Moisture Meter (Roofing)

A moisture meter is a handheld instrument that gives a reading related to the moisture content of a material. In roofing it is used to check wood decks and framing for wetness, to compare dry and wet areas of insulation, to examine core samples during moisture surveys, and to help trace leaks.

Types

• Pin (resistance) meters have two metal pins pushed into the material. Electrical resistance between the pins falls as moisture rises, and the meter converts this into a reading. Pin meters were developed mainly for wood, and ASTM D7438 covers their use for wood-based materials; readings depend on wood species and temperature corrections.
• Pinless (capacitance or impedance) meters have a flat sensor pad pressed against the surface. They send a low-energy electrical field into the material and measure changes caused by moisture, without making holes. Readings are relative and depend on the material's density and the depth of the field.
• Roof scanners are larger, wheeled impedance instruments used across low-slope roofs on a grid, covered by ASTM D7954 (see roof moisture survey).
• Probe attachments with long insulated pins reach into insulation and through membranes in some kits.

Using meters on roofs

Technical guidance on roof moisture testing stresses several points:

• Readings on insulation are relative, not true moisture contents. Scales intended for wood do not translate directly to polyisocyanurate, expanded polystyrene, fibreboard or perlite.
• Moisture is often uneven through the thickness of insulation, so tests should cover the top, middle and bottom of samples and both facers.
• Pin meters only measure the small area between the pins and can miss wet zones.
• Facers, adhesives and foil can distort readings.
• Quantitative results require gravimetric analysis of core samples: weigh, oven-dry, reweigh, and calculate moisture content relative to dry weight.

Practical tips

Take baseline readings in known dry areas for comparison, use the correct scale and temperature correction, and record readings on a roof plan. For wood decks, readings well above the material's normal equilibrium range indicate wetting worth investigating. A meter is a screening tool; its results support, but do not replace, inspection and core sampling.