Repair chimney on the cheap

2 ways to avoid reconstruction

Q: I have a 50-year-old one-story freestanding brick chimney with one flue (see photo).

A chimney sweep recently did an inspection and noted the part of the chimney extending above the roofline could be tilted away from the structure by pushing on it. Apparently, the bond at or near the roof flashing had been compromised perhaps 10 years earlier when a new roof was installed.

The sweep recommended the chimney extension be reinforced by removing the mortar cap, inserting rebar into the four corners of the chimney down to a sill, then filling the void with mortar/concrete.

Is this method of construction considered appropriate for the aforementioned problem or should other methods be considered?

A: Thanks for including a picture with your question. Without it, we’d be confused by your reference to a "freestanding" chimney.

You’ve got a pretty standard fireplace with the chimney on an exterior wall and the firebox opening on the inside of the house. The entire structure is an integral part of the house and not freestanding at all.

The chimney sweep’s suggestion is indeed appropriate — for new construction. It’s the way the chimney should have been built in the first place. But we can’t see how you’ll be able to pull it off as a retrofit. Fear not, though — there are a couple of simpler ways to solve the problem.

Kevin learned the basics of brick fireplace construction from his father-in-law, Ed, a master mason who worked in the trade almost to the day he died. From the bottom up, the chimney rests on a concrete footing that is part of the foundation. The chimney footing is tied to the rest of the foundation with steel rods (rebar). What you see as a freestanding chimney is part of the structure of the house.

Embedded in the chimney footing should be four pieces of 3/8-inch rebar that extend up at the four corners of the chimney. Course after course of brick is set, with the void in the middle being filled with mortar to make a solid mass. At the desired height a firebox is constructed opening into the house.

As the chimney rises, a flue liner, made of short sections of clay pipe, is fitted together with mortar to discharge smoke and heat from the firebox up the chimney. A half-inch air gap is left between the face brick and the flue pipe. Finally, a cap is placed on the top of the chimney to repel rain and critters.

Metal flashing is embedded in the mortar joints to direct water away from the joint where the roof meets the chimney. This seems to be the point of failure, although looking at the picture you’ve provided we don’t see any cracking or other evidence of movement.

If you follow the sweep’s suggestion, you’ll remove the chimney cap and hope that the brick is not solid. If the brick has voids, you’ll try to snake a piece of rebar down the 20 feet or so to the footing. You won’t be able to secure the rebar to the footing.

We also think you’ll run into a fair amount of mortar that’s sloughed off during initial construction blocking the way. This solution seems to us to be the most work and most expensive and won’t provide a better fix.

There are two better ways to address the problem. Option 1 is the simplest: Reinstall the existing metal flashing set into the mortar joints. In our view, this is the best way to go. With a diamond grinding wheel, cut out the mortar around the top of each piece of flashing. Gently bend the flashing back and remove the rest of the mortar.

Once the joint is open, remortar the joint and bend the flashing into the fresh mortar. Let the mortar dry a bit and shape the joint so that it matches the others. This is called tooling. We have used a finger for small jobs but recommend buying a simple, inexpensive tool for this. Any good hardware store will have one.

Repeat the process on each piece of metal flashing.

Option 2 is to stabilize the chimney with a diagonal bracket. Bolt a piece of angle iron or pipe to brackets screwed into the topside of a rafter and the top of the chimney. The diagonal brace may not please the senses, but it will keep the chimney from falling over.

Either choice is better than partially demolishing and reconstructing the chimney.

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