(The 2 Percent/ 4 Percent Rule)
The importance of measuring the moisture content in wood flooring is critical due to the influence moisture has on so many of the physical properties of wood. Moisture content readings may be determined using various moisture meters developed specifically for this purpose. Handheld thermo-hygrometers and electronic moisture meters should be the most important and valued tools in every professional wood flooring installer’s toolbox.
The determination of moisture content is an essential part of quality control within the flooring installation process. At the time of installation, as the installer, you should know the moisture content of the wood flooring, the moisture condition of the substrate, and the ambient conditions in the space. All three of these items will dictate whether the facility and the flooring are properly aligned before being installed.
Some of the most common technical questions we receive about moisture testing are related to the accuracy of moisture meters. The general questioning is not about the accuracy of a moisture meter as a tool itself, but about the accuracy of its ability to read the different products being tested before being installed when relying on the 2 percent/4 percent rule. This is a multilevel question, and the following will address many of the factors that may affect the reading the meter is giving – or not giving – you.
Let’s start with the use of the tool itself. First, consider that moisture meters do not directly measure the amount of moisture in the wood. A value related to the moisture content in the wood is determined from taking measurements of some of the physical properties of the wood that are directly affected by the moisture, or lack of moisture, within.
Whether you are using a pinless (dielectric) meter or a pin-type (electrical resistance) meter, both will give generally reliable readings from as low as 5 percent MC, up to the fiber saturation point (FSP) of the species being tested. There are, however, many user-related variables that can affect the outcome of the readings you are attempting to acquire.
Moisture Meter Reading Variables
The skill of the operator – although moisture meters normally are very simple to operate, many user errors can affect the accuracy and reliability of the readings.
- Read the user manual. Follow all of the meter manufacturer instructions on the proper use of the tool.
- Are the batteries in the meter fresh? A weak battery can affect the readings the meter is giving you.
- Are all of the components of the meter in good working order? Is there damage to the meter housing, pins, or sensors? Are the pins fully screwed in? Are the receiving electrodes and sensor pads cleaned and free of contamination?
Calibration – When was the last time you calibrated your meter? Calibration ensures the meter is giving accurate readings.
- Calibration of a new meter is taken care of from the manufacturer. ASTM D4444 is the standard test method for laboratory standardization and calibration of handheld moisture meters.
- Some meters can be checked for calibration internally or by use of a calibration block supplied by the manufacturer.
- Check with your meter manufacturer to determine if, when, and how to get your meter properly calibrated. Many meter manufacturers will provide a calibration certificate, which verifies NIST (National Institute of Standards and Technology) traceability and that the meter is operating properly. NIST is a non-regulatory federal agency that sets standards for accuracy in meter calibration.
Meter drift – How quickly did you take your meter readings? Meter drift is the decrease (or increase) in true moisture content over a specified elapsed time – record readings from your meter within the first 2 to 3 seconds to ensure accuracy.
Species corrections – Has the meter been species corrected for the floor being tested?
- Most meter manufacturers calibrate their meters to pine/douglas fir/hemlock.
- Species correction is a meter-manufacturer-specified, user-adjusted setting that is made to the moisture meter to compensate for either varying electrical properties (for pin-type meters) or densities (for pinless meters) of the species under test, as compared to the species of the reference calibration.
- Some meters have a species correction adjustment built in, while others include conversion charts to account for species variability.
Temperature correction – What was the temperature of the floor where you took your readings? The temperature of the wood will significantly influence the readings of a pin-type meter.
- Temperature correction is the adjustment that is made to the moisture meter reading to compensate for the phenomenon that the electric conductance of wood increases as the temperature increases, and vice versa. This adjustment, whether manual or automatic, allows for accurate measurements of moisture content even at extreme floor temperatures (i.e., less than 50°F and greater than 90°F).
- You can use the following chart to determine the temperature correction based on the surface temperature of the wood being tested with a pin meter.
Finish type – The finishes used on the wood flooring can also affect the accuracy of the reading you are getting.
- It has been shown that finishes containing metallic components, such as aluminum oxide, can affect the accuracy of your readings.
- Water-based sealers and finishes that are not completely dried can affect your readings. All coatings must be completely dried before evaluating the moisture content of the wood floor.
