From framing and mold prevention to finish carpentry, woodworking, and flooring, there are any number of times when it’s very important that you know how wet or dry the building materials are that you’re working with. If your saw blade is tossing off water when you cut, you can pretty much assume that the wood is wet. But how do you check it with a bit more accuracy than that? The answer is to use a moisture meter.
How moisture meters work
A moisture meter works on the principle of electrical conductance. The battery-powered meter has two small, sharp pins on one end that can be pushed into the material you want to test. If moisture is present in the material, it completes an electrical circuit between the pins. The amount of moisture that’s detected is then displayed; some meters use an indicator needle and a scale to show how much moisture is present, while others utilize a digital readout screen to show moisture levels. Some meters also use colored lights for quick reference, going from green (dry) through red (excessive moisture detected), and may even include an audible alarm when moisture’s detected.
The pins on most moisture meters are very short and very sharp, so they’re ideal for testing wood or drywall since the holes they leave are barely noticeable and are easily repaired. Some meters have interchangeable long probes to use for testing inside wall cavities. Another variation is the hammer-probe, which has larger, longer pins and a sliding weight that’s used to drive the pins deeper into the surface being tested. This allows you to test how deep the moisture has penetrated, which is especially helpful when checking subfloors, framing lumber, and other thick materials.
There are also meters that don’t use pins at all. Called nonpenetrating, noninvasive, or pinless meters, they measure moisture levels without creating holes. Some types of non-penetrating meters have pads on the back that act similar to the pins on penetrating meters; when moisture is present, an electrical circuit is completed and the meter indicates the relative amount of moisture detected. Newer nonpenetrating meters utilize radio frequencies to find the moisture.
When to use a moisture meter
Moisture meters have a number of uses around the house and shop. For example, they can be used to check the moisture content (MC) of framing lumber. Typically, you’re shooting for a MC of 19 percent or less in framing lumber, and, depending on where you live, that may also be a building code requirement. Moisture contents higher than that makes the lumber very prone to warping and twisting as it dries. Beyond warping, excessively wet framing lumber can lead to mold and mildew issues. So checking the lumber prior to installation is a quick way to help prevent some problems later on.
The same holds true for wood used for finish carpentry and woodworking. The higher the moisture content is in the wood, the more likely it is that your joints will open up as the wood dries and shrinks. For best results, you should bring your trim lumber into the actual job site where it’s going to be used and allow it to acclimate, then use your moisture meter to check the wood before cutting. With most trim lumber applications, you’d like a MC of around 6-8 percent.
If you’re doing wood flooring, you want to be sure that the moisture content of your flooring materials and your subfloor aren’t too far apart. For wood strips less than 3 inches wide, the wood and the subfloor should have a MC no more than 4 percent different. For wide-plank flooring of 3 inches or more in width, that MC difference should be no more that 2 percent.
Another very important use for moisture meters is to check for suspected leaks. You can check framing members and sheathing in the attic if you suspect a roof leak, or you can check drywall or flooring around windows, bathtubs, showers, toilets, and other areas where you’re concerned that moisture might be an issue. These quick and easy checks with a moisture meter might detect a small problem before it becomes a major, costly headache.
Where to buy moisture meters
For home use, General Tools makes an easy to use penetrating meter with a digital readout that’s pretty accurate for around $30. Ryobi has recently introduced a nonpenetrating (pinless) meter aimed at the DIY market for about $50. Both of these meters are available at home centers, tool retailers and online sources.
For true professional-grade meters, one of the most trusted names in the industry is Delmhorst (www.delmhorst.com). You’ll pay about $145 and up for one of these meters, but you’ll step up in accuracy and reliability. Check their website for the different products they offer, and a list of where to find them.