Chimney repair in Boulder.

1. Crowns.

   The crown is the concrete slab on top of the chimney. It sheds water away from the flue and the brick, and it takes the worst of a Boulder winter: freeze at night, thaw by noon, again the next day.

   Water in a hairline crack expands about 9% as it freezes, so the crack opens a little more each cycle. Hairline cracks can be sealed while they are small; a crown that has broken up or pulled away from the flue tile needs rebuilding.

   

2. Caps and spark arrestors.

   A cap keeps rain, snow, birds and squirrels out of the flue. Its screen is the spark arrestor: the International Residential Code (R1003.9.2) sets its openings between 3/8 and 1/2 inch, small enough to stop embers and large enough not to clog.

   Wind is the cap’s other enemy here. The NCAR Mesa Lab in Boulder recorded a 137 mph gust on January 17, 1982; downslope winds loosen fasteners and tear screens long before a cap actually blows off.

   

3. Flashing at the roof line.

   Flashing seals the joint where the chimney comes through the roof. Snow piles against the uphill side and melts slowly, which is exactly where lifted counterflashing or dried-out sealant lets water in. The first sign is often a stain on the ceiling or in the attic beside the chimney, not inside it.

   

4. Mortar joints and brick.

   Mortar is meant to wear before the brick does. Once the joints open, water gets behind the face of the brick, and the freeze pops it off in flakes: that is spalling.

   Tuckpointing replaces the failed mortar before the bricks loosen. Bricks that have already spalled or cracked through are cut out and replaced.

   

5. Firebox.

   Firebrick and refractory mortar take the direct flame. Open joints and cracked brick let heat reach what is behind the firebox, so small repairs here are cheap insurance for the framing around the fireplace.

   

6. Flue liners.

   The liner carries smoke and heat out through the middle of the chimney. A cracked or offset tile can let heat reach the framing and let gases into the house, and it rarely shows from the firebox or the roof: it turns up on the video scan of the flue that a Level 2 inspection includes.

   

7. Framed chases and chase covers.

   Not every chimney is brick. A framed chase, sided like the house, ends in a metal chase cover. When that cover rusts or sags, water pools on it and runs down inside the chase. A cover that sheds water, with a cap and screen on the flue, keeps the inside dry.

   

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