Imagine ordering wooden furniture from a factory overseas, waiting three months for production to finish and then discovering that 25 percent of the goods you receive are misshapen due to warping.
Where did the problem happen? And how can warped wood be prevented?
If you’re a manufacturer or importer of wooden products, you may have heard that moisture and humidity levels can impact the shape and integrity of wood. Warped wood can be a serious problem for some products, resulting in customer complaints and returns (related: How Experienced Importers Limit Product Defects in 3 Stages [eBook]). Where some products are concerned, such as building materials, cracked or warped wood that affects a structure can be dangerous.
Here you’ll learn about common types of wood warping, potential causes and ways you can prevent warped wood in your own products.
What is wood warping?
Wood warping is deformity in wood occurring when the moisture content of different parts of a piece of wood changes unevenly. When one part of a wooden board dries faster than another, for example, the drier part shrinks faster and causes stress that changes the shape of the wood. So a board that was meant to lie flat now has a bend in it somewhere. Warping is a product defect that can affect importers of a wide variety of products, such as furniture and wood molding.
Understanding equilibrium moisture content
Before exploring the different types of wood warping, it’s important to understand what causes wood to change shape. The main reason for wood swelling and shrinking is the relationship between moisture content and the relative humidity of the surrounding air.
When wood has a relatively high moisture content, it shrinks as it dries until it reaches equilibrium with relative humidity, sometimes called equilibrium moisture content (EMC). Conversely, wood with a relatively low moisture content will swell as it absorbs water from the surrounding air until reaching EMC. This direct relationship helps predict how wood will behave as it adjusts to particular conditions.
Factors that influence the rate of change in moisture content
Although all wood will generally reach EMC, the process of adjustment can take hours, days, weeks or months. Factors that affect the time needed for wood to reach EMC include:
- Size of the piece of wood (thicker pieces take longer to lose or absorb moisture)
- Coating (coated wood takes longer)
- Grain orientation (end grain wood takes less time)
- Ambient air temperature (warmer air speeds the process of wood reaching EMC)
Ambient air temperature influences the rate of change in wood moisture content but doesn’t significantly affect EMC.
5 Types of wood warping
Wood can warp in a number of different ways, depending on the cause. Understanding these common ways wood warps will help you diagnose potential problems occurring in production or storage:
Note that different types of wood warping are not mutually exclusive. Different warping shown in the same piece of wood could be a sign of multiple problems related to moisture content.
Wood species and warping
Some species of wood are more stable and less likely to warp or crack than others. The following species are known to be relatively resistant to warping:
- Redwood not only has a comparably straight grain pattern but also a natural chemical inside that protects against moisture infiltration.
- Cedar is among the densest wood species, which helps prevent cracking due to changes in moisture.
- Fir is very stable once its moisture content reaches equilibrium with the relative moisture of the surrounding air. At equilibrium, or when the wood is “seasoned”, fir will undergo very little shrinking or warping.
To learn if the wood species you’re using is especially vulnerable to warping and which species are more suited for certain applications, check out the Wood Handbook, a free guide from the U.S. Department of Agriculture.
Ways to prevent wood warping
Several factors affect wood warping, some of which are easier to control than others. Different types of warping also tend to be caused by different factors. So being able to link warp types found to particular causes helps in preventing future warping.
Wood grain and sawing techniques
Grain patterns can influence the way wood behaves as it adjusts moisture content. Wood that’s cut from the heart of the tree is generally more stable against shrinking and warping. Very straight grain with only 1/32 in (about 0.79 mm) between grain lines is less likely to warp, regardless of wood species.
Sawing techniques used in processing the lumber can also influence warping. An understanding of these can help you limit your risk of manufacturing warped wood products. For example, in a quarter-sawn board where growth rings are symmetrical, shrinkage will occur evenly and cup-type warping is far less likely. Then again, a flat-sawn board is less likely to experience crooking because both edges are equidistant from the tree’s core.
Poor saw maintenance and variation in saw speed can result in boards that are thinner on the ends than in the middle, which can cause bow-type warping. It’s also recommended not to saw lumber at an angle to the grain. Sawing so the grain is parallel to the edge of the board can help prevent crooking.
Proper storage techniques to prevent warped wood
The way you stack and store lumber is a major factor for how it will change as it adjusts to EMC. Some best practices for stacking and storage of wood include:
- All boards and the stickers placed between them in a stack should be of uniform thickness, especially when placed in the same layer.
- Stickers should lie flat and be vertically aligned.
- Lumber piles should rest on a flat foundation.
Placing heavy weights on a stacked lumber pile can also be helpful in preventing cupping . The wood also needs proper ventilation, which is best accomplished by spacing the material so all surfaces are exposed to the air.
Store your lumber in a clean, cool, shaded and dry location. If you're storing wood in an area with high relative humidity—80 percent or more—be sure it's wrapped in material that is impermeable to vapor. This prevents the wood from absorbing significant amounts of moisture from the surrounding air. High humidity should be a real consideration for those manufacturing furniture and other wood products in South China and Southeast Asia.
Properly drying or curing wood to prevent warping
Although the process for curing wood doesn’t usually affect crooking, it’s important in limiting other types of wood warping.
Kiln-drying is often used in manufacturing, as it gives the manufacturer more control over drying speed and degree. But all kiln-dried wood will acclimate to relative humidity of its environment. The main advantages of kiln drying over more passive natural drying are that kiln drying stops insect activity and sets the sap in resinous softwoods, which is especially important for wood used as building material.
Experts recommend certain practices for drying wood to prevent warping, such as:
- Don’t allow partially dry lumber to quickly regain moisture
- Don’t dry lumber too slowly (doing so could worsen any bowing and other warping)
- Don’t over dry lumber, which can lead to cracking, splits and end grain checking
Sealing the ends of wood can also help prevent warping caused by uneven drying. Moisture leaves wood ten to twelve times faster from its ends than through its other surfaces. And without sealing the ends of boards of wood, the ends tend to shrink faster than the rest, leading to undue stress that can cause warping.
Monitoring moisture content and checking for warping during inspection
Manufacturers working with wood need to closely monitor both relative humidity of their storage and production areas and moisture content of the wood material. Importers often rely on locally-based, third-party inspectors to visit these facilities on their behalf. But depending on your situation and relationship with your supplier, you may choose to rely on factory staff or high your own full-time staff to inspect instead (related: 4 Ways Importers Conduct Product Inspection [eBook]).
Factory workstations should also be equipped with calibrated prong-type moisture meters to measure the moisture content of the wood at various production stages. Prong-type, rather than surface-type, moisture meters should be used because moisture content can vary one to two percent between the wood's interior and outer surface.
How to check for wood warping
Checking for warped wood is a simple procedure:
- Place a board or piece of wood on a flat surface.
- Check along the edge of the wood to confirm it's level along the full length.
- If one part or end of the board is raised above the rest, the board is likely warped.
Wood warping can be a common problem for those working with this natural but complex and versatile material. And many manufactures find it vital to limit warping, especially when structural integrity or design aesthetics are important in the finished product.
There are many factors that affect the behavior of wood as it acclimates to different humidity levels and environments. But by knowing these common causes and ways to prevent warping, you’re better prepared to predict the way your wood material changes. And whether you manufacture wood siding, wood molding or wood furniture you can be confident how your wood products will take shape.Editor’s note: This post was originally published in July 2016 and has been updated for freshness, accuracy, and comprehensiveness.
Let us know what other tips you have for preventing warped wood in the comments section below!