A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates
hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace
from human living areas. Chimneys are typically vertical, or as near as possible to vertical, to
ensure that the gases flow smoothly, drawing air into the combustion in what is known as the
stack, or chimney effect. The space inside a chimney is called the flue.
The height of a chimney influences its ability to transfer flue gases to the external
environment via stack effect. The height is to ensure the pollutants are dispersed over a wider
area to meet legal or other safety requirements.
As a result of the limited ability to handle transverse loads with brick, chimneys in houses
were often built in a "stack", with a fireplace on each floor of the house sharing a single
chimney, often with such a stack at the front and back of the house. Today's central heating
systems have made chimney placement less critical, and the use of non-structural gas vent pipe
allows a flue gas conduit to be installed around obstructions and through walls.
Most modern high-efficiency heating appliances do not require a chimney. Such appliances are
generally installed near an external wall, and a noncombustible wall thimble allows a vent pipe
to run directly through the external wall.
On a pitched roof where a chimney penetrates a roof, flashing is used to seal up the joints. The
down-slope piece is called an apron, the sides receive step flashing and a cricket is used to
divert water around the upper side of the chimney underneath the flashing.