As carpenters, we’ve been known to invest heavily in tools that make our jobs easier and more efficient. When was the last time you saw a fellow framer or trimmer drive more than a few nails by hand? It’s likely been awhile, as most of us are using pneumatic or gas-powered nailers these days.
How about setting level and plumb lines for a project? Are you still using a water level or transit for such applications? Probably not—the laser level has earned a place in the tool arsenal of most carpenters because it reduces both labor and potentially costly layout errors, thereby saving its owner time and money.
There’s another tool, which, while not yet very common on most jobsites I visit, has the potential to save you even more time and money than laser levels and nail guns combined. It’s a moisture meter.
What Moisture Meters Do
Moisture meters “read” the moisture content of lumber, drywall, concrete—in fact, just about any porous building product, whether already installed or sitting in a supplier’s warehouse. This information allows installers to accurately predict how the materials they’re working with will move in the future. This knowledge is invaluable, as one can then plan the installation accordingly and be secure in knowing they’ve made provisions for the inevitable expansion or contraction of the lumber, or whatever material they’re working with.
Wood flooring installers have been using moisture meters for years. Many will check not only the MC (moisture content) of the flooring they’re preparing to install, but also the MC of the slab or subfloor it’s about to be installed over, and sometimes even the adjacent framing and drywall. This is especially true on new construction projects, where literally thousands of gallons of water vapor will most often evaporate from curing concrete, framing lumber, drywall compound, and paint.
The reason they take such precautions is simple—moisture goes to dryness. If the wood flooring is dryer than the surrounding materials, moisture from those materials will likely work its way into the flooring, causing it to swell, and possibly even buckle, in extreme cases. Conversely, if the flooring has a dramatically higher MC than surrounding surfaces, moisture will be drawn out of the flooring, causing it to shrink. Either situation is a likely call-back (and an expensive one at that), which is why installers often check and double-check conditions until they’re satisfied the MC of all the building materials—flooring, framing, masonry, etc.—in a particular area is similar.
But flooring contractors aren’t the only ones who can benefit from using a moisture meter. Anyone who works with finished products that have a propensity to move due to changes in moisture content will find value in owning an instrument. Finish carpenters, in particular, should carry a moisture meter in their truck and should use it just as flooring contractors do, checking and documenting the MC of not only previously installed components on the job, but all the millwork and cabinets delivered to the site, as well as any lumber taken from an outside source, such as a lumberyard, another jobsite, etc. In short, any material in your finished product that absorbs and releases moisture should be checked prior to installation.
What Moisture Meters Tell Us
Why go through all this trouble? Consider this… a 4% change in the MC of a piece of flat-grained softwood equals a 1% change in its size. This may not seem like a lot, but flooring installers know differently.
To put this into perspective, let’s consider a typical T&G beadboard installation, whether on a porch ceiling, as wainscoting on a wall, or whatever. Assuming 3-in. wide beadboard is being used, it will take 4 pieces, placed edge-to-edge, to cover one running foot on the width of the wall or ceiling. On an 8-foot wide surface, that equals 32 pieces. And if the MC of the beadboard were to increase 4%, that equates to over 1/32 in. of expansion per piece, or about an inch of total expansion, and that’s just in 8 feet! Imagine what happens when a beadboard porch ceiling runs perpendicular to a home!
While we can’t change how nature does her business, we can be aware of conditions and plan accordingly. Fortunately, you need not spend a ransom to acquire a decent moisture meter. There are entry-level models available for as little as $30, though one should probably plan on spending at least $125 and up to acquire a good quality instrument.
Pin or Pinless?
That’s the first choice you will need to make when selecting a meter. Most moisture meters read the moisture content of material through pins that are driven into the surface of the material. The pins are needle sharp barbs, typically around 1/2 in. long. The pins reach into the cells of the material, measuring the electrical resistance of the product. Since moisture is a good conductor of electricity, the greater the amount of moisture present, the higher the reading will be.
Pinless moisture meters typically bombard the material being tested with RF (radio frequency) signals, measuring dielectric properties which change proportionally to how wet or dry the material is.
The advantage to a pin-type meter is that it usually costs less than a pinless. I’ve owned a very serviceable Lignomat pin model for close to 15 years. If I recall correctly, I paid less than $100 for it. A disadvantage to this type of meter is that it’s a bit invasive, leaving two holes wherever it’s pushed into a surface (see image, below). While not very large, these holes can be an issue if the surface is already finished (as is usually the case with cabinetry, for example).
On the flip side, some instruments offer longer pins and even hammer probes for checking the MC of thicker lumber such as logs and timber. This method is very accurate, which is why these are usually the tools of choice for commercial lumber and millwork operations.
Pinless meters leave no holes, but are usually more expensive, with prices starting at around $200, and often selling for much, much more. So, if one can afford it, why wouldn’t a pinless meter be desirable? In a nutshell: Accuracy. Pinless meters scan a cross-section of material, and then average the readings to give you the MC. If more moisture is present on the surface of the material (which is easier for the meter to scan) vs. the core, the MC reading can be skewed.
Additionally, a pinless meter is most accurate when the surface it is reading is dead flat and at least as wide as the scanning portion of the instrument. If you need to determine the MC of a sizable piece of lumber, such as a post or beam, you’ll get a more accurate reading by using a pin type meter. But if you need to verify whether that load of pre-finished, custom mahogany frame and panel interior doors is ready for installation, go pinless.
