Wood Warping – why does it happen.

Have you ever had a woodwork project go backwards because the wood you selected decided to take a turn for the worse? It can be a frustrating thing and can be difficult to wrap your head around at times. It has happened to me many a time and so this article is here to help some of you to hopefully avoid some pretty simple mistakes before you start your next project. It all comes down to a few key points that are listed below.

• Moisture content
• Grain direction
• Grain structure
• Relative humidity
• Temperature
• Air flow
• Wood species
• Planning

Why Does Wood Warp?

The primary cause for wood warping is an uneven rate of drying or rehydration of different faces of a piece of wood. This creates tension and this is shown as warping. Grain density and grain structure also play a large role in the degree of warp that any wood species is susceptible to. The sections below dive into each of the key points above in more detail so read on if interested.

Moisture content

If there is a single primary cause for wood movement, moisture content is the candidate. Wood and moisture are intricately linked from the day the tree seed germinates to the day you get a piece of that tree into your workshop.

Wood is simply the physical structure that moves water within a tree for its entire life, so it is silly to think that this activity and capacity should stop because the tree has been milled and processed into workable lumber.

To better understand the amount of physical movement within wood, consider the following.

There are several recorded instances where tree trunks have split after heavy rain that followed a prolonged dry spell or drought. This was believed to be from the trunk expanding from the excessive water content that was pumped through the trunk and it happened faster than the tree could adjust in real time so the trunk split.

Now fast forward now to your workshop where any and every piece of wood that you have stored is capable of this very same movement.

In some instances the potential for movement is actually enhanced once the tree has been milled as you will discover further down the page where we discuss grain direction and structure.

Experience using wood as a structural element has taught us that removing moisture from wood enhances the stability of that wood and this allows us as wood workers to create the projects that we do.

There is some comfort in the knowledge that the item is likely to stay straight and hold together if we follow some general guidelines that are well proven and practiced.

The primary guideline is moisture content and not just a general percentage written in a book or on an internet page, but real world moisture content where it relates to your piece of wood.

Most pieces of lumber or wood that we use in our projects are typically either square or rectangular in shape, and these sections are cut from any where within the tree trunk.

We will go into this in more detail below because it matters a lot. For now, just think about a 4×2 of wood and consider the 4 faces that are present.

If the wood has a moisture content of 20% and is sitting flat on a bench top, there are only 3 sides really available for moisture to evaporate from, leaving the face that is down still moist. It is this process where a lot of damage to wood takes place within the woodworkers shop.

This piece of wood will warp up at the ends and create a bow in the piece. It can be cured to a degree by replacing the lost moisture and starting again but avoiding the issue in the first place is sensible.

Moisture in wood by itself is a natural event and it is the controlled removal of that moisture that makes all the difference to straight or warped wood.

Grain direction

Grain direction is the next big cause of problems and warping, because it holds hands with moisture content and is directly linked with the milling process, ergo your piece of wood. Trees grow by adding to the girth of the trunk each season and this is expressed in growth rings or annular rings.

Now imagine you are looking down at a cross-sectional slice of a tree trunk so you can see the center of the tree, this is called the pith, and you can also see the rings that radiate out from the center.

Picture this slice getting pumped up with air like a balloon and think how the circular patterns would react.

In my mind they will stretch into larger circles with most of the movement being sideways along the circles circumference. There will be some expansion into a larger diameter but the bulk of the movement will be into the circumference.

Now change the air into moisture, and look at the end grain of a piece of wood that you are planning to use in some project, and you will get a rough idea on how that piece of wood will react to more or less moisture, just by placing the end grain into the trunk cross-section, then matching the grain patterns and picturing the movement as described.

This is the process I use and it has stood me in good stead for many years. There will be variations to the direction of movement, but for a general idea this process works.

Grain structure

All woods have an internal structure that is created as the tree grows, with faster growing trees having a more open grain structure than the slow growing trees. This structure can be understood as the required internal strength of the trunk as the tree grows so it can absorb the stresses that affect the tree over it’s lifetime. This grain structure is also a direct reflection of the method or capacity for the tree to move moisture within the trunk as the tree grows.

