How to Predict Lumber Shrinkage

What Is Lumber Shrinkage?

Lumber shrinkage is when wood naturally shrinks because of a loss of moisture within the lumber.

Of course, lumber always shrinks when it dries, so it’s important to take that into consideration when planning any wood project.

Lumber shrinkage can also happen when lumber reaches its fiber saturation point (FSP).

When wood reaches its FSP, there’s no moisture, or “free water,” in its cell cavities and only its bound water (water chemically bound within the cell walls of the wood) remains.¹ In most wood species, the FSP point is about 30% moisture content (MC), and MC changes below 30% can lead to shrinkage.

But there are other important factors to consider when planning a wood project.

Wood’s Hygroscopic Qualities

Wood is hygroscopic, which means it absorbs moisture from the air. The MC within wood fluctuates based on the temperature and relative humidity of its environment. These fluctuations naturally cause the wood to shrink and expand based on these environmental changes.

When humidity rises, lumber expands. When humidity decreases, lumber shrinks. So some wood movement should be expected because of its hygroscopic qualities.

When the MC of the environment, such as the amount of humidity in the air, is equal to the MC of the wood, then the wood has reached its equilibrium moisture content (EMC). But the EMC will still fluctuate throughout the year as temperature and humidity change with the seasons.

Wood’s Dimensional Changes

In addition to wood being hygroscopic, it’s also anisotropic…wood has different properties based on its grain direction. Lumber doesn’t shrink the same way in all directions, so the shrinkage isn’t uniform.

For example, the end grain of wood absorbs and gives up its moisture at up to 100 times the rate of the face of a wood piece. This is why many wood professionals recommend sealing end grain wood to slow down absorption, whether this happens before a project or before storing lumber for a long time.

The properties in freshly cut green lumber change when it’s dried. The more moisture in lumber, the larger its dimensions. So when it’s freshly cut and in its green state, wood is at its largest. Once wood is dried and moisture has been removed, it goes to its smallest dimensions.

The maximum possible percentage of the shrinkage of wood from green wood to dry wood is referred to as its volumetric shrinkage.

So the way wood is cut affects how much it moves. Wood shrinks the most around the growth rings (tangential shrinkage), about half as much across the rings (radial shrinkage), and the least amount along the grain (longitudinal shrinkage).

The greatest amount of wood movement happens in relation to the growth rings in lumber—anywhere between 5% to 15%—while change in the radial dimension is much less at 2 to 8%. Longitudinal shrinkage is usually very minimal, around 0.1% to 0.2%.

Typically speaking, flatsawn lumber (wood that’s cut in a parallel, tangential direction to the growth rings) will move nearly twice as much as quartersawn lumber (wood that’s cut from the flat side of quarters of a log).

Wood’s radial shrinkage plus wood’s tangential shrinkage equals wood’s volumetric shrinkage (how much wood will shrink). Longitudinal shrinkage is omitted from the formula because it’s so minimal, making it inconsequential to the volumetric shrinkage.

Because of these movement differences, tables with movement values include both the tangential movement value and the radial movement value to give a comprehensive picture of potential wood movement.

Wood’s Species Shrinkage Values

Different wood species have different densities, so they respond to moisture differently. The Wood Handbook prepared by the Forest Products Laboratory includes a formula to help predict calculated shrinkage based on anticipated changes in MC for different lumber species, whether softwoods or hardwoods.

Here’s a simplified version of this formula: Calculated shrinkage = Wood width (metric or imperial) × wood movement value × expected change in MC (%).

Let’s say you’re building a cabinet door in Seattle, WA, out of 24-inch-wide western red cedar. The wood is quartersawn.

Expected changes in MC are based on location and season since temperature and RH fluctuate throughout the year. The Equilibrium Moisture Content for Outside Conditions table can help you predict the expected change in MC in different parts of the U.S.

For example, in Seattle, EMC is lowest in July at 12.2% and highest in December at 16.5%. EMC fluctuates between those two numbers throughout the rest of the year, but EMC is highest during the cooler months and lowest during the warmer months.

The difference between 16.5 and 12.2 is 4.3, giving you the greatest possible MC change for the wood.

Referencing the Dimensional Change Coefficients table in the Wood Handbook, you see the radial coefficient for this softwood is 0.00111.

Using the simplified formula above, you’d plug in your values: Calculated shrinkage = 24 inches × 0.00111 × 4.3

Calculated shrinkage = 0.114552

Using the formula, you can account for this possible calculated shrinkage of under a quarter inch when cutting the wood for your cabinet door.

Using the Dimensional Change Coefficients table and Equilibrium Moisture Content for Outside Conditions table in the Wood Handbook, you can predict wood movement values in flatsawn and quarter boards based on the wood species you’re working with.

If not properly monitored and managed, lumber shrinkage can lead to warping, splitting, or even checking. But with the help of the Wood Handbook and your own expertise and experience, you can predict wood movement before problems arise.

Lumber shrinkage is unavoidable to some degree, but here are some additional steps you can take to minimize shrinkage and be more prepared.

How to Minimize Lumber Shrinkage

Using formulas and referencing tables can be really helpful, but here are some additional practical tips for minimizing lumber shrinkage.

Store Wood in a Dry Place

Store wood in a dry, ventilated area, making sure to stack it with stickers for even airflow.

For best results, store in a room that holds temperatures between 68°F to 74°F and relative humidity between 35 to 50%. This range is considered stable for most wood products.

You may need to use a humidifier (especially in colder temperatures or seasons, like winter) or a dehumidifier (in the more humid summer months) to maintain the RH range and avoid excessive humidity fluctuations.

Good ventilation also helps to manage air circulation and prevent moisture from building up in the air.

Avoid stacking wood under direct sunlight or directly on concrete, as these practices can affect the moisture content of the wood.

Check Wood With a Wood Moisture Meter

Knowing the moisture content of wood can help you know how to address the wood if problems arise, or even before problems begin.

Wood’s hygroscopic structure means it’s inevitable that it’ll release and absorb moisture to reach the EMC of the environment it’s in. It’s best to let this acclimation process finish before starting a wood project.

Wood begins to acclimate to its environment almost immediately, so you should check the MC of your wood as soon as it’s bought or delivered. If the initial moisture content is too high, you can always return the wood or reject the delivery.

Whatever the case, periodically check the wood with a wood moisture meter to keep tabs on its MC. Digital moisture meters provide a quick, non-destructive, reliable way to check the MC in a piece of wood.

Monitoring Moisture

Plan your wood project knowing that wood movement will occur.

The greatest way to protect your lumber is by using reference tables and monitoring its MC to help predict the amount of shrinkage it’ll go through. Shrinkage doesn’t have to put a damper on your work when you intentionally keep track of it.

Woodworking professionals and hobbyists alike can be confident that Bessemeter wood moisture meters provide the accuracy, reliability, and ease needed for any wood project. With a Bessemeter moisture meter, you can find moisture problems and stop them before they cause serious damage.

and make sure your work stands the test of time…and moisture.

1. “Moisture and Wood,” NWFA, 2017, p. 7. (↑)