Chimney repair in Aurora.

1. Crowns.

   Across the top of the masonry lies the crown, a concrete slab that sends rain and melting snow off the roof and clear of the brick and flue. That job leaves it more exposed than any other part of the stack.

   NOAA's normals for the Cherry Creek Dam station give January days between 16.9°F and 47.3°F and 172.0 nights a year at or below freezing. On each such night, ice in a hairline crack forces it a little wider. A hairline crack can be sealed; a slab that crumbles or has come away from the liner must be removed and recast.

   

2. Caps and mesh.

   The cap keeps birds, squirrels, rain and snow out of the flue, and its screen is the part that stops sparks. Under section R1003.9.2 of the International Residential Code, screen openings may measure no more than 1/2 inch.

   Metal caps get dented by hail, loosened by wind and corroded by rust. After a storm, check for dents, backed-out screws, rusted seams, and a screen that sags or is blocked.

   

3. Flashing at the joint.

   Flashing is what seals the gap where chimney meets roof. Snow melts slowly on the uphill side, so leaks tend to start along that edge when it lifts or when the sealant bead wears thin; in a hot summer the sealant can dry out long before winter tests it. The first sign inside is often a brown ceiling stain or damp on the attic wall beside the chimney.

   

4. Brick and its joints.

   Mortar is made to yield before the brick does. When joints open, damp gets in behind the face of the brick, and each freeze can pop off a flake, a failure known as spalling.

   Tuckpointing rakes out the worn mortar and packs in new before any brick moves. A brick split clean through or deeply flaked is cut out and replaced.

5. Inside the firebox.

   Heat and flame bear directly on the firebrick and on the mortar between the bricks. When joints gape and bricks crack, heat can reach the framing behind, which is why a small gap closed early spares the framing.

   

6. The flue liner.

   Between damper and cap, the liner takes smoke and heat away from the house. If a tile has cracked or moved, or a seam of metal liner has corroded, heat can get to the framing and fumes into living space, and from the roof or the firebox the fault generally cannot be seen. A camera passed through the flue shows it, and so does a Level 2 inspection. The median Aurora home dates from 1985, so the typical flue has more than forty years of service.

   

7. Sheet-metal tops on framed chimneys.

   A framed chimney is not masonry at all but a wood box dressed to match the house and finished with a sheet-metal lid. When that lid rusts through or sags, rainwater gathers on it and drains down inside the chase. A flue cap with a screen, fitted below a lid that sheds water, keeps the inside dry.

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