Whether running up an exterior wall or emerging through the
roof, the last few feet of chimneys are completely exposed to
the wrath of Mother Nature. These positions mean that chimneys
are regularly subjected to intense heat, windblown rain,
freezing temperatures, and other environmental insults.
The fact that most chimneys sail through decades of such abuse
with little or no damage is truly a testimony to the durability
of masonry. The one possible chink in the chimney's armor is
the cap (or "wash," as it's called in my part of the
Why Caps Fail
There are four reasons chimney caps fail: First, the cap is
the most exposed area of the chimney and the only horizontal
surface. It's therefore more likely to absorb and collect water
than any other part of the chimney (assuming the roof flashing
was properly installed).
Second, masons usually make the cap out of the same mortar
used to lay the brick. That mixture is too wet for the cap,
which is often 2 or 3 inches thick as it rises toward the flue.
As a result, shrinkage cracks often develop before the mortar
cures (see Figure 1).
Figure 1.Weather has worn away the thin edge
around the perimeter of this cap and numerous cracks have
Third, masons often taper the mortar to a feather edge along
the perimeter of the chimney. Over the long haul, that edge is
too fragile for the harsh environment into which it's placed,
and it invariably wears away, leaving a flat surface that
Fourth, masons rarely isolate the cap from the flue liner.
Like all building materials, brick, mortar, and flue liners
expand and contract in response to temperature changes. In the
winter, when the furnace is running and the family is most
likely to build and maintain a fire, the flue can expand
upward. The cap is exposed to freezing cold and bonded to the
exterior brick. If the cap is also bonded to the flue liner, a
slight upward thrust of the flue can crack the cap.
All four of these problems allow water to enter the cap. Over
the years, the cap deteriorates and a downward cycle develops:
With each winter, more water enters the top of the chimney and
causes more damage to the cap. Eventually, water flows into the
chimney and makes its way into the house — sometimes
in large quantities.
A Sound but Unattractive
Recognizing the shortcomings of the common mortar cap, many
masonry industry experts advocate a reinforced concrete cap
about 4 inches thick, isolated from the flue. This design
solves the last three problems just described and leaves a
tough concrete slab rather than a thin layer of mortar exposed
to the elements. If this kind of cap is made with care, it
should readily shed rainwater and provide many decades of
This design makes perfect sense from a structural point of
view. The only problem is that a concrete slab on top of a
chimney is — putting it charitably —
Chimneys can be beautiful structures and are often a major
focal point for the home's exterior. The way they're
proportioned and detailed can be extremely important for
aesthetic reasons. A concrete cap visible from the ground or
from a window, therefore, is often aesthetically unacceptable.
This is especially true on an older house or one designed along
Best of Both Worlds
To satisfy aesthetic needs and yet build a sound, durable cap,
I've come up with the technique described and illustrated in
this article. This method eliminates the feathered edge and
provides for a reinforced concrete cap that's isolated from the
flue or flues. The main difference between this design and the
one advocated by industry literature is that it hides the edge
of the concrete slab.
First, I make sure that the top of the final flue liner ends
up at least 5 1/2 inches above the top of the final course of
brick. Before laying the final course, I cut the brick along
their length at a 45-degree angle. I've constructed a simple
carriage out of wood that holds the brick at the proper angle
Figure 2.A wood jig clamped to the sliding table
of the saw holds the brick at a 45-degree angle for safe
After clamping the carriage to the sliding table on the wet
saw, I can slice the brick at the desired angle safely and
quickly. To create the illusion that the top row consists of
full brick, I avoid cutting the corner brick all the way
through. Instead, I stop short of the edge, then cut at an
angle across the width of the brick. I then use a 4-inch angle
grinder to clean out the small amount of material that the wet
saw doesn't get.
I return and install the cap a day or two after laying this
top course of brick. I first wrap the flues in fiberglass
insulation to keep the concrete from bonding to them (Figure
3). After taping the insulation to the flues, I fabricate a
rebar grid that fits inside the brick and around the flues.
When the grid is ready, I mix a stiff batch of concrete and
begin packing it into place. After putting a couple of inches
of concrete in place, I install the rebar grid and continue
packing concrete over it.
Figure 3.Wrapping the flue liner with fiberglass
insulation isolates it from the cap.
I begin shaping the cap after reaching the top edge of the
brick, molding it into the shape of a hip roof. I like to make
the lines created by the intersecting planes as crisp and neat
as possible, not just for aesthetic reasons: I want the
surfaces as straight as possible so that rainwater runs quickly
For the same reason, I put plenty of pitch on the cap. The
wash rises 3 1/2 inches from the top edge of the brick to the
side of the flue, which leaves a couple of inches of flue
projecting above the cap — just enough room for a
metal flue cap to clamp onto later, if desired.
After building up and shaping the cap, I let it sit for 30 to
60 minutes. When the cap begins to stiffen, I mix a small batch
of mortar that matches the brick mortar and carefully apply a
thin coat to the cap to make it more water resistant and to
ensure a color match (Figure 4).
Figure 4.The completed cap rises sharply up to the
flue liner and readily sheds rainwater.
I let the cap cure for a few days before returning to pick out
the top inch or two of the insulation, then finish the job by
filling the seam with a gray polyurethane sealant.
John Carrollis a mason builder and writer from
Durham, N.C. This article was adapted with permission