Wood Species: “Same Difference” in Moisture Content

Wood SliceAs every wood flooring professional knows, wood species fall into two main categories: hardwood and softwood. After that, however, it gets more complicated.

Every species of wood has its own unique moisture content (MC) properties, as well as grain patterns, colors, density (or “hardness”) ratings, workability, and chemical makeup that determine its durability over time.

A Primer in Wood Moisture Content

All wood species’ cells have some degree of moisture. Bound water is moisture that is held in the cell wall and free water exists as liquid and vapor in cell cavities (lumens). Even after a tree is cut down, these cells have the capacity to release and absorb moisture in relation to the relative humidity of the air around them.

StrawsThink of drinking straws packed together – no matter how tightly they are connected, there is still room for water to move in and out of each straw. This capacity is what makes wood a hygroscopic material. Densely packed cells (typically a characteristic of hardwood and some exotic varieties) may have a reduced capacity for free water because the inside of each “straw” is smaller, but that capacity still exists and is related to each wood’s capacity for swelling or shrinkage along with other characteristics.

In the wood flooring and construction industries, we talk about each species’ specific gravity (SG), a numerical evaluation of its weight at an optimal MC level.

Moisture Content and Location

Since wood is a product of its environment, a wood’s initial MC is partially predicted by geography. In the United States, wood production is generalized by three regions: Western, Northern and Appalachian, and Southern. Each region may have a predominant wood species, or may produce similar species that vary from region to region based on average rain fall, the length of the growing season, or even the soil composition.

The distinct traits of different wood species are multiplied for wood which is imported from around the world. Each species is a biological structure containing different chemistry and cell types.

Even from the same region, MC in green wood species can be highly-variable. A tree’s heartwood, its wood core, often contains very different wood MCs than its sapwood, the wood underneath the bark. Coastal Douglas fir, a common softwood, carries an average greenwood MC reading of 37 in the heartwood and a whopping 115 in the sapwood.

On the hardwood side, the sweet gum tree has a sapwood MC reading of 137 and a heartwood reading of 79. By contrast, white ash contains almost identical green wood species MCs from heartwood to sapwood. These variations, both by region and internal growth, are why lumber mills invest heavily in bringing green lumber to a standard MC level before it enters the process of manufacturing wood flooring or other wood products.

Wood FloorThat’s all great in theory, but what difference does it make when you’re installing a wood floor?

Accounting for the differences determined by region, climate or species can be a make or break proposition when working with wood flooring of different types. As more and more species are adopted into the flooring industry, these MC subtleties take more knowledge to be sure each floor lives up to its potential.

For any flooring professional, this basic fact stands: The relationship between wood’s MC and the relative humidity of its environment is highly interactive. It can be a hard lesson if you forget that, but it’s smooth sailing once you’ve mastered it.

Moisture Content on the Job Site

Changes in ambient relative humidity cause fluctuations in any wood’s MC, from the forest floor to the newly-installed wood floor on the job site. No matter the species, if the fluctuations are too great, your floor will suffer:

– Crowning

– Cupping

– Buckling

– Gaps and more

Wood of any species reaches its equilibrium moisture content (EMC) when its MC is in balance with that of its surrounding environment. That’s why it is so important to leave your stock on site for an adjustment period before beginning the installation, particularly when moved from one climate region or environment to another, even if that’s just from the local storage facility to the job site.

It’s also why an accurate wood moisture meter is an essential tool for installing any species of flooring. Because each species will react slightly differently, having the settings right on your wood moisture meter is critical. The same is also true of engineered flooring and bamboo flooring, which can have very different SG settings from one manufacturer to another. It’s definitely a case that merits a specialized tool for a specialized task.

The Bottom Line

Bamboo FlooringIf your wood moisture meter has a limited range of species settings or adjustment tables, determining MC becomes a game of potentially expensive Russian roulette. It is important to know the correct wood species’ parameters in order to precisely assess the wood’s MC. A wood moisture meter like the Wagner Meters MMC220 provides pinless technology with extended SG settings for a wider range of MC measurement that won’t mar the finished surfaces.

When working with any species of wood flooring, the principles of moisture measurement remain the same. Be sure your moisture meter will provide the information you need, because only then can you accurately measure and monitor the wood’s MC readings for each distinct wood species, and be sure your customers’ floors will provide the long lasting beauty and strength they expect.

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Tony Morgan

Tony Morgan is a senior technician for Wagner Meters, where he serves on a team for product testing, development, and also customer service and training for moisture measurement products. Along with 19 years field experience for a number of electronics companies, Tony holds a B.A. in Management and his AAS in Electronics Technology.

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