Too many projects either start with warped wood, or are built and then suffer from it. But, it is both preventable and fixable with the proper attention to WHY wood warps. In this article learn how to fix warped wood.
You probably know this, but the #1 enemy of wood is moisture.
To combat moisture damaging wood requires a special drying process to ensure it is ready for use in woodworking projects, proper storage and maintaining the finished product in a humidity controlled environment.
Unfortunately, drying wood is difficult at best and novice woodworkers (or supposed experts) often use wood that isn’t ready for use. And warping, cupping or bowing is the result.
In this article learn:
- What causes wood to warp
- Why moisture matters
- Purpose of kiln drying
- How to fix warped wood before building a project
- And, how to fix a project that is built and THEN warps
Note: Before we begin, if you’re attempting to fix a project that you bought and paid for the first step is to always go back to the builder. Attempting to fix the project first would likely void any actual or implied warranty. Proceed at your own risk!
What Causes Wood to Warp
The most common reason for wood to warp is it has not been properly dried.
While some wood is kiln dried to accelerate drying, other wood is air-dried. But the purpose of all drying is to bring wood to an Equilibrium Moisture Content (EMC) that will help prevent it from taking on or losing excessive moisture once milled.
Which means that, unfortunately, is almost all wood can and will warp if not first dried properly and then subsequently taken care of.
Not Reaching Regional Equilibrium Moisture Content
As you might guess the EMC of wood will vary based on your climate. For example, in arid desert areas like Arizona wood will reduce to almost 6% moisture content before it is considered stable.
On the other hand, higher humidity climates like those found in Seattle have EMC of higher than 13%.
So what does that mean for your project? Well, you’ll need to dry the wood to the proper EMC through a proper drying process BEFORE using it.
Wood Cups and Bows In the Sun and Heat
Unfortunately, even if you have properly dried wood it’s still possible for it to warp if it is subjected to uneven drying. A few common ways this happens include:
- Direct sunlight will dry the top of the board faster than the bottom. While most properly dried wood with finish on both sides will easily endure this, wood that has not been finished or has higher EMC will dry and cup.
- One-sided finishing is a common mistake with some novice woodworkers where the top of a project is stained and finished, but to save time the underside isn’t. As you can guess, the result is a warped finished product as the wood adjusts to relative air humidity unevently
Lacking Proper Support During Drying Causes Wood Warps
More commonly found with exterior lumber, if wood is not properly stacked, end grain protected and weighted down it WILL warp during the drying process.
How to Fix Warped wood that isn’t finished
First, if your wood has not reached the proper EMC, as measured by a moisture meter then you’ll need to stop and address moisture content first.
If your wood is properly dried and ready for use then the following tricks can be applied.
Use a Jointer to Flatten the face
Jointers are designed specifically to flatten curved or cupped wood. And while the most common use is to straighten the edge of a board, with proper technique they can flatten a wide face of a board.
A few considerations:
- Use proper safety equipment and follow your machines instructions
- For straightening a warped edge use the outfeed table as a guide and progressively make passes until the wood passes through with a consistent cut
- However, for a warped face of the wood find the uneven twists and make progressive passes that shave the “high side” off first. Then, when one side is flat, run the board through a planer with the now-flat side down.
Table Saw Sled
Using a table saw sled to straighten bowed wood is my favorite method for longer wood.
As shown in the picture, this method uses a “sled” that is run against the face of the table saw. And then, using toggle clamps , clamps the bowed wood while it is run through the table saw.
For almost all woodworkers this is a jig worth making and having handy – especially if you are using a steady supply of dimensioned lumber.
Glue Warped Wood and Then Surface
If your bowed wood can be edged and made flat for gluing, an alternative to fixing individual boards prior to gluing is to, well, glue them into panels.
By gluing the panel you’ll create a larger, more stable surface area to complete the removal of warps.
The catch? You’ll need to mill the entire assembled panel down (usually) at least 1/8″. And, as a tip, NEVER use clamps to force twists and bows vertically against the glue joints. By doing this you’ll create hidden tension in the panel that will often fail later.
Ironing Bowed Wood
While a popular attempt at straightening wood, ironing and other water-based approaches use more moisture to attempt to fix an issue caused by moisture.
However, if your wood warped, it likely has an existing moisture issue and ironing or sunlight solutions are not going to create a long-term fix.
