All Flat Roofs Should Have A Moisture Analysis Before Coating

The Following are Standard Moisture Analysis Procedures

There are three types of nondestructive testing equipment: impedance/capacitance, infrared and nuclear. One of these methods, or a combination thereof, should initially be used to obtain an overall assessment of the roof system. Technicians trained in the proper use of the testing equipment and employed test methods should conduct the testing.

Impedance/Capacitance

Impedance testing can be conducted in a pattern or at various points throughout the roof area. A higher number of readings provides a more cohesive moisture determination. The testing cannot be completed over wet or ponded areas and modified instruments are typically required for EPDM roof systems.

Infrared

Thermal masses, or "hot spots," are not always an indication of moisture presence. They can be illustrated at under deck heating or cooling vents, venting of hot fumes, moisture on the roof surface (ponded water), or at points of heavy gravel application. Most infrared cameras require clear weather conditions for an extended period prior to and during the testing. This typically includes no recent or current precipitation, heavy cloud cover or windy conditions. Any or all of these conditions could distort the infrared findings.

Nuclear

Generally, nuclear scanning can be completed to depths of as much as 7 inches and testing can be conducted in areas of ponded water. Testing is conducted over the entire roof area by sectioning the roof into grids (5 feet by 5 feet or 10 feet by 10 feet) and recording the readings at each of these locations.

Moisture Verification

In the moisture analysis procedure, the core cuts are extracted to determine both the construction and moisture content of the existing roof system. For these purposes, extraction of the core cuts can be completed in the following manner for all types of roof systems:

1. Identify the appropriate location of the core cut. The proper area should be representative of the entire roof area construction. Do not take a core cut from a previously repaired area.

2. A core cut should be extracted from those areas determined to be dry and areas found to have varying levels of moisture presence: low, medium or high. Facilities with multiple roof areas and/or multiple roof systems require core cuts from each roof area.

3. Identify the locations of the core cuts on the roof plan.

4. Use a template and measure the area to be cut. Core cuts can range from 2 inches by 2 inches to 12 inches by 12 inches, depending on the amount of physical property testing that is required.

5. Following the established pattern, cut the membrane, as well as any insulation and underlayments to the structural deck. Single-ply systems can be cut with scissors. Bituminous roof systems require a box cutter knife or hatchet.

6. Remove all roof system components (membrane, insulation and underlayment) from the opening.

7. Photograph the system components and structural deck substrate.

8. Record system construction components, identifying the method of attachment of each component, including:

• Deck type.
• Underlayment (if used).
• Insulation type, thickness and condition (each layer).
• Method of insulation attachment (each layer).
• Membrane type, thickness and condition.
• Method of membrane attachment.
• Type of surfacing and method of attachment.

Testing the Core Samples

Moisture determination cannot be established solely on a visual (subjective) inspection of the materials. Standard procedures should specify that the exact moisture content of the material is determined through the use of gravimetric testing. Gravimetric testing is conducted by cleaning off the core sample components of debris and placing each component into an identical container. The core component and container is then weighed in its "wet" state. The drying process is accomplished by placing the individual component into a convection chamber for 24 hours at 230°F (114°C). After the drying process - which may include additional drying time - the samples are reweighed while still in the container. Moisture by weight is determined for each tested component by using the following formula:

Dry Basis, % of moisture=weight of moisture/oven dry weight=total weight-oven dry weight/oven dry weight

The result of the gravimetric testing provides the percentage of moisture content by weight of the tested insulation. When insulation becomes wet it loses its thermal and structural integrity. Each insulation has a separate percentage of moisture content at which point it loses its thermal capacity and is considered wet. Studies have indicated that even insulations that have similar chemical properties show great disparities in vulnerability to moisture.

In the early 1990s, Wayne Tobiasson published a study entitled "New Wetting Curves for Common Roof Insulations," which indicated that insulations retained their thermal capabilities at up to 80 percent of their original dry content. Through the use of a Thermal Resistance Ratio (TRR), which is determined by dividing the material's wet thermal resistivity by the material's dry thermal resistivity, each insulation's moisture content capacity was determined. When an insulation's percentage of moisture content enables the TRR to fall below 80 percent, the insulation is determined to be wet.

The chart in Figure 1 provides moisture content by percentage of volume for common commercial low-slope insulations. If the percentage obtained in the gravimetric testing is equal to or greater than the percentage of volume listed in the chart, the tested insulation should be considered wet.

The results of the moisture analysis should be provided in report form. A roof plan accurately identifying all wet areas should be included with a report of findings, test analysis and recommendations. All wet insulation should be removed from the system to deter possible structural deck damage. If the cost of insulation removal exceeds the point of diminishing returns, full roof replacement should be considered.