Why Fence Boards Warp and How to Avoid It


Fences can be tricky to maintain. Especially if your fence is made of wood. While wood is a fantastic option for fencing in many regards, it can always run the risk of warping. Wooden boards are consistently exposed to outdoor weather, meaning that rain or shine, they absorb what’s in the air.

Fence boards made of wood naturally soak in water, causing them to grow in size. When the water is evaporated from the heat, the boards shrink again, which causes warping over time. Warping can be prevented through the use of pressure-treated wood, pre-drying, strong construction, and sealants.

Although this seems straightforward, ensuring that your fence boards don’t warp is a complicated process. Don’t worry because we are here to talk you through it.

Why Do Fence Boards Warp?

Bowing, twisting, curving, and cupping: these terms are used to describe how wood bends in different ways. But, when you describe it, it’s pretty much the same thing.

Any experienced woodworker knows that wood likes to move, but when it comes to building and maintaining a beautiful, and proportioned fence, you need to combat that natural movement.

Naturally, trees are made up of fibers that love to soak up water for later use when it becomes thirsty. When trees are cut down for use as homes, decks, or fences, they can still retain that water. And sometimes, they can retain a lot of it.

Later on, after the wood boards are cut they will always be wanting to get back to absorbing the same amount of moisture as they did when they were full of water.

A way to combat this absorption, and subsequent movement of the wood, is being able to dry out the boards as much as possible, building with it, and treating it afterward.

Technically, wood fibers, the ones that soak up water, move radially and tangentially. Since wood moves twice as much tangentially as it does radially, the width of a board changes twice as much as the thickness. This means that they can bend in curved shapes, which is referred to as warping.

This is why woodworkers prefer to use what are called quartersawn boards, meaning that the rings in the board are perpendicular to one another. This keeps the growth, due to the absorption of water, equal across the board.

But not everyone can afford quarter-sawn wood boards, and it doesn’t look great in every project, anyway.

So here are some pretty easy alternatives to what could be an expensive solution. Keep reading for five things you can do to greatly minimize the risk of warping in your wood fence.

How to Prevent Fence Boards From Warping

Here is what you need to know to avoid having a warped fence.

1. Buy the Correct Materials

Thankfully, nature comes in handy every now and then. There are types of wood that naturally absorb less water, meaning they don’t run the risk of warping as much.

These types of woods include Cedar, which is very popular for its great smell and how it naturally keeps pests like termites at bay. It’s one of the thickest wood types, which helps it to not warp.

Redwood is also a great option seeing as it has a natural chemical in it that keeps it from absorbing moisture naturally. It also has a comparably straight grain pattern, which also aids in keeping warping at bay.

And lastly, Fir is a comparable option in that it’s pretty stable when its moisture content is the same as its environment. This stage where no change in moisture absorption occurs, which is called EMC (Equilibrium Moisture Content,) is much more stable in Fir than it is in other types of trees.

It also helps to find wood that was cut from a particular part of the tree. Anything with perpendicular wood grain is best, though not always possible. If anything, just try your best to avoid the wood that looks like it was cut from the middle of a tree, meaning the grain is almost perfectly circular.

2. Use Pressure Treated Wood

Pressure-treated wood is a fantastic option when it comes to fencing. It’s chemically infused with preservatives that help it to keep from rotting or becoming infested with termites.

Most importantly, though, it becomes resistant to the damp, meaning more protection from warping.

3. Leave the Boards to Allow the Water to Naturally Evaporate or Heat Them in a Kiln

Thankfully, a lot of pressure-treated wood is kiln-dried, so you don’t have to worry so much about having to dry it out. If your wood hasn’t been dried out (which is called seasoning) at all though, a good rule of thumb is to let it dry out for about six months.

If it’s already been previously dried off most of its water content, it’s best to lay it flat crisscross with as much pressure on top as possible for 2-3 days to allow the wood to acclimate to your particular region.

At a minimum, leave the boards to naturally acclimate to a climate before working with them if you plan on building your own fence. This means leaving it upright, where both sides of the board can dry evenly, or wrapping them in plastic to keep them from changing.

You can monitor your drying wood with machines like hydrometers or moisture meters to see the difference in wood hydration at any time, which can help you to pick a spot where they can best acclimate without getting wet or dry too fast.

4. Use Pressure or Building Materials to Keep the Boards Straight

It’s best to keep your wood stored in specific ways to help reduce warping as much as possible. If you can, it’s best to put pressure on boards you are not using immediately in order to keep them as straight as possible, but also not to pack them in a way that moisture can seep into the bottom or in between the boards.

Some ways of accomplishing this are:

  1. Place heavier boards on the smaller ones, or put a heavy object on top of them to keep them from moving.
  2. Allow space in between them to let airflow get between the boards. This will allow equal drying on both sides of the wood.
  3. Cover your wood with materials that don’t allow moisture in, such as plastic or a tarp. This is especially important in the winter.
  4. If you only need a small amount of wood for a project, just clamp your pieces to a workbench or other sturdy object to keep it straight.
  5. Treat the ends of the wood as soon as possible. Moisture leaves wood much more quickly out of the ends of wood boards. In fact, it leaves the ends about ten to twelve times faster, meaning your wood could shrink a lot more on the ends than in the middle, leaving you with wonky looking lumber.

5. Build Your Fence as Sturdy as Possible

If you are building your fence, it’s best to build it as sturdy as possible, so that it doesn’t sag or bend in the long run. Some tips to help with this are:

  1. Use blunt nail tips or screws for your fence, as they cause splitting much less often.
  2. Pre-drill holes in your fence to keep it from splitting.
  3. Put in the back rails (the horizontal boards that keep the fence together) at least 8 in. from the top and the bottom of the fence. If your fence is 8 ft. or taller, put a back rail in the middle, too.

6. Seal in the Moisture Content as Quickly as Possible

Picking what time to seal in moisture in your fence wood can be tricky, though. It’s best to seal the exterior of the wood when the interior has reached what is called the EMC (Equilibrium Moisture Content.) This is when the wood neither gains or loses moisture because it is stable with its outside environment.

If you’re especially worried about any of the many ways your wood can warp, be sure to pick a sealer that is pigmented.

That way you can have the best of both worlds; wood that won’t absorb any moisture, and a beautifully highlighted exterior that will fit any aesthetic.

To keep your fence beautiful for years to come, it’s best to seal/stain the boards with a durable mixture every few years. This will help keep rot, water, and the damaging effects of UV rays (which can also cause warping) at bay.

If you want, you can also paint your fence, seeing as paint offers a lot more options when it comes to style.

To do this, it’s highly advised that you treat the ends of your lumber, as the paint is less effective than a stain in keeping moisture in or out.

Armed with this knowledge, you now can now buy, prepare, build, and maintain your wood fence successfully.

Warping can quickly turn a DIY project into a nightmare, not to mention a waste of money, but with the right materials, and precautions, you can build that gorgeous fence easy-peasy. And you also have full bragging rights when it comes to your friends and neighbors.

For more information and helpful illustrations on how to prevent your beautiful wood fence from warping, check out the youtube video below:

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