When the call came to check out a leaking chimney, it
didn’t take long to find the problem. Measuring nearly 7
feet wide, the chimney sat like a dam at the bottom of a steep
roof, with no cricket to divert water around the sides. The
existing flashing and counterflashing were poorly installed and
had little chance of keeping out moisture.
To remedy a situation like this, I strip off the roofing around
the chimney and tear out the old flashing. After repairing any
damage to the sheathing, I install the cricket that should have
been there in the first place, then flash everything with new
copper. This project was no exception.
Prepping the Roof
Working from the top down, I began stripping shingles high
enough to allow plenty of room for the cricket (see
Figure 1). I used a brick trowel to loosen the
shingles and a small pry bar to pull out the nails.
Figure 1. To make room for the cricket, the author
strips the roofing around the chimney (top left). A brick
trowel is handy for breaking the bond between shingles without
damaging them (top right). The cricket is framed like a small
roof; the author builds it on the ground, then installs it in
one piece (bottom).
When removing shingles for such jobs, I usually take out whole
shingles rather than cutting individual tabs. This makes
tear-out and reinstallation a lot faster. At the chimney, I
removed the flashing and knocked off any globs of hardened tar
that could get in the way.
Installing the cricket. To make life
easier, I generally build the cricket on the ground, then carry
the assembly up to the roof and install it in one piece. I
start layout with the “gable” — the triangle
that butts against the back of the chimney — subtracting
an inch from the total width to allow room for the shingles. I
usually match the cricket’s pitch to the roof, to keep
the framing as simple as possible. The cricket pictured here
had a pitch shallower than the main roof’s because it had
to clear the low shoulder on one side of the chimney.
After securing the cricket, I applied peel-and-stick
underlayment and installed the new shingles, weaving the
valleys between the cricket and the main roof.
Flashing the Cricket
Where shingles butt the chimney, I typically use
7-inch-by-7-inch L-shaped copper step flashing — one per
shingle (see sidebar). To turn the vulnerable
corners of the chimney, I cut and bend pairs of step flashings
that fit tightly together. The first step flashing completes
the section stepping up the main roof, while the second begins
the smaller step up the cricket. When I bend these flashings
into shape, I overbend slightly to ensure that the copper will
lie tight against the chimney when it’s installed. I fill
the corner where the two flashings intersect with sealant.
When counterflashing a brick chimney, I ignore the vertical
joints in the brickwork, because the bonding pattern rarely
matches the pitch of the roof. Instead, I lay the pieces of
counterflashing out so that they remain uniform in size and
follow the roofline.
To do this on the 7/12-pitch cricket roof, I laid a 2x4 on the
roof and scribed a line on the chimney 31/2 inches up from the
roof deck. Starting from the corner, I went up one brick course
from that line and marked horizontally along the mortar joint
until I reached my scribe line. Here I marked a plumb line up
to the next mortar joint; I repeated this process until I
reached the top of the chimney. On the steeper 12/12 main roof,
I skipped up two or three courses instead of a single course. I
used an angle grinder equipped with a diamond blade to cut
1-inch-deep kerfs into these marked mortar joints.
When my local sheet-metal shop fabricates my counterflashings,
they make them slightly oversized, so that I can trim them
on-site to fit. I also have the shop bend a 1-inch-wide
“L” at the top of the flashing, finished with a
U-shaped hook (Figure 2). To anchor
counterflashing into the kerfs in the mortar joints, I wrap
strips of lead around the hook, then use a punch to pack the
lead tight. As I proceed up the chimney, I lap the
counterflashings by 2 inches and use stainless steel
sheet-metal screws (which, like lead, are compatible with
copper) to anchor each piece to the previous one.
Figure 2. Counterflashings have a short
“L” finished with a hook (top left). The author
inserts the “L” into the kerf cut into the mortar
joint, then uses a punch to tightly pack the lead strips
wrapped around the hook into the kerf (top right). Stainless
steel sheet-metal screws fasten the counterflashings together
Finishing the Joints
Either mortar or caulk can be used to help lock the
counterflashing to the masonry and finish the joints.
I’ve been back to redo roofs that I shingled nearly 30
years ago and haven’t noticed any significant difference
in long-term durability between the two. On this project, I
chose to use a grey silicone-based caulk because it has a
slightly greater resistance to water intrusion (Figure
Figure 3. When caulking the vertical and horizontal
edges of the counterflashing, the author uses a tuck-pointer to
pack and smooth the joints and painter’s tape to keep the
brickwork clean (top). The screw heads are sealed with clear
silicone caulk (bottom).
For my final step, I coated the screw heads with clear silicon
John Carroll is a mason and builder in