By John Niggl
Imagine ordering wooden furniture from a factory overseas, waiting three months for production to finish and then finding that one fourth 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. 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?
What exactly 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 part, 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.
Understanding equilibrium moisture content
Before getting into 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.
|Moisture Content vs Relative Humidity|
|Relative Humidity||Moisture Content|
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 will take less time)
• Ambient air temperature (warmer will take less time)
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. That’s why it’s important to understand these common ways wood warps to help you diagnose potential problems occurring in production or storage:
|Types of Wood Warp|
|Bow||Lumber deviates from flatness lengthwise, but not across the face|
|Kink||Lumber deviates edgewise from a straight-line from end to end|
|Twist/wind||Turning of four corners of any face of a board is no longer in the same plane|
|Cup||Lumber deviates from a straight line across the width of the wood|
|Crook||There is movement along one edge of the lumber|
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
Warp warping is affected by several factors, 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 inch (about 0.79mm) between grain lines is less likely to warp, regardless of wood species.
Wood warping is also affected by sawing techniques used in processing the lumber. An understanding of these can help you limit your risk of 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.
|Comparison of Sawing Techniques|
|Cut Type||Cost and Waste Produced||Popularity||Orientation of Growth Rings||Relative Stability||Image|
|Plain / Flat-sawn||Low cost, low waste||Highly common||Intersect face of the board at <30° angle||Low stability and low resistance to warping|
|Quarter-sawn||Middle cost, middle waste||Fairly common, seen as acceptable compromise||Intersect face of the board at 60-90° angle||Middle stability and middle resistance to warping|
|Rift-sawn||High cost, high waste||Uncommon||Intersect face of the board at 30-60° angle (45° optimal)||High stability and high resistance to warping|
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 that grain is parallel to the edge of the board can help prevent crooking.
Proper storage techniques to prevent warped wood
The way lumber is stacked and stored 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.
It can also be helpful in preventing cupping to place heavy weights on a stacked lumber pile. The wood also needs proper ventilation, which is best accomplished by spacing the material so all surfaces are exposed to the air.
When storing lumber, it should be placed in a clean, cool, shaded and dry location. Wood stored in an area with high relative humidity—80 percent or more—should be 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 wood products in South China and Southeast Asia.
Properly drying or curing wood to prevent warping
Although the process used 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 are that it 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 (related: Independent Inspection vs. In-House Inspection).
Factories that produce wooden products should have accurate hygrometers installed to measure relative humidity in storage and production areas. Inspectors should note the humidity in each relevant area. Keeping a record of these readings helps diagnose the causes of any wood warping found in the product.
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 wood moisture content can vary one to two percent between the interior and outer surface.
Checking for warped wood is a fairly simple procedure. Place a board or piece of wood on a flat surface and check that the board lies flat from end to end. If one part or end of the board is raised above the rest, the board is likely warped. Each face of a board should be checked this way to make sure it has a uniform shape.
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.
John Niggl is a Client Manager at InTouch Manufacturing Services, a QC firm that performs product inspections and factory audits in China for US and EU clients. John also writes for the QC-related blog, Quality Wars.