How to use a moisture meter on drywall, and what the readings mean

What the two kinds of meter actually measure

Neither kind measures water directly. Both measure an electrical property that water changes, and each has a blind spot on drywall.

Pin meters push two steel pins into the surface and read the electrical resistance between them. Water conducts; dry gypsum and dry wood barely do. The reading comes only from the material between the pin tips, so a pin meter tells you about the depth the pins reach and nothing more. On drywall that is usually the paint and the face paper unless you push the pins through into the core, which is where the water is. The upside is that pins are hard to fool: a reading at one spot is a reading of that spot.

Pinless meters press a flat pad against the surface and send a weak electromagnetic field into the material, typically 3/4 in to 1 1/4 in deep depending on the model, and read how much the field is disturbed. Water disturbs it a lot; so do dense materials and anything conductive. On a 1/2 in sheet of drywall the field passes through the board and into whatever is behind it: air, insulation, a stud, a screw, a pipe. The upside is speed and no holes; you can sweep a whole room in minutes and find exactly where the wet stops.

The scales are different instruments in disguise. A pin meter in wood mode reports percent moisture content, calibrated for wood. Switched to a gypsum scale (Delmhorst's BD-2100 has one) it reports percent moisture of the board itself, and dry drywall lives below 1 percent. Many professional meters report wood moisture equivalent, or WME: the moisture a piece of wood would hold if it sat in equilibrium with whatever you are touching, so one green-yellow-red band works across drywall, plaster and framing. Pinless meters on drywall almost always report a relative number, 0 to 100 on the Tramex and Klein, 70 to 999 on the Protimeter's search mode, that has no meaning on its own and full meaning next to the reading from a dry wall.

The method that follows works for every scale because it never trusts an absolute number alone.

Set a dry standard before you read anything wet

The IICRC S500 standard that restoration crews work from has a name for the first reading you should take: the dry standard, a reading from the same kind of material in a part of the house the water never reached. The target you dry back to, called the drying goal, is set from it, usually within a few points. Everything else in this guide compares against that number.

Pick a wall of the same construction as the wet one: same drywall thickness, same paint, same side of the house if you can, and an interior wall if the wet wall is interior. A hallway or a bedroom closet two rooms away is typical. Read it at the same heights you will use on the wet wall, with the meter on the setting you will use for the whole job, and write the numbers down with the date and the room temperature. Take three or four points and use the typical value. If your meter has a reference mode (the Protimeter does), store it there too.

Do this before you touch the wet wall, because a dry standard taken after the room has been humid for two days is already contaminated, and because the adjuster will ask what dry looks like in your house, not in a chart.

Tools and materials

• A moisture meter. A pin meter alone works. A pinless meter alone finds the edge but cannot confirm depth or give a defensible number on drywall. Both together is how crews do it; the moisture meter roundup sorts them.
• A hygrometer for the room, so the log has temperature and humidity next to every reading.
• Painter's tape and a fine marker to label each grid point on the wall.
• A tape measure for the 6, 18 and 30 in heights and the 2 ft spacing.
• A phone with the camera date stamp on, for a photo of every reading.
• A stud finder, or a magnet to find the screws, so you know where the metal is before it fools you.
• A notebook or a spreadsheet with one row per point per day.
• Spare pins for the pin meter; they bend in paint.

What the numbers mean, scale by scale

The only universal rule is the comparison with your dry standard. Within that, the manufacturers publish thresholds for their own scales, and these are the ones worth knowing.

Gypsum percent scale (Delmhorst BD-2100 and similar). Delmhorst's own guidance puts normal drywall at 1 percent or lower on its gypsum scale. Between 1 and 2 percent, drywall may be saved if it is dried quickly; over 2 percent at any point, Delmhorst's advice is that replacement is the safer call. This is the most precise scale for drywall because it was built for it; a wood scale that starts at 5 or 6 percent cannot see the difference between dry drywall and drywall that has been wet.

Wood moisture equivalent (Protimeter SurveyMaster and most pro meters). Protimeter's manual defines the bands: 7 to 16.9 is dry and green, 17 to 19.9 is at risk and yellow, 20 and above is wet and red. Dry drywall sits at the bottom of the green band; a drywall reading in the yellow is wet drywall, because gypsum in equilibrium with a normal house does not read anywhere near 17.

Building-materials scale on a consumer pin meter (General Tools MMD4E). The MMD4E reads building materials from 1.5 to 33 percent and its alarm trips at 20 percent in that mode (16 percent in wood mode). General Tools does not publish a dry value for drywall on this scale, so use the dry standard: a dry wall in most houses reads at or near the bottom of the range, and anything that reads several points above your dry wall is wet.