Pin meters have another limitation. They won’t typically read MC under 6%, since material that is drier than that doesn’t have enough moisture present for the meter to detect. This shouldn’t be much of an issue for carpenters in all but the most arid parts of the country, as even the driest material we install is usually in a range above 6%.
If the aforementioned information still can’t help you select a type of meter, there’s always the combination, or dual option. Fairly recent to the marketplace, a dual meter offers the advantages of both types of instrument in one package. It allows the user to quickly scan the surface of the material using pinless technology. If more accuracy is required, it also functions as a pin-type meter, thanks to the probes incorporated on its leading end. This best-of-both-worlds tool usually has a starting price of around $400.
|For 3/4-in. material, the pins should penetrate at least 1/4 in. into the wood. To get an accurate reading on thicker material, the pins must penetrate deeper.|
|This pressure-treated 4×4 was soaked and felt like lead.|
Some meters require manual calibration to ensure accurate readings, while others are factory-set and not adjustable. This issue reminds me of the “fixed vs. adjustable vial level debate,” with each side having its proponents as well as opponents. If you choose a meter requiring calibration, be aware that you may also need to purchase a calibration kit with the meter, and occasionally perform accuracy tests. My feeling is that this is probably overkill for most carpenters as the increased accuracy (if any) is likely not enough to warrant the additional expense and effort.
The more accurate moisture meters require the user to select the type of material being tested. Because not all wood has the same resistance properties, one must set their meter to a predetermined species group. My Mini Ligno, for example, lists over 20 different wood species in two different groups. Setting the meter to the group being tested is clearly outlined in the user manual for the instrument, and usually requires little effort. On my meter, it’s as easy as double-clicking a contact switch that’s between the pins.
In using a couple of the newer models from Lignomat, I was pleased to discover that the more recent entries from this manufacturer also make selecting the species group easy, as they use the same style contact switch as my old meter. I was even more excited to test a model (the SD) that had a setting for determining the MC of not only wood, but plaster and drywall, as well— something my old meter did not allow me to do. This is a particularly useful feature when solid wood paneling is to be installed over a newly finished wall surface, as the wall must be completely dry, or any remaining moisture is likely to wreak havoc on the fresh wood (especially if the backside of the material hasn’t been sealed).
Moisture meters are offered with one of three readout displays—LED, digital, or analog.
LED displays give approximate readings, rounding the MC number to the closest (typically even) number, and illuminate a light, or lights, on the meter that correspond with the number. Some models will illuminate two numbers, should the MC fall between them.
Digital displays, as the term suggests, provide a digital readout of the MC. Some of the more expensive meters even measure to the tenth of a percent! I found these displays to be the easiest to read, with no interpretation required. An additional benefit these displays share with LED displays is their ability to be easily read in low light conditions.
Analog displays use a scale along the lines of those on electrical test meters to display the MC. Some of the screens are a bit small, so those of us with aging eyes may find them a bit of a challenge to read. However, these displays are pretty accurate.
You Get What You Pay For
While doing the research for this article, it quickly became apparent that the old adage, “You get what you pay for,” applies here, as it so often does with tool purchases. I got my hands on a handful of different models of moisture meters from several manufacturers. Since I’d been using an old (by today’s standards) model for so long, I wondered what, if anything, I was missing in not having the latest technology. I also wondered if a bargain basement model would perform well enough that I could recommend it as an option to those who simply can’t, or don’t want to, shell out hundreds of dollars for a device that may not see regular use.
I looked at meters that ranged in price from $30 (yes, thirty!) to around $400. I checked them against each other for accuracy, ease of use, features, etc. In the end, I came to the conclusion that moisture meters are akin to personal safety equipment—something is better than nothing. Even the $30 model I tried was functional, and may well suit the needs of an occasional user. It didn’t have the accuracy, range, or capabilities of more expensive models, but one shouldn’t expect it to, given its ridiculously low price.
Conversely, I didn’t see the value in the $400 instrument for my needs. It appeared to be well thought-out, with wonderful accuracy, but I was perfectly satisfied with tools that cost half as much. I believe the lesson here is to carefully identify what your requirements are, then buy accordingly. If you need the capabilities of a more expensive tool, or just love owning fine instruments, a higher end model may be right for you.
When it was said and done, I decided to upgrade from my 15-year-old LED model to a newer version that is practically identical, though with a digital display. I appreciate the increased accuracy of the newer model compared to the old one, plus the digital readout is my favorite.
I’ve got a 15+ year old Mini-logo LED that has been very useful. Hardwood lumber suppliers will take back wet wood w/o a blink if you have the meter to verify the complaint. We recently got 500′ of 8/4 poplar that metered 14%. The price of a meter is paid for in saving one screwup!.
Great article Greg,
I also think a reason many carpenters don’t consider buying a moisture meter is they really don’t understand moisture content. “I dunno why it shrunk up so much….. It was kiln dried” or some variation of that statement is uttered on too many job sites.
Thanks for the article, Greg. This is important stuff, wood movement, and yes too many carpenters don’t really grasp it.
You’ve inspired me to get me hands on that mini-ligno meter and start carrying it and learn to use it.
Also, anyone interested should look back in the TiC archives for the couple of other good articles on meters and wood movement.
Measuring moisture content in pressure treated wood is best done by weighing and oven drying samples. Preservatives used in treatment of wood contain metals/salts that make electrical conductivity measurements useless.