A fast growing tree will need open galleries in the wood to move moisture faster than a slow growing tree where the moisture can take is own good time.

This explains why many of the densest woods are sourced from the driest climates.

This grain structure is also known as pores.

All woods will warp irrespective of the type of grain structure and should be treated the same where moisture is concerned however the hardwoods will resist moisture penetration better than the softwoods but once wet the hardwoods will dry slower than the softwoods.

They all warp, with some being more susceptible than others.

Relative humidity

This is a challenge for the woodworker because it is outside of our control. Every seasonal change delivers its own relative humidity reading and is just a fancy way of saying the air around us is now more wet or more dry than yesterday.

As woodworkers, we need to be aware of abrupt changes in the relative humidity because it can catch you out if you are not aware of the effect. Abrupt changes can be from the arrival of a large storm like a hurricane or cyclone, or a weather system that turns up on the doorstep unannounced.

This table shows the moisture content change due to relative humidity change with ambient temperature staying constant.

Air Temperature                 Wood Moisture Content at %relative humidity levels

Temp C	Temp F	5%	35%	55%	75%	95%
4.4	40	1.4	7.1	10.4	14.9	24.3
10	50	1.4	7.1	10.3	14.8	24.3
15.6	60	1.3	7	10.2	14.6	24.1
21.1	70	1.3	6.9	10.1	14.4	23.9
26.7	80	1.3	6.8	9.9	14.2	23.6

 mobile users rotate screen for full image

From Forest Products Laboratory. 1999. Wood handbook—Wood as an engineering material.

As you can see in the data there is a huge range of moisture content in the wood when the relative humidity changes.

In the very first section of this article we discussed the effect of having a piece of wood sitting on a bench top with only 3 of the 4 faces exposed to the air and the effect that drying will have in this situation. The faces dried and the bottom face stayed moist and the differential caused the wood to cup upwards at the ends of the board.

When relative humidity rises and the same board is face down in the same location the board will absorb moisture and expand on the exposed faces and will cause the wood to bridge in the center.

The warps caused by these changes in humidity can often cure themselves if the wood can be allowed to have an even airflow around it so keeping projects on spacers as you work on them will go some way to avoiding the problems that can turn up.

Temperature

When we think of temperature it is often related to the ambient temperature or seasonal temperature. This is easily compensated for in woodworking and has limited effect on the stability of any piece, however the object of this section is to highlight the effect of direct sunlight on wood.

Direct sunlight will almost certainly warp wood because the warming of the wood dries the exposed face while also heating the wood, partially softening it so when the drying and the softening are added together you will get warping and it can be severe.

It can be reversed with some creativity but avoiding the problem is the safest bet.

Airflow

As we have noted above, airflow allows any piece of wood to dry at an even pace on all faces, and it allows for an even exposure to changes in relative humidity. When undertaking a project it is best practice to place spacers under the work piece overnight to avoid any of the issues that can arrive with a simple change in the weather.

Wood species

Different species are more or less susceptible to excess movement than others and before attacking the next project it is a good idea to research the behavior of the selected wood just to get a heads up.

Some woods have what is called interlocked grain and this wood can move and warp a lot. Sometimes this type of wood will move no matter what you do and it will have stresses in the trunk that can bind the saw blade or chainsaw bar as it is being milled.

These woods are best left for the wood turners amongst us because it can be near impossible to achieve a flat straight piece of wood no matter the temperature, the humidity or the moisture level of the stuff.

Give this wood to another woodworker as a friendly gift and sit back and smile.

Planning

So this is pretty much the end of this article and as you might have noticed there are a few reasons why any particular wood will warp and twist.

It is one of the many challenges that we woodworkers face so as we set out to build any project it is a smart thing to sit down at a screen and do some research on the wood you plan to use and understand how stable it is when handled sensibly. That is a good place to start with.

Most of the issues above are avoidable if care is taken.