creosote can combust in a flash fire, often producing temperatures in excess of
the chimney top, and in some cases lower a vid-
2,000°F. For homeowners, a chimney fire is a terrifying experience, for it may liter-
eocamera to inspect the flue linings. That video is
ally roar for extended periods inside the entire flue, flames shooting skyward from
helpful if the chimney needs relining because
the chimney top as though from an inverted rocket. if there are cracks in mortar or
homeowners can see the damage for themselves
flue tiles—or no flue tiles at al —those superheated gases can “breach the chimney”
and make an informed decision.
and set fire to wood framing. at that point, the whole house can go up in flames.
Because chimney cleaning takes serious
you can prevent chimney fires simply by inspecting and cleaning the chimney
elbow grease, working atop the roof is often the
most effective way to brush clean a chimney. But
regularly. in general, don’t burn green (unseasoned) or wet wood. give a fire enough
working on a roof is inherently dangerous to you
air to burn completely. each time you start a fire, open the dampers and air controls
and your roof shingles, which can be easily
until the fire is burning well. don’t burn christmas trees (whose unburned resins col-
abraded, torn, and dislodged, leading to leaks.
lect as sticky masses inside flues), wrapping paper, or glossy-coated papers because
Because many people put off cleaning a chimney
their emissions can corrode stovepipes and attack mortar joints. above all, never
until it’s almost heating season, they frequently
use chimneys whose tiles or mortar joints are cracked or chimneys that have no
go aloft when the weather is inclement or the
flue lining.
roofs are slick after a rain. For all these reasons,
if you’re considering buying a house, have its chimney professionally inspected if
you’re probably better off hiring an insured pro-
you see signs of a chimney fire such as creosote flakes on the roof or the ground;
fessional who is certified for cleaning and
scorched or cracked flue liners or chimney crowns; warped dampers; or charred studs
inspecting.
or joists near a chimney. Many local codes require inspections before homeowners
If you are determined to clean the flues your-
fire up new wood-burning appliances.
self, start by turning off the furnace and other
appliances (such as water heaters) that vent to
the flues and disconnecting their vent pipes.
Using duct tape, tape plastic over the thimbles
that open into living spaces, to prevent dislodged
soot from entering. If you have a fireplace, open
its damper to allow dislodged soot to fall into the
fire pit. Then firmly tape sheet plastic around the
fireplace opening. But before you start, suit up.
Dislodged creosote and soot are highly carcino-
genic, so wear a respirator with replaceable car-
tridges, tight-fitting goggles, gloves, and dispos-
able coveralls.
To clean a chimney thoroughly, you’ll need
special brushes, which scrub flue surfaces with-
out damaging them. Today, many professional
This flue tile and mortar cap were
sweeps favor polypropylene brushes to clean
cracked by a chimney fire in a flue that
sooty flues and stiff steel-wire brushes for flues
was overdue for cleaning.
with heavy use and creosote buildup. These
brushes come in various sizes to match the most
common flue shapes. You can screw them onto a
series of 3-ft. to 4-ft. rod sections or to a continu-
ous flexible rod (on a reel) up to 50 ft. long.
After you’ve brushed the flues well, allow the
dust and debris to settle before removing the
plastic covering the fireplace and other openings.
Shovel up the soot and debris at the bottom of
each flue and from the fireplace, then vacuum all
areas thoroughly. Don’t forget the soot that may
Masonry
225
Installing a new crown. Before you start,
spread sheet plastic around the base of the chim-
ney to catch falling mortar. There are essentially
two types of crowns. If your region gets a lot of
precipitation, pour an in-place concrete crown,
which overhangs the chimney 1 in. to 11⁄2 in. and
acts as a drip cap, keeping rain and sleet off
bricks near the top. Otherwise, use a trowel to
build a sloping mortar crown that runs flush to
the chimney faces. A flush crown isn’t as durable
as an overhanging crown but is much quicker
to build.
To construct an overhanging crown, as shown
on the facing page, measure the outside dimen-
sion of the chimney top and build a frame from
2x2s that slides snugly over the chimney top.