When this happens, I tend to consider the wood as scrap or cut down into a size for drawers or other projects that won’t make the bowed wood an issue.
How to Fix Warped Plywood
When it comes to plywood it’s number one structural feature is the laminated layers that add strength.
But, with this lamination, an undesirable mixture of different materials, structural defects and wood species can create opportunity for plywood warps.
While not all plywood needs to be flattened prior to use, if you have a severe case the best cures are:
- Cutting the plywood to finished size to see if the warp impacted the use in the project
- Or, use moisture from a sponge and a sufficient amount of weight to attempt to relieve the “dry side” and let it expand to flatten the sheet.
- Lastly, and for thinner 1/4″ plywood, I’ve found simply forming it into shape on the back of a cabinet or piece of furniture is enough. Over time it assumes its proper shape.
How to Fix Warped Wood in Finished Projects
While fixing warped wood BEFORE its finished is ideal, the harder fix is when wood warps AFTER its finished.
If you remember back to EMC, if wood has not been properly dried and is then placed in an environment (indoors or outdoors) where it is exposed to a moderate to high imbalance in humidity the result is almost always a warp, twist, bow, cracked surface or cupped wood.
Which, if you’ve paid for a wood project, is beyond frustrating and usually extremely costly. And, if you made the project, disappointing as the fix isn’t always guaranteed to work.
Reverse the Cause of Twisted Wood
A classic cause of finished wood projects warping is the top of the project was finished and exposed to the sun. And, if you catch it soon enough, one way to reverse the issue is to reverse the exposure. While this may not always work due to grain patterns, its worth a try if you catch it quickly.
Rip and Redo
If a finished piece of wood has a bow or cup then odds are the wood will not be recoverable. When this happens, you’ll need to evaluate a few things:
- When the entire piece is new, generally the entire project might be at risk.
- If the underside of the wood (eg. on a table) was NOT finished then consider immediately applying finish to the un-impacted wood to prevent it from warping or cupping
Rather than trying to fix a cup or a warp, if the piece of wood is relatively easy to remove then taking it out and replacing is a prime option. Just make sure you replace it with properly dried wood.
Use Weight and Time (and Repeat)
For fixing cabinet doors or other glued up projects that can “flex” one of the tricks you can deploy is using weight to rebalance the wood.
To begin, find a flat surface that will hold the entire project and enough weight that it will flex the project (without breaking it). Then:
- First, insert a small shim or magazine under the corners that are “low”
- Then, place a small amount of weight on the edges of the door that are high until you see a deflection in the wood.
- Leave the weight on the door at least 24-hours, then remove and check to see if the door has straightened.
- Lastly, repeat this process using more time or weight until desired results are obtained.
Choosing Lumber That is Dry
While design of a project is often the #1 focus, choosing stable wood is the foundation to a successful project.
To avoid instability in wood, consider:
- Kiln dried wood is best for almost all projects from cabinets to furniture and tables. Use a hardwood supplier you can trust, and a moisture meter.
- Air dried wood takes a year per inch of thickness. Meaning, if you are wanting to use that tree you sawed into lumber you’ll need to wait.
- Store wood in a lumber rack if dried.
Frequently Asked Questions
Why does wood warp?
Wood most commonly warps due to moisture and heat unevenly impacting the piece of wood. For example, wet wood that is exposed to sunlight will dry on one side faster than the other. When this drying occurs the wood will physically shrink and “pull” the wetter wood, creating a twist, bow, cup or even split.
Can plywood bows be fixed?
Fixing a warped piece of plywood is not always possible since, by design, plywood is composed of multiple layers of wood that are banded and glued together to provide rigidity. This rigidity, when the plywood warps, makes it difficult to evenly straighten the plywood.
Where is the best place to store wood to avoid warping?
For woodworkers, the best place to store your lumber is off the floor and on a lumber storage rack that has supports every 24″ to ensure material is flat. Concrete, surprising to many, is a major source of moisture and will almost immediately start transferring water to the wood. For that reason, wood should never be stored flat or on end over a concrete floor.
There’s fewer disappointments in building or buying a wood project only to have it twist, cup or fail. While learning how to fix warped wood is important, sometimes it’s just not feasible to fix the wood.
Which can lead to an expensive redo of the project.
Last update on 2022-06-28 at 02:13 / Images from Amazon