What is the average size of a chimney?
What drives chimney size
- Number of flues. Each fireplace, furnace or water heater needs its own flue. A chimney serving a fireplace plus a furnace is wider than a single-fireplace chimney.
- Flue liner size. Clay liners come in common nominal sizes such as 8 by 8, 8 by 12 and 12 by 12 inches, sized to the fireplace opening.
- Masonry thickness. Brick walls around the liner add several inches on each side.
- Framed chases. Many Georgia homes with factory-built fireplaces have a wood-framed chase covered in siding or stucco, often wider than a brick chimney, with a metal chase cover on top.
Height rules
The residential code's termination rule, often called the 3-2-10 rule, requires a masonry chimney to extend at least 3 feet above the highest point where it passes through the roof and at least 2 feet higher than any part of the building within 10 feet. Factory-built chimneys follow their listing instructions.
Why width matters for your roof
A wide chimney on a slope acts like a dam. Water and debris pile up on its uphill side and work under the flashing. The code requires a cricket, a small peaked roof that diverts water around the chimney, when the chimney is more than 30 inches wide parallel to the ridge and does not intersect the ridge. Many older Georgia chimneys that leak on the uphill side are wider than that and were built without one.
Flashing scales with size
Larger chimneys need longer runs of step and counterflashing and more careful corner details.