Relative scales on pinless meters (Tramex, Klein, Extech pinless mode, Protimeter search mode). There is no dry number. A Tramex ME5 reading of 30 or a Protimeter search reading of 300 means nothing until you know the same meter read 15 or 150 on the dry wall. The pattern is what you read: the wet area is a plateau of high numbers with a gradient at its edge, and the edge is where the number returns to the dry standard.

Framing behind the wall (any pin meter in wood mode). When the baseboard is off or the wall is open, read the sole plate and the bottom of each stud. Framing in a dry house typically reads in the single digits to low teens; restoration practice treats 16 percent as the wet line, and the MMD4E's wood alarm and Protimeter's yellow band both start near there for a reason. Plates against a slab are the last thing to dry.

False readings, and how to tell them from water

Both kinds of meter read conductors and density, not just water. Protimeter's manual says it plainly: salts, carbon and metal can give false positives in pin mode, and conductors other than water within the material can drive search-mode readings high. The common ones on a drywall job:

• Screws and nails. Every 12 to 16 in up each stud. A pinless reading that spikes in a vertical line 16 in apart is the fastener pattern, not a leak. Move 2 in sideways and read again.
• Corner bead and metal edge trim. Every outside corner is wrapped in steel or metal-reinforced tape under the mud. Read 6 in away from corners, never on them.
• Nail plates, electrical boxes, pipes, ducts. Anything metal in the cavity shows up on a pinless meter as a wet patch the size of the object. A pin reading of the board at the same spot will be dry. That disagreement between the two meters is the tell.
• Foil-faced insulation and foil tape. A wall insulated with foil-faced batts reads high everywhere on a pinless meter. Switch to pins.
• Studs. A pinless meter reads the stud as part of the wall; the reading over a stud is slightly higher than between studs even when both are dry. Keep the grid between studs, or at least compare like with like.
• Fresh paint, new joint compound, wallpaper paste. All read wet until cured, which can take weeks for compound. If the wall was recently repaired, your dry standard needs to come from an equally recent repair or you wait.
• Surface condensation. A cold exterior wall on a humid morning can be damp on the face and dry in the core. Pins pushed through the paint read the core; Protimeter notes that surface moisture has little effect on its search mode. Wipe, wait ten minutes, read again.
• Vinyl wallpaper, tile, enamel paint. Pins cannot penetrate, so they read the finish, not the board. Score the surface, or read from the other side of the wall or from the cavity once the baseboard is off.
• Your own hand. A hand wrapped around the back of a thin partition or a door, or fingers near the sensing pad, moves the reading. Hold the meter by its handle.
• Temperature. Pin meters are calibrated near room temperature and read low on cold material. If your dry standard came from the same house at the same temperature, this washes out, which is another reason not to borrow a dry number from a chart.

The habit that catches all of these: when a reading surprises you, move the meter 2 in and read again, then confirm with the other kind of meter. Water makes a gradient; metal makes an edge.

Dry in place or cut it out

The readings are one of four inputs, and our wet drywall guide walks through all of them. In short: drywall wet by clean water (Category 1 under IICRC S500), wet less than about 2 ft up by the meter, with an empty cavity behind it and found within a day or two, dries in place with the baseboard off, vent holes at the plate, air movers and a dehumidifier. Drywall wet by contaminated water, wet higher than that, backed by wet insulation or a vapor barrier, wet for more than about 72 hours, or whose readings stop falling for 48 hours, gets cut above the wet line. The meter also sets the cut: 12 to 24 in above the highest wet reading, rounded to 16, 24 or 48 in from the floor.

The meters we hand people

For a single leak, a pin meter with a calibration check is enough to run this method; add a pinless meter when you want the edge found in minutes or you expect to do this again. These are the two we point homeowners to most often, and the roundup covers the professional options.

When to call a pro

Call a restoration contractor if the grid keeps finding wet readings in rooms you did not know were affected; if readings at any point have not fallen in 48 hours with equipment running; if the wet area runs above 2 ft, across a ceiling, or into an exterior wall with insulation; if the water was from a drain, a toilet, or outside; or if the loss is an insurance claim and the adjuster wants a moisture map with daily psychrometric logs, which a crew produces as a matter of routine. Call a plumber or a leak-detection service if the readings are rising rather than falling, because the leak is not fixed; our hidden leak guide covers finding it. The form below reaches vetted local restoration contractors through HelpMeNow.com; send them your readings and photos and they will know what they are walking into.