How Can I Tell My Chimney Is About To Fall Down?

Q: Many of my neighbors have had portions or all of their chimneys rebuilt in the past several years. All our homes are roughly the same age. How can I tell if my chimney needs work before it literally falls down on my neighbors, my roof or me?

A: You'll find the most obvious chimney damage and earliest deterioration will be at or near the top. The exhaust in any chimney cools progressively as it rises, sometimes reaching the dew point and leaving condensation on the inside of an unlined flue. This supply of water and exhaust tends to eat away at the mortar holding the exterior facing brick. The higher on the chimney you look, the cooler the exhaust, the more condensation. Lined chimneys (concrete, metal or clay usually) aren't prone to mortar softening as readily; the flue liner keeps the cold outer chimney out of contact with the warmer moist exhaust, reducing the chance of condensation and allowing any that may occur to fall lower into the flue and evaporate. Many readers have written in the past about mysterious leaks at their chimneys they assumed were rain; quite often this can be traced to condensation in an unlined chimney, particularly after converting from oil to gas heat. The moisture can literally travel through the brick to the outside. Sun and rain of course are huge contributors to deterioration of mortar as well.

If you can't climb up on the roof to inspect the chimney itself, you may be able to view it with the help of binoculars. As the chimney ages, the mortar softens and falls out. The bricks begin to shift from their original positions as they loosen, falling out in extreme circumstances. The top, and all rows, of bricks should be in place and not out of line. Scrape a screwdriver into the mortar joints to check for security. The joint should be firm. When it's just softening or powdery. it can usually be "tuckpointed." In the worst cases, the mortar is just plain nonexistent, requiring a rebuild.

Older chimneys will be wide open at the top, while newer chimneys will have mortar "caps." Many of these caps are cracked from freezing and thawing, allowing moisture to enter and cause damage. While you're on the roof, check the flashing or tarring at the roof/chimney intersection.

Like humans and sunflowers, chimneys tend to lean toward the sun as they age. When you see a leaning chimney, it's almost always leaning to the south, lacking other structural deficiencies. You likely have deterioration to some degree. Fully exposed chimneys on the south side of a building (as opposed to a chimney coming up through the middle of a roof) suffer this fate the worst. Look at the intersection with the roof from above and below; it should be tight and uniform along the entire height. If the chimney passes through an attic, this is the place to check for leaks around the flashing at the roof.

The firebrick in the firebox should have intact mortar lines, the bricks themselves snug. It's rare not to see cracked individual bricks and limited missing mortar. This doesn't make the fireplace unusable until the bricks are broken completely, loose and falling out of position.

Q: A few double pane windows in my home have fallen victim to BB's. The insulating value is obviously compromised, but can I seal the hole between the panes with silicone?

A: You could do this, yes, but you will likely suffer that typical fogging that occurs on windows with failed seals. I'd just leave it open (only one side is broken?), budget for replacement window panes and go hunting for a neighborhood kid with a BB gun.

Several readers contacted me regarding my Nov. 20 answer to a question about poor hot water flow at the tub and shower (cold was fine) where hot water flow was excellent throughout the rest of the home. Not knowing the type of supply piping (I assumed galvanized) or the valve type or configuration (I assumed mixer valve - 1971), I suggested a pipe between the valve and the last tee was partially blocked. My assumption about the valve being operational could be wrong even with good cold flow on a mixer valve. Removal of the valve assembly may reveal a broken O-ring or other blockage on the hot side. Thanks plumbers, especially Bill, for keeping me straight.

Ask The Expert answers readers' questions every Saturday. Send questions to Ask The Expert, Seattle Times, P.O. Box 70, Seattle, WA 98111, or call 206-464-8514 to leave your questions on Ask The Expert's recorded line. E-mail address is dhay@seattletimes.com Sorry, no personal replies.