Building a Chimney Crown
Chimney Crowns
Introduction:
A properly built chimney is
constructed with a crown or top, sometimes called a
chimney cap. Building a chimney crown is one of the
most important elements of the chimney. Like a
house roof keeps the house dry, the crown on the
chimney is designed to keep the chimney dry.
Even the smallest amount
of water that seeps into a
chimney can cause damage, especially if the chimney
is exposed to freezing weather. The water that
soaks into the masonry can and will freeze, causing
it to expand. As it expands the pressure works to
break the bonds of the mortar used to construct the
chimney.
During
chimney construction the chimney mason builds the
chimney crown with an overhang to shed the water.
The
overhang prevents water from flowing off the top of
the chimney and down the sides of the chimney. Over
time this will cause chimney stains.
The chimney crown is made
using real concrete not the mortar mix used for the
brick or stone construction of the chimney. The
mortar mix doesn’t have any stones in it. The stone
in the concrete is what really give it its
strength.
Most
chimneys are built with a clay flue liner. The
crown or top should never touch the flue liners.
The flue liners get hot from the wood stove fire,
furnaces, boilers and they will expand. This
expansion often cracks the top or crown.
Normally the mason will wrap
the flue liners with a thin foam insulation about
three-eighths of an inch thick to isolate the flue
liners from the concrete crown. Once the crown is
hard, the insulation is cut away and replaced with
a special caulk. The crown is reinforced with steel
or wire mesh to prevent large cracks from forming
in the concrete. In addition, the crown is sloped
so it sheds water.
When the crown is completed it
needs to be cured for several days to gain maximum
strength. After
the crown is cured, it's a good idea to apply a
clear penetrating water repellent on the sides of
the chimney as well as the chimney crown. This
sealant will really help keep wind driven rain from
getting into the chimney.
Related articles:
Fiber
Reinforced Concrete
Foundation
Footings
Foundation Repairs
How to
Make Stronger Concrete