Accuracy Based on Averages
If every plank in a pallet of flooring were at the same moisture content and moisture meters gave the same readings on every plank of wood of the same species at the same moisture content, the moisture content of the entire pallet could be determined by one single reading. The actual moisture content of the planks within any pallet of flooring will vary from plank to plank, and even within each board. Moisture meters can also give various readings on different boards as well as different parts of the same board even at the same actual moisture content. Using the average from a large population of readings, you can determine with confidence that your test results will give you a good snapshot of what is accurate. The higher the number of moisture tests you can take, the more probable it is that the average value will give you an accurate perspective of the floor as a whole.
What does an “average” moisture reading mean?
First, take a look at what is necessary to determine the MC of an individual board. Using your moisture meter, if you were to take 10 readings from that board, you would likely get a range of high readings, low readings, and readings somewhere in the middle (depending on the characteristics of the board). Then take the sum of those readings and divide that by the number of tests to get the average MC of that board. You can run an oven-dry test on that board to validate the accuracy of your meter. This is not too difficult to do and will get you quantifiable information for that board.
Now take a look at the entire floor as one unit (which is more meaningful for installation purposes). If you were to take a moisture reading using the same process as detailed above but of every individual board that you are going to install in the floor, and then take the sum of those readings and divide that by the total number of boards tested, you would get the average MC of the entire floor. In this scenario, you’d also want to remove any extreme, outlying test results (and boards). That average would give you an accurate perspective of what the overall MC is of that floor, but measuring every board is a tedious task.
NWFA Guidelines suggest taking a minimum of 40 readings per 1,000 square feet of the flooring being installed, and then averaging the results to get a good representation of the actual MC of the entire lot of wood. More readings are better than less. Record, date, photograph, and document all of your results.
Moisture Testing the Right Species
Do you know which species you’re testing? Do you know the proper correction?
Moisture meter manufacturers use calibration curves (which are found as an average of 100 measurements for the same wood species) to determine corrections for testing that species. Most meter manufacturers have calibration curves for most of the species and subspecies used in the flooring industry and are normally pretty accurate to within +/-1 to 2 percent.
If you are installing an oak floor be aware that there are more than 600 different species of oak. More than 30 oak species are commonly used for wood flooring, each with unique characteristics that can affect the outcome of the moisture readings you are taking. Ask the supplier for adequate detail about the species of wood sold to you. A couple of very simple questions can help you identify the species or subspecies of the flooring you are testing.
- Who is the manufacturer?
- Where is the wood grown? (Northern-grown oak is quite different from southern-grown oak.)
If your supplier can’t get you this information, the manufacturer details should be included on the packaging, in the packaging, or on the underside of the flooring itself. Contact the flooring manufacturer to ask about the species. They may also be able to give you proper moisture testing procedures and species corrections for their product. If not, contact the meter manufacturer for accurate species correction settings.
Moisture Testing Engineered Wood Flooring
Your moisture meter will give you a reading when you test engineered wood flooring. But do you know if your reading is accurate? If you are not accounting for all of the variables necessary when testing engineered wood flooring, it is highly probable your readings are not accurate.
When moisture testing engineered wood flooring, you can’t simply set your meter to test the wear-layer species and then run the same testing you would on a similar solid board. The wear-layer species may not directly coincide with the conglomeration of material being tested. All of the materials within the depth of signal penetration can influence the readings the meter is taking. This can include the core and backing components (different species or composite materials), adhesives, and even substrate composition, which can all have different densities, specific gravities (pinless), or electrical resistances (pin-type). This will affect the accuracy of the end reading you are looking to acquire.
A pinless meter will take into account all of the material it is affected by within the depth of the scanning range it can receive. When testing engineered wood flooring using a pinless meter, you will need to know the specific gravity of the entire piece of flooring itself. Check with your meter manufacturer to determine what setting it recommends for the flooring being tested.
One useful method you can use to find an approximate specific gravity of the material is by the floatation method. The floatation method is a rapid method of determining the specific gravity of a piece of wood, although it is not extremely accurate, and the results of this test can be scrutinized. This test involves placing a board on end in a narrow vessel of water. The percentage of the total length of the board that is below the water line is numerically equal to the specific gravity of the specimen. This method may get you enough useful information to be able to determine what setting to use for your pinless meter.
A pin-type meter can take readings of engineered flooring, when you are physically able to place the pins directly into the wear layer, and isolate your readings to that layer of wood, then change the species correction to test the core and backing. Accuracy comes into play when either of the pins comes into contact with the adhesives and varying core materials within the construction of the engineered floor. In general, the adhesives and resins found in engineered flooring (as well as plywood and OSB) are more electrically conductive than wood itself. When using a pin-type meter to test these products, depending on where the pins lie within the specimen, the electrical resistance can be less than that of dry wood. This means your moisture readings could be higher than the actual MC of the flooring sample.