Shim from below to wedge the 2x2 frame in
place so its upper face is flush to the top course
of bricks. This 2x2 frame (actual dimension,
A professional chimney sweep needs
11⁄2 in. by 11⁄2 in.) creates a 11⁄2-in. overhang.
a variety of brushes. The reel at right
contains a 50-ft. flexible rod that can
Next, cut strips of plywood 31⁄2 in. wide and as
be resting in thimbles or on the fireplace smoke
push and pull brushes.
long as the sides of the frame; using a screw gun
shelf (see p. 230).
or an impact driver, screw these plywood strips
to the 2x frame so that the strips stick up 2 in.
rePlacing a cHiMney croWn
above the top of the frame. The resultant ply-
A masonry crown is a beveled flue collar at the
wood frame keeps the concrete in place and cre-
top of the chimney, sloping gently to direct water ates a 2-in.-thick edge.
away from flue tiles. When crowns weather and
Wet the bricks with a brush or a spray bottle,
crack, water can drain between flue tiles and
and you’re ready for the concrete. Use a cement-
brick, seep into mortar joints, and freeze, thereby rich, fairly stiff concrete mix: 1 part portland
cracking flue tiles, bricks, and mortar joints. In
cement, 2 parts sand, and 2 parts 3⁄8-in. gravel is
warmer seasons, this water can leak into living
about right. To prevent cracks, mix in a handful
spaces, stain walls, and linger as acrid combus-
(1⁄4 cup) of fiberglass fibers, which concrete sup-
tion smells. Replacing the crown is easy enough
pliers carry. As you place the concrete into the
if the chimney is not too tall—and the roof not
plywood frame, use a trowel to force it into the
too high or too steeply sloped. Otherwise, you’ll
corners and to drive out air pockets.
need rooftop scaffolding—which a pro should
Important: Whether you build a mortar crown
install—before tearing down the chimney to
or pour a concrete one, wrap the flue liners with
sound masonry and rebuilding from there.
polyethylene bond-breaker t
ape or closed-cell
In warm regions, uncovered flues and crowns
foam strips. This prevents the mortar and con-
are common. However, for winters in cold cli-
crete from bonding to the tile liners and thereby
mates, flues should be capped to prevent the
provides an expansion joint. Without this gap,
entry of rain and sleet that can damage flues dur-
heat-expanded flue tiles can crack a new crown
P R O T I P
ing freeze-thaw cycles.
in a single heating season.
In most cases, a few hammer blows will dis-
If you’re building up a sloping mortar crown,
Few home vacuum cleaners
lodge deteriorating mortar crowns. Put the debris
have the power to suck up a year’s
use a premixed mortar mix. For a slightly more
into a bucket. Then sweep the top of the chimney
worth of soot or the filters to
flexible, crack-resistant crown, substitute a liquid
clean. If some mortar joints need repointing,
confine it. instead of destroying
latex fortifier for part of the water.
attend to that. If many mortar joints are soft and
the house vac, rent a commercial-
Both types of crowns should be sloped away
badly eroded, tear down the chimney to the
grade canister vacuum with
from the flue liners and troweled to a smooth fin-
superfine filters.
roofline, clean the bricks, install new flashing,
ish. In addition, both will cure slower and stron-
and rebuild it. If bricks are cracked or broken,
ger if you cover them with damp burlap or plastic
replace them with new SW grade bricks. It’s okay to protect them from rain and sun. Caulk the gap
to reuse old bricks if they’re solid, but if you must between the flue liners and the crown with a
replace more than a handful, rebuild the chim-
good urethane sealant.
ney with new bricks. They’ll look more uniform
Chimney caps should match the style of the
and last longer.
house: Chimneys on colonials and capes are
often protected by slabs of bluestone bedded into
226 chapter 9
corner tiers of mortared brick. On newer homes
or those with stainless-steel liners, a stainless-
steel or copper cap may be more appropriate. If
zzzzzz overhanging chimney crown
Caulk gap with
there are multiple flue liners, you may need mul-
Flue tile
urethane sealant.
Crown slopes away
tiple caps or an overall custom cap.
from flue tile.
Bond breaker
tape
relining a cHiMney
Plywood frame
confines concrete.
While inspecting a chimney, you may find that it
has no flue-tile lining or that existing tiles are
cracked or broken and too inaccessible to
2 in.
replace. Because superheated gases can escape
through gaps, such a chimney is unsafe to use. In
this case, your options are:
11/2-in.
overhang
Seal up the chimney so it can’t be used,
2x2 frame
and add a new, properly lined chimney
Shims keep
elsewhere. Or tear out the old chimney and
frame in place.
replace it.
Install a poured masonry liner. In this
procedure, a heavy-gauge tubular rubber
balloon is inflated inside the chimney, and the
void is then filled with a cementitious slurry.
A simple 2x2 and plywood frame creates a 1½-in. overhang and a
After the mixture hardens, the tube is deflated
2-in.-thick edge for this site-built concrete crown. Join the frame
and removed. Poured masonry creates a
with drywall screws, and it will be easy to disassemble.
smooth, easily cleaned lining and can stiffen a
weak chimney. Poured masonry systems are
usually proprietary, however, and must be
installed by someone trained in that system.
