Settling a heated debate about fireplace fixes

Q: I used my fireplace for the first time this year and noticed that the brick lining is cracked. A friend told me that I should never use mortar to repair the bricks, but he didn't know exactly why. He only knew it was not a good idea. I decided to wait until I could find out more about repairing the fireplace. Can you help?

A: Mortar is mixed with water to form a paste, which is then used to set bricks or to repair the joints between bricks. When mortar is used in a fireplace, the water trapped in the mortar becomes steam and can explode, spewing hot ashes from the fire all over the hearth and floor. Not a good idea!

First, loose or damaged firebricks should be replaced, which is usually a job for a professional mason. Creosote, an oily substance and highly flammable byproduct of burning wood, can build up in the cracks and loose joints of the firebricks. Over time, the creosote can become brittle and hard to remove without doing further damage to the fireplace bricks. The creosote can also migrate beyond the cracks in the bricks to the space between the firebox and the chimney structure. If ignited, a chimney fire can quickly spread to become a house fire. As you can see, proper replacement is very important.

If you simply need to patch loose joints that sometimes form between the firebricks, use a refractory fireclay, purchased at home stores and fire shops.

In a process known as "tuckpointing," a stiff, small-bladed pointing tool is used (you can also use a butter knife or a pocketknife) to force the fireclay into the loose joint. First clean the joint and remove any loose refractory cement. Use a knife to make a small groove in the joint to accept the new clay. There are also high-temperature caulks available, but I have found them difficult to apply in a way that achieves a professional-looking repair. The caulk smears and is hard to remove once it has dried.

While you are repairing the firebox, take a look at the flue liner to make sure it is clear of debris and that there are no visible cracks in the liner. Make sure the damper is working properly and, if you have a chimney cap, make sure it is attached firmly and the protective screening is in a good condition. Repair the flat part of the chimney, the wash, using a silicone caulk or a nonshrinking cement to prevent water entry, which can damage the exterior of the chimney. Outside, remove overhanging tree limbs that can burn or restrict the drafting of the chimney. Fireplace and chimney safety is paramount.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. E-mail questions to him at d.Barnett@insightbb.com. Sorry no personal replies.