Water Leaks around Brick Chimneys
What are the Causes and Cures?
by Joe Mehaffey
Many homeowners have problems with water leakage associated with
brick chimneys. This can manifest in a number of ways such as:
1) Water runs down the flue.
2) Water runs down the brickwork inside the house or attic.
3) Exposed chimney brickwork gets damp and white lime deposits form on
the surface of the brick. In this case, the water may/may
not run out. It can be that the brick just gets damp when it
There can be a number of causes for this.
1) It may be that the flashing around the chimney is inadequate,
worn, corroded, or that it was improperly installed in the
first place. This is USUALLY verifiable fy the simple test of
running a low pressure hose around the chimney to roof intersection and
see if any water comes inside. If you see any water at all
repairs are needed.
2) It may be that the top of the chimney is cracked or loose and that
water is entering at the top and running down inside the chimney and
coming out anywhere along the way. How can this happen you ask?
Aren't chimneys SOLID brick. The answer is yes and no.
Chimneys are usually solid brick but sometimes concrete blocks or
other solid filler materials are used INSIDE the chimney and the
outside is brick veneer. Sometimes the inside of the chimney is
built up of brick that are "just thrown in" and haphazardly filled in
with mortar. While this is usually structurally OK, it can
leave channels, cracks and crevices where water can migrate from the
of the chimney all the way down. These crevices can, in the
aggregate, hold many gallons of water and you may have a big rain one
week and the chimney brick inside your house can stay damp for weeks..
There are a sevaral answers to this problem. a) In a new chimney
installation insist that the bricklayers use great care to COMPLETELY
fill the interior with brick and mortar so that there are no cracks and
crevices to begin with. This will likely take the average crew an
extra half day on a typical chimney. b) Have the brick masons use
hydraulic cement for mortar for the last 5 feet or so of the chimney
near the top. Hydraulic mortar is non porous and if placed
solidly will not allow water penetration. c) If the chimney is
already built then a) and b) are not practical. In such case,
have the entire top of the chimney covered with a lead coated
copper sheet cover. This
will act as a roof on the chimney. This will prevent water from
the top of the chimney in the first place through cracks in the mortar
brickwork. This is also a good time to put a bird screen and cap
the flue if you do not already have one..
3) Many "fashionable" brick used these days is rough and porous
on the surface.
If you find you have a leak around the flashing and the flashing is
glued flat to the brick (NEVER the proper way) instead of the
flashing being slipped into a one inch slot (previously filled with
caulk), then you may as well get ready because this flashing WILL
leak. The flashing should be either copper (good) or lead coated
(better) or lead (best) if it is in contact with brick. Aluminum
(frequently used) will usually corrode in as little as 5 years in
with wet masonry. Copper flashing will usually last 20+ years,
coated copper 75 years and lead 250 years. All these times
PROPER installation by a craftsman versed in his trade and not just the
handyman. Some roofers may propose "sealing" your porous brick
some kind of paint product. This cannot HURT, but most
paint products will fail quickly. One product which I have used
for about 5 years that seems to be holding up well is "Lifetime"
Silicone waterproofing sealant. I have used this product on
concrete and on brick and it works very well and seems as good at five
years as when new. If it will go the claimed 20+ years, I cannot
If your leaking chimney is positioned so that a good bit of water
impacts the "up the roof" side of the chimney, you should have a
back of the chimney. The cricket is a small "roof" type structure
causes water flowing toward the chimney from upwards on the roof to
to either side of the chimney and not impact right into the chimney and
associated flashing. Crickets can be added after the house is
if necessary, but they are best and most cheaply installed at the
a house is constructed.
If you have lightning rods installed in your house while it is being
built, you can have the masons bury the wire to the lightning rod
protecting the chimney INSIDE the chimney structure as they build it.