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Water, Water Everywhere
Dan Fiala - March
2001
Proper roof design and
effective roof drainage can solve the problem of ponding
water.
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While there may be a few
uncontrollable reasons for unexpected premature
failure of a roof system, the life expectancy of a
roof can be determined greatly by three basic
factors; installation, drainage and ongoing
maintenance. Of these three very important
factors, drainage may be the one most often
compromised by building owners and designers.
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Why
Drain the Roof
One commonly asked question is
“What will a little water hurt?” A
better-phrased question might be "What won't a
little water hurt?" Water, over a period of
time will, to some degree, degrade virtually all
building materials. Therefore, it is very
important to keep water from sitting on building
materials for extended periods, including the roof.
The roof serves to protect the entire building
interior from the elements, however, it is not meant
to be a holding tank for water. One of the
best ways to protect your roof and experience
longevity in the roof system is through proper roof
drainage. Ponding water will accelerate the
degradation of your roof system. With ponding,
the membrane itself may degrade or seams may fail.
Furthermore, ponding water will increase the risk of
major damaging leaks as the water remains on the
roof surface even after the rains stop, and
continually enters through the defect area until the
pond is gone.
Once water enters the roof
system, then a whole new list of problems arise.
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 Ponding
water can be a contributing factor to premature roof
failure.
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Roofs that are asphalt-based,
such as built-up asphalt roofs or modified asphalt
roofs, can be affected even sooner by ponding water
than single-ply membrane roofs, such as PVC or
EPDM-type membranes. Through periods of
rainfall and sunshine, cycles of ponding water on
asphalt-based roofs accelerate degradation of the
asphalt. The sun creates ultraviolet
radiation, which chemically degrades asphalt through
photo-oxidation. Photo-oxidation increases the
number of asphaltenes and water-soluble products,
which results in the migration of the asphalt
products to the surface of the roof. The
products then harden and are washed away, and a new
surface is exposed to the photo oxidation to begin a
new cycle.
In the summer, roof surface
temperatures between the ponded areas and dry areas
can range to more than 60 degrees Fahrenheit.
This extreme temperature difference results in
erratic movement within the roof membrane.
Erratic movement in the membrane from contraction
and elongation can create membrane wrinkles.
Furthermore, wetting/drying cycles of ponding areas,
can reduce the tensile strength of roofing felts to
less than 20% of their original strength. The
combination of reduced membrane strength, along with
erratic membrane movement, promotes membrane
splitting and cracking. Ponding water may also
promote plant growth on the membrane. The
roots of these plants can penetrate an already
weakened membrane, resulting in holes through
the roof. Water entry around the
penetrating roots may start out small, however, when
the plant dies or is removed, water entry will
substantially increase.
In colder climates, ponding water
can cause additional problems. These ponded
areas will freeze, and the ice can be very damaging
due to the high coefficient of thermal expansion.
The ice can rip seams apart, tear flashings and
erode membrane surfacing.
Too much
ponding water can also cause structural failure and
roof collapse. Not only will this be a
disaster to the structure, building contents, and
interior operations, it could cause human injury or
fatality. In short, ponding water on a roof
can be nothing but trouble. Proper Design for Drainage
The roof designer must have a
good drainage layout design or they have fallen
short of a good roof design. The designer must
decide between interior and peripheral drainage
systems. However, this may be pre-determined
by structural slope or the lack thereof. If
the structural roof deck is not sloped, then the
designer must use other materials within the roof
system to create slope, such as tapered insulation.
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Roof drains should be specified
at a minimum of two drains for a total roof area of
10,000 square feet of roof area thereafter.
However, additional drains may be required for roofs
with obstructions or irregular shapes. Drain and
drain leaders are often located at columns so that
the leader can be hidden within the column.
Unfortunately, when the roof deck settles or
deflects, the columns often produce high points in
the roof deck and the drain may no longer be located
in the lowest point of the roof. The added
cost for installing lateral leaders to get the drain
away from a column or load-bearing wall is minimal
compared to the cost down the road that the
resulting ponding will cause. When possible,
drains should be sumped into the roofing system to
prevent localized ponding directly around the drain.
Existing Drainage Problems
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A siphon
drainage system is one solution to
existing ponding problems. |
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With existing roofs, which are
already inadequately Sloped and pond water, there
are possible modifications that can be made to help
protect the roof against ponding. When
feasible, a drain or drains could be added.
This may seem to be a costly approach, but when
compared to a prematurely failed roof system, the
cost is trivial. Furthermore, at the time of
eventual roof replacement, it is highly likely that
this drain would need to be added anyway.
Another way to eliminate a ponded
area is to install a siphon drainage system.
The siphon drainage system is a pump or a set of
pumps placed in the lowest point of the ponded area.
When the ponded area reaches a certain depth, the
pump activates until the drainage line is primed and
siphoning takes over. The drainage lines
should be run from the pump into an existing roof
drain, gutter or simply over the edge of the roof.
However, a power failure during a rainstorm could
temporarily shut an electric pump down. There
are solar powered pumps, which can also be utilized.
Cycling radiant solar energy, siphons water off of
the roof much like the electric pumps. Solar
radiation is absorbed in the solar powered pump,
which increases the internal pressure, and cooling
reduces the internal pressure. Vacuum pressure
from each heating and cooling cycle siphons water
from the pump out through a drainage line.
One other approach to a ponding
problem is to install a roof surfacing which better
withstands the affects of ponding than what is
presently there. One commonly used surfacing
is a combination of asphalt and gravel. The
ponded area is flood coated with asphalt or coal-tar
pitch waterproofing, and then gravel is embedded to
prevent the waterproofing from cracking.
Coal-tar pitch, when compared to asphalt, is far
superior under ponding conditions. However, a
study conducted by the National Bureau of Standards,
indicated that even gravel surfaced roofs with
ponding conditions required major repairs within 10
to 15 years of service.
As one can see, ponding water on
the roof will do nothing but cause trouble and prove
to be very expensive to the owner. Regardless
of the approach, whether it is proactive with proper
roof design during the original installation, or if
it is reactive by adding drains, pumps or an
additional surfacing to an already ponding roof, any
approach to get the water off of the roof will be
beneficial in the long run.
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