What is chimney capping, and should chimneys be capped?
Short answer. Chimney capping means installing a cap, a metal hood with mesh sides, over the top of the flue to keep out rain, animals, leaves and sparks, and most working chimneys should have one. The term can also mean sealing off an unused chimney entirely, which still needs some ventilation so moisture does not build up inside.
Cap, crown and chase cover
- Cap: the metal cover with a hood and screen that sits over the flue. Stainless steel and copper last longest, while galvanized steel rusts.
- Crown: the concrete slab across the top of the masonry that sheds water off the brick. A cap does not replace a sound crown.
- Chase cover: on a framed, sided chimney, the metal pan over the whole top, with the cap sitting over the flue pipe.
Why most chimneys should be capped
- Rain: an open flue lets water fall onto the damper and smoke shelf, rusting the damper and soaking the masonry.
- Animals: birds, squirrels and raccoons nest in uncapped flues, a common spring nuisance in Georgia.
- Debris: leaves and twigs can block the flue, which is a fire and carbon monoxide hazard.
- Sparks: the mesh acts as a spark arrestor, helping keep embers off the roof and nearby trees.
- Wind: a cap can cut down on wind blowing down the flue.
Capping an unused chimney
If a fireplace is no longer used, some homeowners have the chimney capped off completely. A sealed chimney still needs a little ventilation, or condensation can build up inside and dampen the walls around it. If any appliance, such as a gas furnace or water heater, still vents into the chimney, it cannot be sealed, so have a chimney professional confirm what uses the flue first.
Sizing
Caps are sized to the flue or to the full crown, with enough screen area and height that they do not restrict the draft. A chimney with several flues can take a single cap over all of them.
A cap keeps water out of the flue, but a leak where the chimney meets the roof is a flashing problem; see chimney leaks and crickets.