Moisture Meters for Home Inspectors

by Scotty Bane, InterNACHI^® Certified Professional Inspector^®
and Kate Tarasenko, Editor-in-Chief

Moisture is the primary cause of the deterioration of a structure. One tool that can be used to provide some detail and accuracy in testing for this condition during a home inspection is a moisture meter.

A moisture meter is used to measure the percentage of water in a material. The inspector can use it to test the roof decking, support beams, pillars, and other elements for their moisture content. It’s common to find areas on the roof where the flashing and roof covering are in less-than-optimal shape. In these areas of questionable maintenance and integrity, if certain areas are suspected of leaking, the moisture meter can be used to clarify their true condition. 

The meters come in two styles:  pin-type and pinless. Pin-type moisture meters leave small indentations, which can mar or even damage the surface. This type of meter can be used when checking the roof decking from the attic, or deck posts, which are located outdoors. Pinless meters can scan a larger area quickly without damaging the surface. 

Readings typically take five to 10 seconds and should be done at least twice to ensure accuracy. If an area is found to have moisture, the edges of the problem area should be located, if possible. 

High-quality moisture meters are accurate to within less than 0.1%. Low-end moisture meters can be very inaccurate, so multiple points of examination maybe needed to confirm your findings. Readings of 5 to 12% are considered normal, and up to 17% is considered acceptable.

Wood starts to rot and is susceptible to mold when its moisture content reaches 35 to 50%. Wood can typically hold up to 25% without showing signs of deterioration. Moisture meters work well with concrete, wood, tile, vinyl, gypsum/drywall, plaster, and many other building materials. 

If excess moisture is found, the humidity of the room should also be considered. An acceptable level of indoor humidity is generally 30 to 50%, which prevents the growth of microorganisms, such as mold. Excess moisture promotes mold, mildew, fungi, bacterial growth, and possibly viruses. Contaminants can degrade the indoor air quality and cause health problems.

Moisture meters are not affected adversely by humidity. These tools do not need to be calibrated but should be kept clean. If the meter’s case is cracked or broken, the tool should be replaced. Pins and electronics can be safely cleaned with isopropyl alcohol and a soft rag. If the meter is dropped, it still doesn’t require calibration, but a basic functions test should be conducted to make sure it’s still working properly. 

A typical moisture meter can cost anywhere between $30 to $250. Professional-level meters usually cost around $60, and a product warranty should be considered with your purchase to protect your investment.

A typical moisture meter runs on AA or AAA batteries. The device maintains its accuracy even if the batteries are low, and it will simply not function when the batteries do not hold sufficient power. The meters can use alkaline or lithium batteries; to save money, rechargeable batteries are recommended. It is also recommended by most manufacturers that the batteries be taken out of the tool when not in use to prevent corrosion of the internal metal materials.

Moisture meters function the same way a battery does in that they contain two different metals, and when moisture is detected, it acts as an electrolyte, completing a circuit. The more moisture that is detected, the stronger the reading will be. If moisture is not present, the circuit will not be completed, thus giving no reading. 

When moisture is found, the source of the leak should also be explored. An area that looks darker and feels damp would merit further investigation. Water stains and discoloration are also clear signs of a possible current or previous moisture intrusion issue. Another common place to find evidence of a water leak is under the sink in the kitchen and bathroom(s).

Finding excess moisture in a structure should be reported, with recommendations for the appropriate contractor to correct the situation, if warranted. It’s possible in many situations to address the excess moisture or leak by simply repairing the leak and drying out the area, without the need for major demolition or repairs. 

A photo of the device as it displays its reading can be easily included in your home inspection report. Clients appreciate clear photos of the affected area with supporting photos of the meter readings as proof of your findings. The method of taking a photo of the area and the meter reading of the condition is a strong addition to any home inspection report.   

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