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How to Flatten, Acclimate, and Store Thin Wood

Just a few moments ago you were filled with excitement as you entered your workshop to begin a new project. But now that flat board you left on your work table overnight has hopelessly warped into a “C” shape. Do not despair! This is a fairly common occurrence, and you can remedy it easily.

Cupped board

Why Wood Warps

Wood is a natural material that constantly adapts and changes with the humidity level. Learning how this works is important if you want to properly manage the wood’s movement and successfully store lumber.

Wood is constantly trying to equalize to the same humidity level as the surrounding air. In a high humidity environment, a dry piece of wood acquires moisture from the air through pores on its surfaces. In a dry environment, the reverse happens, and the wood may release moisture.

As the moisture content changes so do the size of the wood fibers. If the board takes on moisture from the surrounding air, the fibers swell and the board will expand slightly. And as dry air sucks the moisture out of the wood the fibers shrink.

The key to understanding why your nice, flat board warped in your absence is understanding that this swelling and shrinking doesn’t always happen at the same rate throughout the entire board. When left on a workbench or shelf with one face more exposed to airflow, the moisture content changes faster on that face than on the opposite side. This disparity in moisture and swelling is what causes the board to twist or cup into that all too familiar “C” shape. The thinner a piece of wood, the less resistance it has to this warping force and the faster and easier it will warp.

Is Kiln Dried Wood Stable?

Kiln drying is an effective way of drying green (freshly cut) lumber in a controlled environment to a specific moisture content. When done correctly, the resulting dried wood is fairly stable and stronger. But any wood, no matter the drying method, will continue to change moisture levels throughout its lifetime.

How to Store Wood to Prevent Warping

Now that you understand why wood warps you are well on your way to understanding how to keep it flat. Since we can’t completely prevent the wood's constant swelling and shrinking, the key to keeping your board from warping in the first place is to make sure it happens at an even rate throughout the board. There are several ways to do this:

Stacked and spaced wood

#1 Use Spacers to Ensure Airflow on All Sides of the Board

One way to ensure even airflow around the entire board, and thus even swelling and shrinking, is to stack your boards with small sticks (called stickers) between each board. Use scrap pieces about 1/4” thick (a pencil thickness) laid across the grain between each board in a stack. This ensures that each board can receive an equal amount of airflow on each side, and any moisture change to the wood will happen at the same rate around the entire piece.

stacked boards

#2 One Tight Stack

Another method is to simply stack the boards in a tight stack with no gaps between them. You can then place a flat piece of plywood on the top board. This method assumes that little or no air flow around the board is just as good as a lot—as long as that airflow is even. This method works just as well as spacers and sometimes better for very thin boards (1/16” or 1/8” thick).

Does Weight Help?

Regardless of which method you use, some weight on top of your boards can help hold the wood firmly in place. But since moisture causes the changes in the wood, weight alone won’t keep a board flat or flatten an already cupped board.

Acclimating Wood

When you bring a new shipment of wood to your shop, chances are that the environment your wood came from differs from the air in your shop. Your new boards will immediately try to equalize to the surrounding humidity level and, if not stored properly, may cup or twist in the process. Use one of the above methods to store the wood, and once the boards have been in your shop for several weeks, the wood will have mostly acclimated to its new environment, and the swelling and shrinking will have stabilized. Now the wood will retain its current shape (which ideally is flat) until it experiences another large humidity change.

How to Flatten Cupped or Warped Wood

To flatten a warped piece of wood, you’ll need to change the moisture content on one side of the board. Look at your warped board and identify the inside face of the “C” or cup. The wood fibers on this side of your board are dryer and have shrunk. You can use water to relieve the tension and allow the board to flatten.

Step 1: Dampen the Dry Side

Using a damp cloth or paper towel, evenly wet this entire face of the board as if you were applying a stain or finish. You can also use a spray bottle here. Ideally you should dampen the face of the wood without leaving any standing water or droplets. How much water you should use depends on the piece’s thickness and how much it has warped. Thicker pieces usually need more water, while thinner pieces require less. Either way, be sparing with the water and repeat the process if necessary. This is better than using too much and damaging the wood.

Step 2: Leave the Board Overnight

Set the damp board, wet face down, on a flat workbench or countertop. This prevents the wet side from drying too quickly and exposes the dry side to the air. Some weight on top of the board here can help coax the wood flat but isn’t always necessary. Let the board sit for 24 hours or overnight and check it again. If isn’t completely flat yet, start back at step #1 and repeat the process.

Step 3: Re-acclimate

Now that your board is flat, give it some time to equalize to the surrounding air and stabilize so it doesn’t warp again. Jump up to “How to Store Wood” and use one of the two methods to store the wood for a few weeks before trying to use your newly flattened board.

When This Method Doesn’t Work

In some instances, this method for flattening wood won’t work. If the wood was not dried correctly to begin with, it might carry too much tension in it’s fibers and stubbornly decide to always stay cupped. Or if the board is twisted and bent into an “S” or other odd shape, often the board won’t return to a flat state. Let us know if you’ve received a piece like this from us. We will make it right.

Finishing

Once you’ve completed your project you should seal the wood with a finish. This seals the pores in the surface of the wood, helps to keep out moisture, and further stabilizes the wood. You can use any number of wood finishing or sealing products. A rub-on, oil finish is easy to apply and retains the wood’s natural matte finish and color. Be sure to evenly finish all faces of your project to create an even seal! Finishing just one side of a board usually results in an imbalance in moisture content causing your wood to warp.