Install the stove or fireplace insert before install-
S a f e T y a l e R T
This method is expensive.
ing the flue liner.
Assemble the flue pipe on the ground, joining
Install a stainless-steel flue, a sensible
The most dangerous part of
pipe sections with four stainless-steel sheet-metal
choice if you want a solution that’s readily
roof work is going up and down
screws per joint so the sections stay together as
available, quick, effective, and about one-third
the ladder. Minimize ladder trips
you lower them down the chimney. Although pop
the price of a poured masonry liner. Interchange-
by lowering buckets of mortar
rivets could theoretically be used, they are likely
able rigid and flexible pipe systems enable instal-
debris to a hard-hat–protected
to fail under the stress and the corrosive chemi-
lations even in chimneys that aren’t straight.
helper on the ground, who will
cals present in wood smoke.
later fill upward-bound buckets
Installing a stainless-steel liner. Steel flue lin-
Next, insulate the flue pipe, as necessary, with
with concrete mortar, cleaned
ers and woodstoves are often installed in tandem, heat-resistant mineral batts and metal joint tape.
bricks, and so on. (empty 5-gal.
correcting flue problems and smoky fireplaces at
Heat ratings vary. Temperatures inside flue pipes
joint-compound buckets are per-
the same time.
intermittently reach 2,000°F. Flue pipes are insu-
fect.) never stand on tarps or
Start by surveying the chimney’s condition,
lated to keep temperatures constant inside and
drop cloths when working on a
including its dimensions. After steel flue pipe is
prevent condensation, which also prevents accre-
roof because they’re slippery and
installed, there should be at minimum 1 in. clear-
unsafe. roof-side workers should
tion of creosote, which is corrosive. Generally,
ance around it. Thus a 6-in. pipe needs a flue at
also wear safety harnesses.
the first pipe section coming off the woodstove is
least 7 in. by 7 in. Note jogs in the chimney that
not insulated because temperatures are so high
might require elbows or flexible sections. Also
that there’s little danger of condensation. Toward
note obstructions, such as damper bars, that
the top of the pipe, stop the insulation just before
must be removed before you insert the pipe.
the pipe clears the chimney—you don’t want to
If you’re installing a woodstove, too, measure
expose the insulation to the elements.
the firebox carefully to be sure the stove will fit
Carry the flue-pipe assembly onto the roof
and that there’s room for the clearances required
and lower it down the chimney. This is a two-
by local code and mentioned in the stove manu-
facturer’s instructions. You’ll also need room to
end of the flue pipe nears the woodstove, a
insert the stove, with or without legs attached,
coworker below can fit the lower end over the
and raise it up into its final position. Stoves are
woodstove’s outlet.
heavy—300 lb., on average—so give yourself
Although installation details vary, a metal
room to work. Fireboxes often need to be modi-
top plate centers the pipe within the chimney
fied to make room for a fireplace insert or stove.
and is caulked to the chimney top with a high-
Masonry
227
temperature silicone sealant. The juncture
Measure. Measure the firebox before you start
between the pipe and the top plate is then cov-
tearing out old bricks. Note its height, width,
ered with a storm collar, which typically uses a
depth, and angle at which sidewalls meet the
band clamp to draw it tight. That’s caulked as
back of the firebox. And if the back wall also tilts
well. Finally, cap the top of the flue pipe. The
forward, take several readings with a spirit level
monsoon cap shown below in the bottom right
to determine the slope. Finally, note the height
photo maintains a fairly uniform updraft even
and dimensions of the chimney throat, the nar-
when winds shift suddenly.
rowed opening at the top of the firebox usually
covered by a metal damper. Knowing the location
reBuilding a FireBox
and dimensions of the throat is particularly help-
ful—it tells you the final height of the back wall
If you can see broken firebricks or missing mor-
of the firebox. To keep all of this information
tar inside your fireplace, it’s time to rebuild the
straight, you’ll do well to take digital photos of
firebox. You’ll need to decide which bricks to
the firebox, print them out on typing paper—and
leave and which to replace. But you’ll almost cer-
write angles, dimensions, and whatnot directly
tainly need to replace the back wall, which suf-
on the prints.
fers the highest temperatures as well as the most
physical abuse.
Tear out. Starting with the back wall of the fire-
This job requires a respirator, eye protection,
box, use a flat bar to gently dislodge loose fire-
and—at least during demolition—a hard hat. A
bricks—most will fall out—and place them in an
Renovation 4th Edition Page 53