Fortunately, most moisture meter manufacturers have conducted a multitude of tests and can guide you through proper use of their meter when testing a variety of wood flooring products. Check with your meter manufacturer for testing protocol and appropriate species correction values when testing any engineered wood flooring product.
Moisture Testing the Subfloor
Your moisture meter will also give you a reading when you test a wood subfloor. But again, is your reading accurate?
When was the last time you walked onto a jobsite, looked at the OSB subfloor, and knew whether it was made of aspen or pine, and from where it was sourced?
Due to the variability in the materials used in the manufacturing processes of wood subflooring, such as the different species of woods used (southern yellow pine and aspen), the location from which the wood is sourced, and the variety of nonwood resins and adhesives used to manufacture them, it is difficult to get an accurate moisture reading of these materials.
The OSB and plywood manufacturing process can also affect their ability to absorb moisture readily in comparison to their solid wood counterpart. The manufacturing process fundamentally alters the wood cell wall chemistry. When the sites that water used to be able to enter through and bind to are now occupied by adhesives and/or if the wood is so altered that those sites are not accessible, EMC will be lower because fewer water molecules can get into the product at the same RH and temperature conditions.
The accompanying chart provided by the APA (the Engineered Wood Association) indicates the differences in EMC of each product category (solid wood, plywood, and OSB) at 70°F (21°C). The data provided for plywood and OSB has been scrutinized for accuracy by the wood science community, but in general, most would agree that EMC for solid wood is higher than EMC for plywood/engineered wood, which is higher than OSB.
Fortunately, many of our moisture meter manufacturers work closely with some of the OSB manufacturers to determine proper settings, testing methods, and correction values for their specific subfloor material. It is your responsibility to know your meter when testing wood subfloor materials. It is your meter manufacturer’s prerogative to help you determine the best setting for the material you are testing.
When in doubt, check the moisture content of other properly conditioned wood materials (2x4s, newel posts, wood beams, etc.) within the structure to get an idea of where the EMC is in comparison to where it should be, and then compare this value with the EMC chart as a baseline for testing the subfloor.
After calibrating your meter for the subfloor materials being tested, check the MC in a minimum of 20 areas per 1,000 square feet and average the results. Pay special attention to exterior walls and plumbing. In general, more readings will result in a more accurate average. The MC of the subfloor should coincide with the expected in-use (e.g., normal living) conditions of the facility, based on the EMC chart. Anything outside of this range would be considered unusually high. Record, date, photograph, and document all of your results.
Unusually high MC readings must be identified, documented, and addressed to establish the size and magnitude of the problem area. Installation should not proceed until the origin of the moisture has been identified and remedied.
IMPORTANT Never install a wood floor over a known moisture condition. A known moisture condition is one that you are aware of and could pose future damage to the flooring, the building, or the occupants. It is best practice to always test for moisture regardless of conditions so that any unknown conditions can become known conditions, which then can be handled appropriately.
2 Percent/4 Percent
NWFA Guidelines state that solid-strip wood flooring should be within 4 percent MC of the subfloor and solid-plank wood flooring should be within 2 percent MC of the subfloor. This is a general guideline that has proven to be a useful and acceptable tool for determining when it is OK to install wood floors and when it is not. When taking into consideration the many variables that can affect an accurate moisture reading, including the functionality of the tool itself; the temperature of the material being tested; the +/- 1 to 2 percent accuracy of the meter itself; the differences in each species or sub-species; the non-wood components of engineered flooring, plywood, and OSB; and the overall “skill of the operator” factor, it is necessary to understand what your readings really mean. With any of these factors taken into account, you could see a 2 percent to 4 percent (or more) variation in what is accurate and what is not.
So why follow a rule that appears to be based on a general concept in many scenarios rather than on quantifiable data? Refer back to the “Accuracy Based on Averages” section of this article.
It is also your responsibility as a professional in the wood flooring industry to do your due diligence when installing a wood floor with the professional tools made available to you. Due diligence is defined as “an action that is considered reasonable for people to be expected to take in order to keep themselves or others and their property safe” (Cambridge Dictionary). By taking moisture readings of the wood floor and the subfloor, with all of the previously mentioned factors taken into account, along with following the manufacturer’s instructions and NWFA Installation Guidelines, you are doing your part.
The 2 percent/4 percent rule is a general guideline that is based on decades of experience. Does it fit every scenario? No. Does it work most of the time? Yes.
Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at firstname.lastname@example.org.