Last month, I brought home some plywood. A month later, it’s visibly cupped:
Wood warp is an inevitable part of woodworking. It’s impossible to completely avoid, however there are a few easy things you can do to reduce the severity and prevent the wood from warping as much as possible.
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1. Store Wood in a Cool and Dry Place
Wood warps because of uneven absorption of moisture. If water is spilled on one side of the board, the water will soak into the wood and cause one side of the board to expand, while the other side of the wood stays the same size.
This causes the board to bend to accommodate the slightly larger side.
While spilling water on a board is an obvious case of a change in moisture content, the atmosphere around the wood can cause the level of moisture to change as well.
Humid air has more moisture to absorb. Dry air has less moisture.
The more moisture that’s available, the more the lumber will absorb. And since single pieces of wood generally come from different portions of the tree, the wood rarely absorbs the moisture evenly.
So storing your wood in cool and dry place helps prevent wood from absorbing moisture and warping.
And if it’s already too late and your wood is a warped mess, check out this post on saving warped plywood!
2. Lay Wood Flat During Storage
While moisture absorption is the primary reason wood warps, other forces can cause wood to bend as well, gravity being a key one.
When lumber is leaned against the wall, gravity is pushing the wood toward the ground. In the short-term, this is unlikely to bend the wood.
But if you’ve ever left a piece of wood leaning against the wall in your basement, only to come back a few months later to find a cupped board, you know that over time, gravity will bend the board.
Therefore, it’s best to lay long boards flat during storage. Someday, I hope to have shelves in my workshop to hold my boards.
In the meantime, they’re just stacked flat on the floor.
Not great, but still better than storing the vertically.
3. Use Stable Types of Wood
Some wood is less-susceptible to warp than others, because the grain is tighter and therefore less likely to absorb moisture.
Redwood and cedar are both known for their ability to reduce warp.
Additionally, plywood tends to be more stable than wood. To be honest, I have a hard time believing this, because plywood seems to warp easily. That photo at the top was plywood.
However, some of that is a mind trick. Plywood appears to warp more than wood because pieces of it tend to be large, which makes any warp more obvious.
But it’s actually more stable, because the alternating layers of wood help stabilize the material. Wood only expands along it’s width, so by alternating the direction of the layers, the adjacent layers can help keep the plywood from moving.
So when working with smaller plywood pieces that are more comparable to lumber, plywood is a more stable choice.
4. Use Sealant on the Wood
As we’ve discussed, moisture is the primary cause of warp in wood.
So if you can find a way to prevent any moisture from reaching the wood, you’ll prevent warp.
This is part of the reason you rarely see warp in finished wood products. The finish reduces the moisture absorption enough that warp isn’t easily visible.
Any quality wood finish will prevent warp. Thick finishes that form a film on the top of the wood, like polyurethane and resin, will do this best, but I love shellac as well.
When you apply the finish, make sure to apply it evenly across all parts of the wood, not just the easily visible areas.
For example, if you were finishing a table, finish the bottom of the table as well as the top.
By applying sealant evenly throughout the entire piece, you’ll ensure that any moisture is absorbed evenly.
If finish was only applied on one portion of the wood, the other portions would absorb moisture more rapidly, causing the wood to warp.
5. Acclimate the Wood
Wood primarily changes size when it’s introduced to a new atmosphere, because it responds to the new air conditions, absorbing moisture until the balance of moisture in the wood matches the surroundings.
By moving the wood to its final location before actually building the project, you give the wood time to absorb or release moisture before you begin.
This gives you a more stable piece of wood work with.
Otherwise, if you were to begin the project immediately, the wood would likely change size after you built the project.
This can lead to warp or unsightly gaps as the wood tries to move while being bound in the build.
So allowing your wood to acclimate to the environment helps reduce warp in the final project.