Does sealing wood prevent warping and expansion?


Since the dawn of woodworking, carpenters, builders, and woodworking enthusiasts have had to deal with wood warping, expanding, or shrinking. This can be disastrous if not handled correctly and can cost you a pretty penny and a lot of time to fix. With modern technology advancing at breakneck speed, one has to wonder; Does sealing wood prevent warping and expansion?

Sealing wood will not prevent it from warping or expanding. Sealing both ends of the wooden plank may help prevent the ends from expanding or shrinking faster and exacerbating the warping of the plank. However, there is no guarantee treating wood with a wood sealer will prevent it from warping or expanding.

Although stopping wood from warping, shrinking, or expanding is impossible, you are able to manage it with the proper care and maintenance from time to time. For this reason, research and knowledge about the wood that you plan to use and how it is harvested are critical. Below, we will discuss how the wood you use is manufactured, wood movement fundamentals, wood care, how to fix warps, and which woods warp the least.

How is lumber harvested?

In order to understand why wood warps and moves, it is essential to comprehend how lumber is harvested, cut, and treated.

Lumber is a generic term used to describe various lengths of wood used for construction or building materials. Pieces of lumber are usually cut lengthwise from the trunks of trees in rectangular squared sections. A portion of lumber that consists of a thickness of fewer than 2 inches is called boards. In comparison, those sections that are between 2 – 5 inches are considered dimensions. Lastly, pieces of lumber with a thickness of 5 inches and higher are classified as timbers.

Manufacturing process

The process starts with the selected trees being felled. Depending on the wood species being felled and the area in which the wood is being harvested, this is done with chainsaws. Once the trees are felled, they are loaded on transport trucks and transported to a sawmill.

Some felling sites have heavy machinery that de branch the logs before they are loaded on the transport trucks’ bed, while other de branch the wooden logs when they arrive at the sawmill. Water is sprayed on the beds of the trucks to prevent the wooden logs from drying out during transport.

Sawing

At the sawmill, the logs are cut into boards, dimensions, and timbers. This is done by placing the logs on a moveable carriage that slides lengthwise on a set of rails. A computer scans the wooden logs to determine any abnormalities or visible defects and then selects a cutting pattern that will yield the most pieces of lumber from the wooden log. The machine that does these complicated cuts is called a Headrig saw. It is located at the end of the rails, upon which the wooden logs are mounted.

The first cut that is usually made to these logs removes a section called a slab. A slab consists of the wooden log’s outer surface, which has the original tree trunk’s curvature. This cut is usually discarded and ground into mulch to be used in the production of paper. The carriage upon which the wooden log is clamped returns to its original position and the other slabs are removed. For each cut, the wooden log is re-evaluated by the optic sensors and the Headrig sawyer or Headrig operator to ensure there are no defects or abnormalities in the wooden log.

Seasoning or drying

The lumber that has been cut into boards and dimensions is moved to an area in the sawmill for drying or seasoning. This process usually consists of letting the wood dry to prevent decaying and to let the wood shrink and reach its equilibrium moisture content.

Because of the large size of timbers, they are usually sold wet or green. This is because it is extremely difficult to dry these large pieces of lumber thoroughly. The other pieces of lumber will be dried using either air drying, kiln drying, or vacuum drying. The method of drying depends on the sawmill and the type of lumber being dried.

Air drying

As the name states, air drying is the process of stacking the pieces of lumber with spacers in between to let air move through them. The area is usually covered to prevent rain or moisture from seeping back into the lumber. Air-dried lumber generally has a moisture content of 20%.

Kiln drying

For this process, wood is stacked in an enclosed area where heated air is circulated. The temperature of the air circulating through the lumber is usually around 110-180⁰ Fahrenheit or 44-82⁰ Celsius. Because the wood is stacked in an enclosed space with the constant hot air circulating, the moisture content is significantly lower than that of air-dried wood. Kiln-dried lumber usually has a moisture content of lower than 15%. For this reason, kiln-dried lumber is usually specifically used for interiors, floors, doors, and molding, where the risk of shrinkage should be minimal.

Vacuum Drying

Vacuum drying is a method of drying lumber that consists of placing wood inside an airtight vessel or container under less than atmospheric pressure while heat is transferred to the wooden material through specific heating methods. It is not a new method of drying lumber, but rather a very expensive one. For this reason, not many lumber companies employ it. However, it has been proven to be the most effective and fastest drying method and is primarily used for more high-end wood species.

The primary differences between vacuum drying and conventional drying methods are that the main driving factor behind vacuum drying is the total atmospheric pressure difference. Because the airtight container lowers the total atmospheric pressure surrounding the wood, the boiling point of the moisture inside the lumber is lowered considerably. This causes the movement of the moisture inside the wood to change to water vapor bulk flow, which means that more moisture moves down the length of lumber to escape out the ends.

Vacuum drying has many advantages over conventional kiln or air-drying. One of which is that lower temperatures can be used to dry the wood due to the lower atmospheric pressure inside the vacuum chamber. This means that there are fewer chances of warping or damaging the internal structures of the timber. It also greatly reduces drying times, maintains the color of the wood, and sawmills are able to dry larger cross-sections.

