Moisture Meter Readings Explained: What Counts as Dry?

Moisture meter readings for wood and drywall mean different things, so one number cannot be called dry for both. On wood, the meter gives a true moisture content, and indoor framing normally sits somewhere around 8 to 14 percent. On drywall, most meters give a relative number, and the real test is whether it matches a reading from a dry part of the same house.

Pin and pinless meters

There are two kinds of meter, and they measure in different ways.

A pin meter has two sharp probes that you push into the material. It measures how easily a small electrical current passes between them. Water conducts, so wetter material gives a higher reading. Pin meters read the exact spot and depth where the pins sit, which makes them precise. With longer insulated pins they can read the middle of a stud or the subfloor under a finished floor. They leave two small holes.

A pinless meter has a flat sensor that you press against the surface. It sends a signal a couple of centimetres into the material and reports an average for that zone. It leaves no marks and is fast, so it is the tool for scanning a whole wall or floor to find where the wet area starts and stops. It is easier to fool, which is covered below.

A proper inspection uses both. The pinless meter finds the suspect areas and the pin meter confirms them.

Moisture meter readings for wood

Most meters are calibrated for wood, so a wood reading is a real percentage: the weight of the water compared to the weight of the dry wood. As a general guide for framing and subfloor inside a heated house:

• About 8 to 14 percent: normal. It runs lower in winter when the furnace dries the air and higher in a humid summer.
• About 15 to 19 percent: elevated. Something is adding moisture, or the wood is still drying. It needs watching and usually needs drying.
• 20 percent and up: wet. Mold growth becomes likely if it stays there.
• High 20s and above: saturated. Wood that stays this wet for a long time can start to rot.

Readings shift a little with wood species and temperature, and good meters let you correct for both. For drying decisions the small corrections matter less than the trend and the comparison to dry wood nearby.

Moisture meter readings for drywall

Drywall is gypsum with paper faces, and it holds far less water by weight than wood does. Dry drywall has a true moisture content of well under one percent. A wood-scale meter pushed into drywall will still show a number, often somewhere from 5 to 12 on dry board, but that number is not a real percentage. It is a relative reading.

Some meters have a separate drywall or gypsum setting, and some have a plain reference scale from 0 to 100 for non-wood materials. Whichever scale is used, the method is the same. Take a reading on the same kind of drywall in a part of the house that never got wet. That is the dry standard. Then compare. Drywall that reads clearly higher than the dry standard is wet, and drywall that matches it is dry, whatever the number happens to be.

If the pins slide in with no resistance, or the board feels soft or crumbly, the reading hardly matters. That board has lost its strength.

What counts as dry

The working rule in restoration is to dry materials back to what is normal for that house, not to a number from a chart. A stone farmhouse near Lucan in August and a new build in Kilworth in February will have different normal readings, and both can be perfectly healthy.

For wood framing we also use a fixed ceiling. We keep drying until framing reads under 16 percent, and we log the readings every day so the trend is on paper.

Readings that fool people

• Metal: corner bead, screws, nails, foil-backed insulation, pipes and wiring behind the surface all send a pinless meter high.
• Salts and old stains: mineral deposits left by past leaks, and the white powder on basement concrete, conduct electricity and can read wet when dry.
• Dense or unusual materials: plaster, tile, brick and concrete all read high on a wood scale. Use the reference scale and compare to a dry area of the same material.
• Surface wetness: condensation or a recently washed wall gives a high pinless reading over a dry core.
• Depth: a pinless meter can miss water deep in a thick assembly, and short pins can miss a wet bottom plate behind a dry-looking baseboard.
• One reading: a single spot proves little. Readings are taken in a pattern, low on the wall and working up and outward, until they return to normal.

If you own a meter, the most useful thing you can do is build a baseline. Take readings on a few walls and floors while the house is dry and write them down. When something leaks, you will know what your own normal looks like. If you would sooner have it checked, a moisture inspection with a thermal camera and both meter types runs $249 to $549.

Common questions