Roof Moisture Survey

A roof moisture survey locates and maps wet insulation and substrate in a low-slope roof assembly. Water that enters through a leak can spread through insulation well beyond the visible leak and remain for years, reducing thermal performance, rotting decks and degrading adhesives. Surveys help owners decide whether to repair, recover or replace a roof, because many codes and manufacturers do not permit recovering over wet materials.

Methods

• Infrared thermography detects temperature differences at the roof surface, typically after sunset when wet areas retain heat longer than dry ones. ASTM C1153 covers infrared imaging of roofs for wet insulation. It is fast over large areas but depends on suitable weather and roof construction. (See infrared roof thermography.)
• Nuclear moisture surveys use a gauge containing a radioactive source that emits fast neutrons. Hydrogen atoms in water slow the neutrons, and the gauge counts the slowed neutrons that return. Readings are taken on a grid, commonly spaced about 5 to 10 ft (1.5 to 3 m) apart, under ANSI/SPRI/IIBEC NT-1. Operators must be licensed for the radioactive source.
• Impedance (capacitance) scanning passes a low-frequency electrical signal into the roof and measures changes caused by moisture. Readings are taken on a grid or continuously, and ASTM D7954 covers the method. Results are comparative and depend on calibration to the roof being surveyed.
• Core cuts remove samples of the roof assembly for inspection and gravimetric testing (weighing wet, drying, and weighing again). Moisture content is calculated from the difference between wet and dry weight relative to the dry weight.

Why cores matter

Infrared, nuclear and impedance readings are relative indicators, not direct measurements of moisture content. Technical guidance in the industry stresses that anomalies must be verified with cores, and that moisture is often uneven through the thickness of the insulation, so samples should be checked at the top, middle and bottom. Without verification, surveys can produce false positives that lead to unnecessary removal of sound materials.

Reporting

A survey report usually includes a roof plan marking wet areas, core locations and results, photographs, and recommendations. Repeating surveys over time helps track deterioration and plan budgets.