Aerial Infrared Roof Moisture Surveys
INTRODUCTION
IR P/PM (infrared predictive/preventive maintenance) is not
limited to annual infrared surveys of electrical switchgear. For
instance, infrared thermography can be a very effective tool for
roof asset management. IR roof moisture surveys are performed on
roofs to quantify the extent of roof moisture (water) that is
inside the roof system. Infrared thermography is not leak
management. No matter how the water got into the substrate, the
purpose of this type of survey is simply to find and document
where the water is located. Extending the life of a roof will
save the owner the expense and aggravation of re-roofing or
re-covering. Re-roofing means that the roof is taken down to the
decking and replaced completely. Re-covering means that the
waterproofing layer(s) are removed, the wet insulation is
removed and replaced and a new waterproofing layer is put down.
The cost of an infrared roof moisture survey is three to five
CENTS per square foot. It cost between three and five DOLLARS
per square foot to repair/replace roofs, so knowing the exact
location of the subsurface water is extremely useful
information, since only those areas that are damaged need to be
repaired. This information is used to plan budgets and when
needed, as a bid document for contracting repairs and/or
replacement of the roof.
Keywords: Infrared, Thermography, Roof, Roof Asset Management,
Infrared Roof Moisture Survey.
ROOF MAINTENANCE IN GENERAL
The ravages of sun, wind, rain, snow, chemicals, leakage, rapid
changes in temperature and time - will eventually cause every
roof to fail. Some roofs last 40-50 years…when they are well
maintained. Owners may believe that a roof warranty will somehow
protect them from having to do maintenance. Not so, as roof
warranties are written by roofing manufacturers for the purpose
of protecting themselves from liability. For example, often a
warranty is written so that if improperly installed or defective
roofing materials are used on a roof and water leaks into the
electrical switchgear room causing an explosion, the roofing
manufacturer will replace the materials, the roofer will
reinstall the materials, but the building owner has to pay for
the replacement of the switchgear and any downtime that resulted
from the failure. Also, the roofer’s and roofing manufacturer’s
liability, in the case of roof failures are also reduced by
vaguely written roof warranties, which do not define words like
“regular” or “routine” maintenance. Not accepting the roof
warranty is not the answer, since the roof will not be installed
unless the owner agrees to the warranty. To eliminate these
problems, the building owner should have an agreement with a
qualified roofer or roof consultant to inspect and maintain the
roof (in accordance with the terms of the warranty) at least
once a year.
Waterproofing problems manifest themselves in two ways: Leakage
and entrained moisture contamination. Leakage is pretty simple,
although the leak inside the building rarely directly relates to
the exact spot on the roof, since the water flows down the slope
of the roof to a spot that is not sealed and into the building
at that point. Most leaks occur where the waterproofing is
sealed or where there is a penetration of the roof. Since most
types of roof systems absorb some amount of water, it is harder
to find the exact spot of water contamination in the insulation
because it may not leak into the building until it has absorbed
all the water it can hold. There are three types of surveys that
are used to find water in a roof. Nuclear gauges-which count
neutrons, capacitance meters-which measure resistance, and
infrared-which measures heat. Both nuclear gauges and
capacitance meters are used to take spot readings on a 10′ X 10′
or 20′ X 20′ grid on the roof. These measurements are used to
extrapolate where the water is from the readings obtained from
the gauge. They are good for types of roofs that do not gain or
lose much solar energy and therefore, do not lend themselves to
infrared.
BASICS OF INFRARED ROOF MOISTURE SURVEYS
During the day, the sun radiates energy onto the roof and into
the roof substrate, and then at night, the roof radiates the
heat back into outer space (See Figure 1). This is called
radiational cooling. Areas of the roof that are of a higher mass
(wet) retain this heat longer than that of the lower mass (dry)
areas. Infrared imagers can detect this heat and “see” the
warmer, higher mass areas, during the “window” of uneven heat
dissipation.
Figure 1) Areas of the roof that are wet retain heat longer than
dry areas.
Some roofs and insulation types or combinations do not absorb
any water. These roofs leak straight into the building. Even
roofs, which have insulation types that do absorb water, some do
not exhibit a good infrared signal, primarily for two reasons.
1) The surface is too reflective, and/or 2) the roof’s ballast
is so thick (or dense), that daylight radiation is not absorbed
into the substrate (insulation), therefore it cannot be emitted
back into the atmosphere at night. Even with a strong infrared
signal, factors on the roof can affect the analysis and
interpretation of the data. Some of these factors: water between
multiple layers, old patches, heavy flood coats, reflective
coatings, heat-producing equipment under the roof -or heat
blowing down onto the roof, stains, ponding water on the roof,
heavy build-up of ballast at parapet walls and along edges, etc.
These roofs should be inspected by other methods as described
above.
