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Published: Sunday, 5/6/2012

Chimney crown needs special attention

Q: My chimney needs repair. I had a few contractors come to assess its condition, but none made me feel very comfortable. They all said the chimney top (some called it a crown) is in bad shape, but each recommended a different way to fix it. If you were going to repair or rebuild the top of your chimney, what would you do?

A: If you want good quotes, you have to tell the contractors exactly what you want done. But you don't yet know that.

The top or crown is supposed to keep the chimney dry. Water that seeps into a chimney can cause all sorts of damage, especially if the water that soaks into the masonry freezes.

Water expands as it freezes, and this pressure will destroy the chimney's mortar.

Many bricklayers build chimney crowns the wrong way.

The chimney crown should be made of real concrete, not the mortar mix used for the brick or stone. The mortar mix rarely has any stones in it. The stones in concrete are what really give it its strength.

If the chimney has clay flue liners, the concrete material should never touch them. The flue liners get hot, and they expand, which can crack the crown.

The flue liners must be wrapped in foam about three-eighths of an inch thick to isolate the flue liners from the concrete mix.

Once the crown is hard, some of the foam is cut away, and the gap is closed with special masonry caulk.

The concrete for the crown must contain reinforcing steel or mesh to help prevent large cracks, and the top of the crown must be sloped so it sheds water.

The underside of the crown overhang must have a groove all the way around to stop water from rolling down the sides of the chimney.

It should be at least 1/4 inch wide and at least ⅜ inch deep.

A flashing should be placed on top of all the brick that separates the crown from the masonry of the chimney itself. The flashing is the last line of defense should water make it through the chimney crown.

The flashing can be made from asphaltic-rubberized membranes or metal that is soldered to make one solid piece. It should extend past the face of the chimney on all four sides and be turned down at a 45-degree angle so water that runs off drips out past the face of the chimney.

The concrete of the chimney crown must be cured properly to gain maximum strength. It should be covered tightly with plastic for three or four days after it's poured. The mason can spray on a clear curing compound should he not want to put on the plastic.

Once the work is complete, it's a great idea to apply a silane-siloxane clear penetrating water repellent on the sides of the chimney as well as the crown. This sealant will help keep wind-driven rain from getting into the chimney.

Tim Carter is a former builder of custom houses. For free home-improvement information go to www.AsktheBuilder.com and sign up for Tim's free newsletter. To ask Tim a question, click the Ask Tim link on any page of the Web site.



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