Q: Our 1940 home has a brick fireplace and chimney. We had it inspected recently and were told by several people that it has a crack near the roofline and needs to be taken down and rebuilt. The estimate to rebuild it is about $15,000. Are there less expensive ways to replace this chimney that do not involve rebuilding it from the ground up with brick?

A: It sounds as if at some point your chimney may have sustained some earthquake damage that went unnoticed until the chimney inspection.

A total rebuild from the ground up might sound a bit excessive if the crack is at the roofline. But for maximum earthquake protection when using traditional materials, this is probably the way to go.

To make an informed decision about how to proceed, you need to assess the risk, whether it be fire hazard, earthquake mitigation, or both. To do this, you also need a primer on how chimneys work.

Whenever a fire burns, whether in a fireplace or a gas-fired furnace, it produces toxic gases that must be directed out of the living area to the outside. The purpose of a chimney is to release these gases.

For many years chimneys have been constructed with flues, also known as liners or flue liners. Masonry chimneys are lined with 2-foot segments of clay pipe that are mortared together during chimney construction. A chimney liner starts at the source of the fire and extends to the chimney top. Modern masonry chimneys are reinforced by rebar — 3/8-inch steel rods embedded in the chimney’s concrete foundation and mortared into the chimney. The purpose of the liner is to prevent gases from leaking from the chimney into the living area. We have seen older chimneys without clay liners.

The inside of the chimney was covered with mortar to provide an airtight seal. These can last many years, but are susceptible to dangerous cracking and leaking and are often unsafe by today’s standards. Sometimes, after a buildup of creosote and a resulting chimney fire, the clay liner gets so hot it cracks. If that happens, the seal is broken and the liner must be replaced.

In your case, you describe a crack at the roofline, so we suspect that rather than a chimney fire, you suffered some earthquake damage. We believe that your chimney is not reinforced and that there may or may not be a flue liner.

There are alternatives other than tearing down the chimney and rebuilding it from the ground up. Ask the inspector and the others you’ve consulted why they recommend a complete rebuild instead of having a mason deconstruct the chimney to the point of the crack and rebuild the chimney above the roofline.

If the problem includes a damaged clay liner, that can be replaced in the process. If there is no liner, a new one can be installed.

If the answer is earthquake damage prevention, this alternative does not provide increased protection against earthquake damage.

You have another alternative, depending on the building code in your city: Tear the existing chimney down to the firebox and reconstruct it with lightweight framing, install a metal flue, then install brick veneer over the plywood sheeting attached to the framing.

This, of course, is a middle ground between a complete rebuild and simply repairing the upper portion of the chimney with non-reinforced masonry.

For more information on this alternative check out www.abag.ca.gov/bayarea/eqmaps/fixit/chimneys.html.  There is a slide show at this site with lots of good information.

Also, your local building department is a great resource. Your taxes pay for it, so use it.

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