WALK-ON INFRARED ROOF SURVEYING
To perform a walk-on or on-roof survey properly, a crew of three
to four people is needed: an experienced infrared thermographer
and helper, an experienced roof consultant or roofer and the
building owners’ representative for access and security. The
crew walks around the roof(s) and when an area of suspect
moisture is found, the roof consultant verifies it is wet and
then the helper marks the edges of the area directly on the roof
with marking paint as instructed by the thermographer. The crew
needs authorization and access to all areas and levels of the
roof(s) from either ladders or roof hatches and plenty of time
to collect data. The infrared images are stored on videotape or
digital media and printed (See Figure 2).
Figure 2) On-roof IR image.
The next day, the thermographer goes back on the roof(s) to take
matching visual photographs of the marked areas that contain
subsurface moisture. The problem with on-roof infrared is that
when one is standing on the roof, eye-level is at best six feet
over the surface. Even with the best hand-held infrared camera
available, there is virtually no way to get large areas (800 sq.
ft. blobs or 100′ long striations) of moisture contamination on
the screen in one infrared shot. Taking multiple shots is
labor-intensive and makes the report confusing. When performing
on-roof surveys, many times “you can’t see the forest for the
trees.” Marking the roof is fairly easy, but it is very
difficult/time-consuming to produce accurate drawings of the wet
areas from painted lines on a roof. Often the drawing supplied
by the owner is outdated, incorrect or even non-existent.
AERIAL INFRARED ROOF SURVEYING
There is never a time when on-roof imagery is better than aerial
imagery. While the best IR imagery of a roof is taken from the
air, the same laws of physics apply to both aerial IR and
on-roof IR…like a dry roof, low winds and no rain on the night
of the survey. Also, the “window” when the roof is radiating
heat differently from wet and dry areas is longer with aerial
infrared because slight nuances of temperatures over large areas
are distinguishable. The high angle of view allows the aerial
thermographer to produce more usable imagery and therefore
accurate CAD drawings. The cameras that are used for on-roof
surveys are not of sufficient spatial resolution to obtain good
imagery from flight altitudes of 1,200 - 1,500 feet above the
roof, so high-resolution, large format IR cameras (See Figure 3)
are required.
Figure 3) Large format infrared imager, fixed-mounted in a light
aircraft.
Once the aircraft is over a building, very little time (five
minutes per 200,000 square feet, about 25 minutes for two
million square feet) is required to fly over making multiple
passes. The imagery is recorded on digital videotape. Visual
photographs are taken earlier in the day or the next day. After
returning to the office, the photos are printed and the
thermographs are saved on the computer. The raw video imagery,
thermographs and photographs are used to make an edited
videotape copy of the passes over the building. Both visual and
infrared images are used to do the analysis by overlaying the
CAD drawing of the roof ‘over’ the digitized photographs and
thermographs. The drawings always need to be corrected, because
rooftop equipment has been removed, moved or added since the
last update of the drawings. Then, areas of suspected moisture
contamination are drawn on the CAD file. The result is a report
where visual, infrared and CAD components (printed and video)
are well matched and lined-up. The report is given to a roof
consultant who verifies the wet insulation during the day, while
making other condition notes on the roof.
Fixed-wing aerial infrared imaging provides many advantages over
on-roof infrared imaging:
Access to multiple levels of the roof is not a problem.
High-angle, straight down infrared images lessen reflection
problems. High-resolution images capture large areas at once,
making report writing easier and less expensive to produce.
Plan-view imaging allows for infrared images, visual images and
AutoCAD drawings to be reconciled closely. As a result, the
report is clear, concise and easy to understand (See Figure 4a,
4b, 4c). Plan view imaging allows accurate marking of areas of
suspect roof moisture contamination. The printed CAD drawings
can be used on the roof to paint areas of moisture contamination
directly on the roof (after verification), if desired.
Figure 4a) Photograph of a roof.
Figure 4b) Thermograph of a roof.
Figure 4c) Scaled CAD drawing of a roof.
The aerial infrared thermographer can wait for a good night for
imaging, surveying many roofs under good conditions. The
trending of roof moisture becomes possible. An aircrew of two
can easily survey many millions of square feet in a single
night. Processing the data is done in the office, not on the
roof. Report components can be purchased as needed. Aerial IR
allows the building owner to buy only the report he needs at
that time.
The biggest advantage of aerial infrared is on roofs that are
the most difficult to image from any distance or angle. Roofs
that, for instance, have a lot of ballast, are covered with
reflective coatings or for whatever reason are impossible to
image while standing on the roof. With high-resolution, plan
view aerial imagery, slight nuances of temperature can be seen
from far enough away to actually see the pattern of heat and
make a determination of where the problems are.
CONCLUSIONS
Every day millions of square feet of perfectly good roofing
materials are disposed of in our landfills. Why? Because roofs
are often replaced because know one knows where exactly the roof
is damaged until it is too late. If you want your roof to last,
it must be regularly maintained by professionals. Infrared roof
moisture surveying is the best method of non-destructive testing
on roofs, and aerial infrared is the best platform for
performing infrared roof moisture surveys. Improvements in IR
cameras and flight methodology, aerial infrared thermography and
aerial infrared reports are getting better and more useable
everyday.
