Portland cement mortar. This is harder and stronger in compression, but it’s also more brittle. Portland cement doesn’t flex with building movement the way lime does, making it more prone to cracking under stress. It’s also denser and less breathable, which can trap moisture inside walls and accelerate decay of adjacent materials. In seismic zones, that brittleness becomes a serious liability.
Why URM Is Vulnerable in Earthquakes
Unreinforced masonry has lower strength and ductility than reinforced concrete or steel. Ductility is the ability to bend or deform without breaking. Steel-reinforced walls can flex during shaking and absorb energy. URM walls cannot. They are stiff until they crack, and once cracking starts, there is nothing inside the wall to hold the pieces together.
Earthquake damage to URM buildings follows predictable patterns. Studies of post-earthquake damage, including a detailed survey after a 2019 earthquake in rural Turkey, identified four main categories: total collapse, corner damage, out-of-plane wall failure, and in-plane wall damage. Out-of-plane failure is the most characteristic URM hazard. This is when an entire wall section falls outward (or inward) because there is nothing tying it to the floors or roof. The wall essentially peels away from the building.
The root causes are consistently the same: weak connections between walls and walls, between walls and roofs, and a lack of bond beams (horizontal reinforcing elements) within the walls themselves. Without these connections, each wall acts independently during shaking rather than working as a unified structure. Corner damage occurs where perpendicular walls pull apart from each other. In-plane damage shows up as diagonal cracks running through the wall face, following the mortar joints in a stair-step pattern.
Parapets and Chimneys: The First Things to Fall
You don’t need a major earthquake to shake loose parts of a URM building. Parapets (the short walls extending above the roofline) and chimneys are the most immediately dangerous elements because they sit at the top of the structure where shaking is amplified, and they’re often completely unbraced. Even moderate ground motion can topple them onto sidewalks, cars, or adjacent roofs.
If you live in or near a URM building, the Earthquake Country Alliance recommends checking chimney mortar with a screwdriver. If it crumbles when you pick at it, the chimney is likely a hazard. During earthquakes, everyone in the household should stay away from chimneys and fireplaces. One practical protective measure is installing plywood panels at roof level or above ceiling joists to prevent falling brick from crashing through into living spaces below.
How Many URM Buildings Still Exist
Major cities in seismic zones have spent decades inventorying their URM buildings. California law requires local jurisdictions in the highest-risk seismic zone to identify all potentially hazardous buildings and establish mitigation programs that include notifying building owners. The numbers give a sense of scale: San Francisco identified roughly 2,000 URM buildings, of which about 100 have been demolished and around 150 were still not in compliance as of 2008. Oakland counted 1,612, with 89% either demolished or retrofitted by 2003. Berkeley identified 700, and all but 22 had been retrofitted.
Cities in the Pacific Northwest face similar challenges. Seattle has its own URM inventory and has been developing retrofit standards, though compliance timelines vary. Across the country, many smaller cities and towns have never conducted formal inventories, meaning thousands of URM buildings remain unidentified and unaddressed.
How URM Buildings Are Retrofitted
Retrofitting doesn’t turn a URM building into a modern reinforced structure. The goal is more modest: keep the walls from separating from the floors and roof so the building holds together long enough for occupants to escape. The most common techniques target the specific weaknesses that cause URM failures.
Wall anchoring is the foundation of most retrofits. Steel anchors, typically at least half an inch in diameter, are driven entirely through the masonry wall with a bearing plate (at least 30 square inches) bolted on the far side. These anchors connect walls to floor and roof framing at every level, resisting the out-of-plane forces that would otherwise let walls topple outward. Rosette-style anchors, which spread the load across a wider area of the masonry face, are a common choice.
Parapet bracing addresses the falling-hazard problem at the roofline. Parapets that don’t meet current standards must be removed, stabilized, or braced to stay in position during shaking. Bracing systems are typically spaced no more than 8 feet apart along the parapet and are designed to resist lateral forces calculated from the building’s expected seismic load. For parapets that exceed safe freestanding height, a steel bracing frame anchored to the roof structure supports the top of the wall.
Other common retrofit elements include adding plywood diaphragms to strengthen floors and roofs so they can transfer lateral forces to the walls evenly, installing steel moment frames or braced frames within the building to add overall stiffness, and reinforcing wall corners where perpendicular walls tend to separate. The specific combination depends on the building’s size, condition, and the seismic risk at its location.
Living or Working in a URM Building
URM buildings are not inherently dangerous in everyday conditions. Many have stood for well over a century. The risk is specifically tied to seismic events, and it scales with the severity of shaking and the condition of the building. A well-maintained URM building in a low-seismic zone presents minimal concern. The same building type in Seattle, San Francisco, or Salt Lake City is a different calculation entirely.
If you’re trying to determine whether a building is URM, look for solid brick or stone walls (not brick veneer over wood framing), especially in buildings constructed before the 1950s. Multi-story brick buildings with no visible evidence of steel framing are strong candidates. In many cities, URM inventories are public records, and your local building department can tell you whether a specific address has been identified and whether it has been retrofitted. Retrofitted buildings are significantly safer, though they still won’t perform like modern construction in a major earthquake.