Elevated Moisture in Your Inspection Report:
What a Moisture Meter Reading Means
A meter reading is one number, at one spot, on one day. Here’s what Russ reads into it, and what it leaves out.
In a Vancouver crawlspace in late August, Russ got a reading of about 16 percent on the framing and wrote it up as elevated: at the end of a dry summer, that wood should have been at its driest. Two weeks later in Battle Ground, a reading of 15 to 16 percent went in as the high end of normal, with a note to keep the vent openings and vapor barrier in shape.
Same number, two different write-ups. The number is easy to write down. What it means depends on the material, the spot, the season, and what else is going on around it.
How a moisture meter reads a wall
The meter Russ carries is pressed flat against the surface. It sends a small signal a short way into the material and reads how it responds. Water changes the reading, so a wet spot reads higher than a dry one.
On wood, the number is a moisture content percentage, roughly the weight of water in the wood compared with the wood itself. On drywall, plaster and most finishes, the scale is relative. A drywall reading of “20” isn’t 20 percent water. What counts is how the spot compares with the same wall a few feet away.
Meters can be fooled. Tramex, a meter manufacturer, warns that metal within the meter’s reading depth can cause false high readings, and corner bead is a common culprit. Nails, screws, foil-faced insulation and the metal lath behind old plaster do the same thing. A high reading in a straight line down a wall corner is usually the corner bead. A high reading in a soft, stained spot at the base of a toilet usually isn’t.
Normal numbers for wood, and the ones that get written up
Framing inside a lived-in, heated house usually reads in the low teens or below. Research from the USDA Forest Products Laboratory found decay fungi need wood above about 20 percent moisture to keep growing, and 20 percent has long been the rule of thumb for keeping framing safe from decay.
Russ’s reports flag wood earlier than that in crawlspaces and attics, because the reading is a snapshot. Readings around 15 to 16 percent at the end of summer can mean the space is holding moisture during the driest part of the year, and wet season will push the number up. Posts sitting on or near the soil with nothing between wood and ground were the clearest case: one Sandy crawlspace had support posts reading high from moisture wicking up out of the dirt. The crawlspace moisture post covers why that happens in this climate.
Attic sheathing gets the same treatment. Rusted nail tips, dark staining, and sheathing reading 16 to 17 percent (as one attic did) point to moisture the attic isn’t venting out. The post on attic moisture and condensation explains the usual causes.
Why a stain can read dry
In about 30 of Russ’s recent reports, a stain or water damage read dry on the meter. It’s the most common moisture-meter result he writes up, and it’s the one buyers misread most.
A dry reading at a stain means the water isn’t there today. Russ’s reports give three common reasons it might come back anyway:
- The weather. An Oregon City basement had stains along the baseboards that read dry in July. The report noted that’s normal in the dry season, when the soil around the foundation holds less water, and that this kind of intrusion tends to come back with heavy rain.
- Nobody’s home. A vacant house doesn’t run its showers. One Lake Oswego attic had apparent mold around a disconnected bath fan duct, and the meter read low because the house had been empty.
- The leak only runs sometimes. A toilet that seeps when flushed, a shower door that leaks when it’s used, a roof flashing that lets in wind-driven rain. Between events, the drywall dries out.
So the report on a dry stain usually reads two ways at once: dry at the time of inspection, and a sign of past or intermittent water. Both parts are true. The post on ceiling water stains walks through how Russ reads a stain by where it sits.
Real Inspection Photos
Moisture Readings From Russ's Inspections
Moisture meter and thermal camera checks from Russ's inspections, wet and dry.
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An elevated reading at the base of a toilet, with gaps in the flooring around it. The report pointed to a failing wax ring or a toilet leak. -
Crawlspace framing reading about 16 percent at the end of a dry summer. Russ wrote it up as a sign the space isn't drying out. -
A stained basement baseboard that read dry in July. The report noted this kind of intrusion tends to come back in heavy rain. -
Behind a loose baseboard tile next to a shower, where the floor read wet. The report referred it to a plumber and a mold remediation specialist. -
A cold spot on the thermal camera that read dry on the meter. The report traced it to a supply duct leaking air into the ceiling.
Where readings come up wet
When the meter reads wet, the leak is there now, and the report says so directly. In Russ’s reports, the most common place by a wide margin is the floor at the base of a toilet. About a dozen houses had elevated readings there, usually with gaps, swelling or staining in the flooring, and several had matching stains on the subfloor seen from the crawlspace or basement below. The usual cause is a failed wax ring or a loose connection.
Other wet readings in his reports:
- Bathroom floors and walls next to showers and tubs, often with failed grout or caulk nearby
- A garage wall patch near a hose bibb, reading wet behind the earlier repair
- Basement and garage ceilings under bathrooms
- A garage ceiling saturated under an active roof leak, with apparent mold on the drywall
A wet reading doesn’t say how far the water has gone. The meter reads a short distance into the surface. It can’t see the subfloor under tile, the framing inside a wall, or whether there’s rot or mold behind the drywall. Opening the wall or floor is a repair contractor’s job, not part of a visual, non-invasive inspection. That’s why these findings end with a referral: a plumber for a toilet or shower, a roofer for a roof leak, a contractor to open it up and look.
Moisture meter vs. thermal camera
The two tools get confused, and they do different jobs. A thermal camera shows surface temperature. Damp drywall often reads cooler, because evaporating water pulls heat out of it. That makes the camera good at pointing to a spot worth checking.
It can’t tell you the spot is wet. In one Portland house, the camera showed a distinct cold patch on a living room ceiling. The meter read dry, and the cause turned out to be a supply duct leaking cool air into the ceiling cavity. Same picture on the camera as a leak, different problem. The thermal imaging post covers what the camera picks up and what it misses.
So the order is camera first, meter second. A cool spot that reads wet goes in the report as moisture. A cool spot that reads dry goes in as something else, or not at all.
- Check whether the finding says the area read wet, or read dry with signs of past water
- Note where it is: under a bathroom, at a toilet base, below a roof penetration, in the crawlspace
- Ask the sellers about past leaks and repairs in that area, and look at the disclosure
- Look at the recommended trade (plumber, roofer, contractor) and what they’re asked to check
- If you move in, look at the spot again after heavy rain, or after running the shower above it
A moisture reading is one number at one spot on one day. Wet means water is there now. Dry means dry that day, not fixed. The meter can’t see behind the surface, which is why wet readings come with a referral to the trade that can.
For what happens when a reading comes with apparent mold, see the mold finding guide. To set up an inspection, you can book online.
Moisture Meter Readings FAQ
