May 18, 2016
There is much dissention in the marketplace today on the best way to detect moisture trapped in a roofing system. There are many different options, both destructive and nondestructive. We are going to analyze three of the major nondestructive techniques: nuclear, dielectric, and infrared.
There is much dissention in the marketplace today on the best way to detect moisture trapped in a roofing system. There are many different options, both destructive and nondestructive. We are going to analyze three of the major nondestructive techniques: nuclear, dielectric, and infrared. Nuclear and dielectric techniques both utilize a grid system. The roof is marked off in an intersecting grid, usually on 5', 10', or 20' centers. The most commonly used are 7'-10' centers. The operator moves the instrument to each point and records the instrument readings at each one of the intersecting grid points. The infrared technique "photographs" 100% of the roof surface. It is important to note that the quality of the results of any of these types of analyses may vary greatly depending upon both the quality of the instruments being used and the competence of the user. We will assume that both the instruments and the operator are competent. Also note that all three of these methods generally employ the use of destructive testing in the form of core sampling to verify results.
The first method of moisture detection is the nuclear technique. The theory of this technique is quite
complex, but basically a nuclear meter measures the slow reflected neutrons from a fast neutron source. Nuclear meters contain radioactive material in order to release these neutrons. When the neutrons are released from the meter, they collide with other particles within the roof. If the particle is a hydrogen atom, the neutron's speed will be reduced. Since water contains hydrogen atoms,
the amount of moisture in a roof system should be proportional to the number of "slow" neutrons
reflected back from the roof system. This process is complicated by the fact that roof components may
also contain hydrogen atoms, but presumably the readings should be fairly consistent for all dry roof
areas, with a marked change for areas containing moisture.
There are both advantages and disadvantages of nuclear analysis. One advantage is that the data
acquisition is rather insensitive to climatic conditions, enabling this survey to be done during the day
and during virtually any part of the year, as long as there is no standing water on the surface of the roof. Another advantage is that it can be used on virtually all roof types. One disadvantage is that the meter contains radioactive material which requires special handling and licensing. Another disadvantage is that data acquisition is time consuming and laborious. Also, the meter readings are angle sensitive and must be properly placed on the roof. The readings can also be affected by inconsistencies in the roof components. Finally, a major disadvantage is that since the nuclear meter samples about 2 square feet of roof at each grid point and most surveys are taken on a 10' x 10' grid, a nuclear moisture survey sees only about 2% of the roof. Even if the grid size is reduced to 5' x 5', the coverage is only increased to 8% and the work is multiplied by 4.
The next method is dielectric or capacitance detection which uses
a capacitance meter. The meter creates an alternating electrical
current in the roof system below. Capacitance is the property of
a substance which permits the storage of electricity when there
are differences between two conductors. Capacitance is
measured as a dielectric constant. The dielectric constant of a
water saturated roof system will be much higher than that of a
dry system (with intermediate constants for intermediate
mounts of moisture).
The advantages and disadvantages of the dielectric method are very similar to the nuclear method. This applies to the climactic advantage, which is exactly the same. The advantages is that the equipment is less sophisticated, easier to operate, and generally less expensive than the other two methods. One disadvantage is that it is virtually impossible to determine the actual boundaries of a known anomaly. Also, due to its electrical charge operation, there are numerous conditions which will prevent the equipment from obtaining any accurate or useful readings. Finally, the major disadvantage is that, like the nuclear method, capacitance meters sample about 2 square feet of roof at each grid point and most surveys are taken on a 10 x 10' grid. Therefore, a dielectric survey sees only about 2% of the roof.
Infrared scanning, the final method of detection, senses the temperature of the surface of the roof. Wet insulation changes the ability of the roof system to store and conduct thermal energy, thus causing changes in the temperature of the roof surface. With this method, 100% of the roof surface is viewed through a scanner or camera. Areas with wet insulation will appear lighter than dry areas due to an increase in temperature. The data generally is recorded through digital imagery. Outlines of the wet area can also be painted on the roof surface.
The advantages of infrared surveying are many. The first advantage is that the data is generally extremely accurate if obtained by an experienced operator with appropriate equipment. The second advantage is that the data is acquired over the entire roof and not just at grid intersection points, so the entire roof surface is seen. This method tends to be less time-consuming than the other two methods, offering cost advantages, particularly for large facilities. This method can also be used to identify conditions requiring remediation on exterior walls and window panels.
There are, however, disadvantages to this method. The first disadvantage is that it is highly weather dependent, with many variables including surface moisture, wind speed, daytime vs. nighttime temperature, etc. The second disadvantage is that these scans generally need to be performed at night, increasing safety issues. The readings may also be affected by interior heat sources or differences in insulation thickness. The final disadvantage is that the results are qualitative rather than quantitative, and therefore are highly dependent on the experience of the operator.
In conclusion, roofing experts generally agree that the most reliable method of moisture detection in a roof system is using an infrared survey. The disadvantages of this system can be overcome by an experienced operator, enabling accurate results for 100% of the roof surface. Verification procedures are recommended, including test cuts, moisture probes (Delmhorst), and use of a Tramex meter. It is important to note, however, that the results from an infrared survey are highly dependent upon many different variables, requiring the direction and interpretation of a trained expert in order to achieve usable results.
StructureScan, a division of StructureTec, performs nondestructive evaluations of facilities using infrared scanning. If you are responsible for the care and maintenance of the exterior of your facilities, contact us today to schedule your StructureScan infrared survey!
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StructureTec is a building envelope, pavement, and roof consultant specializing in the waterproofing and restoration of existing building envelopes, roofs, pavement and concrete structures.
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