Cupping in lumber is defined as a bow along the face of the board. Wood is a natural product that absorbs and releases moisture easily, depending on the relative humidity of the environment. While it's not possible to completely prevent lumber from cupping, these tips will help minimize cupping, warping, and crowning in many situations.
Things You'll Need
- Dimensional lumber such as boards, 2x4s, posts.
Choose wood that has been kiln dried and store it properly. Lay the boards on stickers up off the ground or floor, and align all stickers throughout the pile. (Stickers are short pieces of dimensional lumber, usually scraps, all the same size for even piling.) Support the ends well. Line up all boards in the pile, and try to create a cubical stack (same height as width). If possible, band the stack together.
Store wood in a building or shed where it will experience fewer sudden changes in relative humidity. Allow the wood to acclimate to the place where you will eventually use it by stacking it near the project area for a week or more.
Use quarter-sawn lumber rather than flat-sawn lumber wherever possible, as it is less prone to cupping. Flat-sawn lumber has large rings (grain), and this makes it prone to absorbing moisture from the air.
Use the "rip-and-flip" method to help wide boards stay true in situations where matching grain isn't critical. Simply rip the board in half, flip one half end-for-end and glue the two pieces together with wood glue. While the wood may still cup, it will be less noticeable.
Use the narrowest boards possible, and choose hardwoods. Wide boards tend to cup more than narrower boards, and softwoods cup more than hardwoods.
Finish the wood properly. Apply stain, paint polyurethane and other products to all surfaces of the wood, including end grain. This may require you to finish the wood before you assemple the project, but it will minimize distortion caused by changes in relative humidity.
Tips & Warnings
- Proper initial seasoning and storage are the two most important things to consider when working with wood. If a board is badly cupped, discard it because there is little you can do to minimize the cupping.
- Photo Credit lumber 1 image by jimcox40 from Fotolia.com