A Quick Look At Roof Moisture Surveys
This article is meant to provide general information that will be of
practical use. Over the years many very good technical articles have
been written about roof moisture surveys. Interested individuals
are encouraged to make an effort to find and read those articles.
Such articles can shorten the time needed to learn how to perform an accurate
roof moisture survey. Just because you have equipment available doesn’t
mean you will be accurate or efficient.
When a roof moisture survey is used to find water trapped within a roof
system, hundreds of thousands of dollars in deferrable costs can be saved,
if that information helps you decide to repair rather than to replace an
existing roof system.
It has been proven that some roof systems can dry out roof insulation
under certain conditions. However, moisture in a roof system
can still contribute to a number of problems that can affect the performance
of a roof system. Such problems include: reduced R value of the roof
insulation, metal deck corrosion, accelerated aging/deterioration
in insulated built up roofs, deterioration of adhered laps on some single
plys, insulation deterioration, fastener corrosion in membrane laps and
where used to secure the insulation, etc.
The mission of a roof moisture survey is usually to identify the size
and location of any subsurface moisture. Alternate goals may be to
find leaks, set maintenance priorities for multiple buildings, check roof
system condition before a warranty expires or to help decide whether to
repair or re-roof the building.
There are three common types of non-destructive roof moisture surveys:
Infrared, Nuclear and Capacitance.
Infrared
During the day, the entire surface of a typical roof reaches very close
to a uniform temperature. A two-man crew (minimum, for safety) will
go up on a roof after dark when the roof has started to cool off.
Because of the thermal storage characteristic of water, wet areas will
hold heat for a period of time while dry areas will cool off relatively
rapidly. At some time there is a difference in the surface temperatures
of the wet and dry areas, that period of time is described as the window.
An infrared camera produces images of these temperature differences.
A good rule of thumb for how long a window will last is 4 hours.
Some types of construction when combined with a particular weather pattern
can make the window longer or shorter. Structural concrete with a
gravel surfaced built up roof on a hot summer day tends to lengthen the
window. A white single ply with minimum insulation over a wood deck
on a mild day can shorten the window significantly.
Typically, the perimeter of the suspected wet areas are painted on the
roof and a roof plan showing the location and outline of suspected wet
areas is supplied.
The Infrared technique has limitations. “Hot spots” can also be caused
by interior heat sources, excessive gravel or roof top equipment. The camera
only “sees” apparent surface temperature, so the roof surface should be
dry and not covered by leaves or other debris. Also, an infrared
camera will not produce usable images if a 10 to 15 mph breeze is blowing
across the roof or if the surface has a fresh aluminum coating on it.
On the positive side of the technique, it surveys the entire roof as
the camera operator pans the roof surface. This means that even small localized
areas of sub-surface moisture can be found. Infrared surveys are
very effective on large roofs since the operator is "scanning" the
roof and only needs to "look" closely at hot (suspected wet) spots.
An infrared camera in the hands of an experienced operator can be very
accurate and cost effective.
Nuclear
In a nuclear roof moisture survey a grid pattern (usually 5' or 10') is
established on a roof plan and on the roof. A nuclear moisture gauge
is placed at each grid location and a reading is taken. The readings
displayed by the gage are a measurement of the quantity of hydrogen ions
that are present within a detectable area beneath the gage. Since hydrogen
ions are present in water, these readings can be correlated to moisture
content, once a base line reading for a dry area has been established.
The readings are used to create a map of the wet areas on the roof plan
which again show the location and outline of the suspected wet areas.
Tapered insulation, multiple roof systems or numerous repair attempts
are examples of construction which affect nuclear survey results.
The nuclear readings can miss small areas because the actual reading
are taken on a very small proportion of the total roof. Large roofs
take significantly more time than the infrared or capacitance techniques.
Nuclear roof moisture surveys are performed during the day and are not
affected by debris on the roof, winds or aluminum roof coatings.
Capacitance
Capacitance meters use a low-frequency signal to identify conditions, such
as sub-surface moisture, which will conduct electricity. A device
is pulled or pushed across the roof following a pre-established pattern,
often rows 5 to 10 feet apart. It takes continuous readings of the
area that it passes over. An audible alarm alerts the operator when
a wet area is passed over, while a meter gives relative readings.
Once again the suspected wet areas are marked on the roof and/or a roof
plan.
Multiple roof systems, thick insulation, metal flashing, a wet surface
and EPDM can cause erroneous readings.
Verification Techniques
The results of any roof moisture survey should be verified by cross checking
with other techniques or core cuts. Core cuts and delmhorst meter readings
can be used to verify the presence of moisture found by any survey type.
Small capacitance meters are sometimes used to cross check suspected wet
areas found by the infrared or nuclear roof moisture surveys.
Roof Moisture Survey Costs
Fees for roof moisture surveys generally range from $.02 per square foot
to $.05 per square foot, fees are affected by a number of factors.
Most service companies have a minimum fee regardless of the size of the
project. Contact an experienced provider for advice or more information.