Why does wood warp, shrink, and expand?

The answer is more straightforward than you think. All wood comes from trees, which at one stage were living organisms and relied upon a source of water, minerals, oxygen, carbon dioxide, and solar radiation to grow.

This is more or less like humans if you think about it. So what happens when you open your hand and clamp down with the other hand around your wrist, you will notice that your fingers slowly start to close as the flow of blood is interrupted.

A similar process happens to wood when harvested, except that some of the moisture and minerals remain inside the wood’s fibers. The moisture and minerals are susceptible to temperature, humidity, and exposure. Depending on the intensity of these elements, the moisture level within the wood will increase or decrease.

Increased moisture levels within the wood will expand the wood fibers causing the lumber to swell in size. While decreased moisture levels will cause the wood fibers to contract and shrink, possibly bowing the wood inwards depending on the wood’s grain structure and direction. This process is known as wood movement and is also our next topic of discussion to better understand why wood warps and moves.

Wood movement

There are four ways in which moisture moves when drying wood. These moisture movements have a direct impact on how wood moves and will be explained below.

  1. The liquid water or moisture content contained within the wood moves through the wood’s cell structure by capillary action. This means because wood consists of narrow passages or capillaries, the moisture moves due to the intermolecular forces between the liquid and the surrounding cell structures. This is also known as free water bulk flow.
  2. Water vapor bulk flow is the movement of moisture from a high-pressure zone to a low-pressure zone and is yet another way in which moisture moves through wooden cell structures.
  3. Another way water moves in wooden materials are vapor diffusion due to differential relative humidity gradients. Vapor diffusion is water molecules’ movement through a vapor-permeable material such as the wood’s porous cell structures. Relative humidity gradients mean that the humidity levels outside of the lumber’ cell structure are higher or lower than inside, causing the moisture to increase or decrease inside the cell structure.
  4. The last way in which water or moisture moves within wooden materials is through diffusion due to moisture content. This is generally due to the temperature outside the wood being higher than inside, essentially causing the wood to release its moisture content. This occurs typically longitudinally, meaning throughout the ends of the exposed lumber rather than transversally.

The constant fluctuation of the moisture content within wood due to the relative humidity of the surrounding air is due to the fact that wood is hygroscopic. This essentially means that the molecular structure of wood depends on the level of humidity in which it exists, better known as relative humidity.

Meaning as we established above, that if the environmental or relative humidity increases, the molecular structure of wood expands. In contrast, the opposite occurs when the humidity decreases and the wood shrinks. This relationship between wood and its environmental humidity is called equilibrium moisture content.

Equilibrium moisture content is predictable and can be invaluable when working with any type of wood to ensure minimal warping.

Understanding equilibrium moisture content

As we established before, the moisture content of wood is directly tied to the surrounding air’s humidity. The higher the humidity, the higher the moisture content of the wood. So, for instance, if you harvested wood in an environment with high environmental humidity and transport the lumber to an area where the humidity is lower, the moisture content of the lumber will adapt accordingly to correspond with the environmental humidity.

For this reason, it is always best to let lumber or wood acclimatize when transporting it over large distances to allow it to reach its EMC or equilibrium moisture content and take regular moisture readings. To better understand how relative humidity affects the moisture content of wood, have a look at the table below:

Relative HumidityMoisture Content
0%0%
25%5%
50%9%
75%14%
99%23-30%

Thus, you can accurately determine how lumber will move when installed when understanding the equilibrium moisture content principle. For this reason, carpenters and woodworkings rely heavily upon it.

How wood warps, expands and shrinks

A few factors come into play when attempting to accurately predict how wood will move, besides understanding the equilibrium moisture content.

Width of the timber boards or planks

The width of the material plays a pivotal role in how the wood will move. The wider the board or plank of wood, the more the movement will occur. The amount of movement in the wood is directly proportional to the size. In other words, a board or plank that is 4 inches wide will move half as much as a plank or board that is 8 inches wide. Gluing pieces of wood together will not prevent the wood from moving either, as the wood will move as one piece of lumber.

The grain orientation of the timber boards or planks

Another factor that comes into play is the grain orientation of the wood. Timber boards are usually sawed in either characterized as flat sawn or quarter sawed.

Quarter sawed lumber is also referred to as rift sawn or vertical grain cuts and consists of planks that have been cut with annual rings between 45-90 degrees to the board’s face. Quartersawn lumber shrinks and expands only half the amount that flat sawn timber does.

Most timber that you will purchase over the counter is flatsawn and, as we mentioned, shrinks and expands more than quartersawn timber. Flatsawn timber consists of planks that have been cut with annual rings between 0-45 degrees to the board’s face.

The humidity inside and outside the timber storage structure

The average house in America has humility control. However, the same cannot be said of sheds, garages, and most structures where the average person will store their lumber. What this means is that most wood after being bought has to adjust to ambient humidity to reach the equilibrium moisture content.

