How Chimneys Built Before Modern Seismic Bracing Move During an Earthquake

A Structure That Moves on Its Own Terms

The San Fernando Valley sits close enough to the Hollywood Fault zone that even a moderate quake reaches homes throughout Studio City, Sherman Oaks, and the neighboring communities on a regular basis. Most residents think about how their house held up afterward: did anything fall off a shelf, did a wall crack, did the foundation shift. A masonry chimney rarely enters that mental checklist, yet it is one of the parts of an older home most likely to respond to shaking on its own, separately from the rest of the structure. This piece walks through why that happens on a chimney built decades ago, and what the resulting damage tends to look like once a technician gets a close look at it.

Why a Masonry Chimney Behaves Differently Than the House Around It

A house frame flexes as a connected system during ground motion, distributing force through studs, joists, and sheathing that were engineered to work together. A masonry chimney is a different kind of structure entirely: a tall, heavy, largely rigid column of brick and mortar that sits beside the house rather than inside its frame, tied in at the roofline and sometimes along an interior wall. Chimneys built during the decades that produced much of the housing stock in Studio City and Burbank went up before steel strapping and anchor bolts connecting the chimney to the roof framing became standard practice. Without that connection, the chimney and the house can move independently of each other when the ground shakes, and independent movement between two structures joined at one point is exactly the kind of stress that produces damage.

What a Separation at the Roofline Looks Like

The most visible sign of this kind of movement shows up right where the chimney meets the roof. A gap opening between the brick and the surrounding stucco or siding, fresh cracking in the material at that seam, or a chimney that appears to lean slightly away from the house when viewed from the yard are all signs worth a closer look. On a hillside property in Sherman Oaks or a canyon lot in Encino, this separation can be harder to spot from ground level, since the angle of the roof and the height of the stack make the joint difficult to see without getting up there directly.

Crown Cracks That Look Different From Weather Cracks

Every masonry crown develops some hairline cracking over the decades simply from sun exposure and temperature swings, and a technician who works on older chimneys regularly learns to recognize that pattern: a fine network of shallow cracks spread fairly evenly across the surface. A crack that appears suddenly after a shake event tends to look different, often a single sharper line or a crack that has widened enough to catch a fingernail, sometimes running toward one edge of the crown rather than spreading evenly. Distinguishing a fresh seismic crack from years of accumulated weather cracking matters because the fresh one is actively letting water into the chimney right now, while the older pattern may have been stable for years.

Flue Tile Joints That Shift Out of Alignment

The clay tile liner inside an original masonry chimney was installed section by section, each length mortared to the one below it as the structure went up. Those joints are the weakest points in the liner, and a shake event can nudge one section slightly out of line with the next, narrowing the passage or opening a gap between sections that was tight before. None of this is visible from the firebox looking up. A liner that shifted during a quake months ago can sit undetected until a video scan runs the length of the flue and shows a joint that no longer lines up cleanly with the section above or below it.

Step Cracking in the Exterior Brick

Earthquake damage in masonry tends to follow a recognizable pattern: cracks that zigzag along the mortar joints in a stair-step shape, moving diagonally up or down the face of the chimney, rather than cutting straight through the brick itself. This is different from the vertical cracking that sometimes appears from ordinary settling, and different again from the fine surface crazing that comes from decades of thermal cycling. Recognizing that stair-step pattern on the brick face of an older Studio City or Burbank chimney is one of the clearer signs that a recent shake event, not gradual wear, produced the damage.

Why the House Passing Inspection Does Not Mean the Chimney Did

Homeowners often assume that if nothing structural turned up wrong with the house itself after a quake, the chimney came through fine as well. That assumption misses how differently the two structures are built and connected. A house engineered as a single frame can absorb shaking in ways an older masonry stack, tied to that frame at only one or two points, simply cannot. A chimney can sustain real damage, a cracked crown, a shifted flue joint, a loosened roofline connection, while the rest of the house shows nothing worth mentioning to a general contractor.

What a Post-Event Assessment Actually Covers

When we check an older chimney after a shake event, the visit works through the roofline connection first, looking for gaps, fresh cracking, or a lean that was not there before. From there we move to the crown, checking for the sharper, more localized cracking pattern that separates recent damage from long-term weathering. A video scan of the flue interior follows, since tile joint separation almost never shows up any other way. We finish with a walk around the exterior masonry, checking for the stair-step cracking pattern along mortar joints that signals ground movement rather than gradual settling.

Chimneys With a Prior Repair History

An original masonry chimney that has already had a section relined or a crown patched at some point over the decades does not necessarily respond to shaking the same way throughout its full height. Older mortar and a newer repair mix can have different flexibility, which means the boundary between original construction and a later repair is sometimes exactly where new movement damage shows up first. Homeowners who know their chimney has a repair history should mention it when scheduling a post-event check, since it tells the technician where to look closest.

What Homeowners Can Watch For Between Professional Visits

A technician's assessment covers the details most people would miss, but there are a few things a homeowner can keep an eye on between scheduled visits. A hairline gap opening along the seam where the chimney meets the exterior wall, fresh cracking in stucco or siding right at that joint, or a chimney that seems to lean a little more than it used to when you glance up from the driveway are all worth noting and mentioning at the next inspection. Inside the house, a smell of dampness near the fireplace that was not there before, or a fireplace that suddenly seems to smoke more than it used to, can both point back to a shifted flue joint rather than anything wrong with how a fire is being built. None of these signs require a homeowner to diagnose the problem themselves. They are simply reasons to move up the timeline on a professional look rather than waiting for the next annual visit.

Why Waiting Rarely Helps on a Shaken Chimney

A crack or joint separation caused by ground movement does not tend to heal on its own, and it usually does not stay the same size either. Rain working into a fresh crown crack after a shake event accelerates whatever damage the shaking started, and a flue joint that has shifted slightly can continue opening further with every subsequent fire cycle or minor aftershock. Addressing this kind of damage while it is still limited to one crack or one joint is generally a smaller repair than waiting until water intrusion or continued movement has spread the problem into surrounding brick and mortar that were otherwise sound.

Booking an Assessment After You Feel Shaking

If your home is served by an original masonry chimney and you have felt any recent shaking, even a quake that caused no obvious damage elsewhere in the house, it is worth having the chimney looked at on its own terms rather than assuming it rode out the event along with everything else. Our CSIA-certified technicians check the roofline connection, crown, flue interior, and exterior masonry as a complete assessment, not just a glance from the ground.

Contact Blackstone Chimney Sweep at (820) 345-0340 to schedule a check. We serve Studio City, Sherman Oaks, Encino, Woodland Hills, Burbank, and Simi Valley, working with the older masonry housing stock common throughout this part of the valley.