On average, the humidity where you would store your lumber, assuming it is in a storage structure that has no humidity control, is between 25 -65% relative humidity. This means the lumber you store can undergo a 6% change in moisture content. This roughly translates ¼ inch movement in a 12inch maple wood board.

Some composite wood materials have less movement depending on the relative humidity but can still experience as much a 1/10th of movement that occurs with solid woods.

The moisture content of the wood when delivered

In order to measure the moisture content of wood accurately, a moisture meter is needed. Because the relative humidity can vary significantly from the timber retailer to your home, it is highly recommended to take regular moisture readings before putting the timber to use.

The wood type determines movement amount

The species of wood has an impact on the amount the degree to which the wood will expand, shrink, or warp. Some species or types of wood move more or less depending on some of the factors mentioned above. However, genetics certainly play a role as well.

A 12-inch board made from western red cedar will fluctuate 1/8 inches, while the same sized board made from maple will fluctuate ¼ inches. To calculate how much your timbers’ movement can fluctuate, you can use the online shrinkage calculator at http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=shrinkage.

You can manually calculate the amount wood will move by using the general rule of thumb that most types of wood, cut in flat grain, will move 1% for every 4% change in moisture content. For quarter sawn cuts of wood, the movement will be approximately 0.5% for every 2% change in moisture content.

Below is a table of the most popularly used woods in North America and Canada and how they move across the grain or radially.

SpeciesRadial expansionType of wood
SpeciesRadialType
White oak0.00180Hardwood
Maple0.00165Hardwood
Yellow popular0.00158Hardwood
Red oak0.00151Hardwood
Pine0.00148Softwood
Black cherry0.00126Hardwood
Western red cedar0.00111Softwood
Northern white cedar0.00101Softwood

What can increase movement risk or warping

Many factors can contribute to wood movement, most of which are caused by human error. The best way to prevent most of the causes listed below is to consistently measure the moisture content and be aware of the specific wood’s required equilibrium moisture content.

Excessive moisture content in delivered materials

The best moisture content for wood that is intended for interior use is between 6-8%. In the best-case scenario, wood that is purchased or delivered from a retailer will have a moisture content of 9-10%. Remember, a preferable difference of 2% in moisture will have minimal wood movement. This is considering that the wood will be store indoors or under cover away from the elements. However, it is recommended that a moisture content measurement is taken as soon as the wood is delivered. And a period of at least a week is given for the wood to reach its equilibrium moisture content.

Which woods warp the least and why

As seen on the graph above, we can safely determine that white and red cedar are the more stable wood species for interior trim, wooden structures, and furniture.

Northern white cedar

Northern white cedar is commonly found in the reaches of the north of America. The wood is light, strong, and decay-resistant due to the natural oils it contains. These trees grow up to be around 49ft or 15 meters tall and 2.11 ft or 0.9 meters in diameter. It is popularly used, as mentioned above, for wooden posts, siding, boats, and furniture.

The wood is has a coarse grain structure, is soft and strong for its weight. It is brittle, however, and should be treated with wood sealer to prevent cracking. Screws should be driven into pilot holes and consist of non-corrosive aluminum or brass. When cutting or resizing, a multi-toothed saw should be used slowly to avoid tear-outs and splintering.

The presentation of northern white cedar is, as the name suggests, is pale yellowish to light brown and takes well to staining, painting, and clear varnish, wax, or lacquer treatments.

Western red cedar

Like its northern white cousin, Western red cedar is a softwood that grows in America’s northwestern parts. It’s a sturdy wood used mainly for exterior purposes such as construction, decks, fences, and furniture. Interior uses include wall treatments, millwork, cabinets, and moldings. The wood lacks pitch or resin, which is one of the reasons why it withstands warping and wood movement.

The texture of the wood is medium to coarse-grained. It has a low shock resistance but is highly stable. The color of the heartwood can range from dark red-brown to pale yellow. When left untreated, it will turn silver-gray with age.

Unlike northern white cedar, this wood does not hold nails well but glues easily. When sanding or planning, use low grain sandpaper or a shallow pass, as the wood’s soft grain tears easily. Western red cedar holds paint and stain when coated with multiple layers. Inside it presents best with clear wax, lacquer, or varnish finish.

Different types of warping

Five types of wood warping occur due to wood movement.

  1. Bowing occurs when the wooden material warps along the length of the lumber, bending the whole plank, dimension, or timber. When this happens, it usually bends the thinnest face of the material in a curve.
  2. Crooking is what happens when the wooden material warps along the length of the lumber but bends the thicker face of the lumber in a curve.
  3. A kink warp is when a section along a straight piece of plank, dimension, or timber bends suddenly to one angle. It happens typically along the width of the lumber and shows as a pronounced kink.
  4. Cup warps are identifiable by the c-shape it forms along the wooden material’s length, usually bending the two longitudinal edges closer together.
  5. Twist warping can easily be spotted as it looks like the wood has been wrung with one end twisting or winding in one direction and the opposite end turning in the other.

How to fix warping

Fixing a warp in any wood is not an easy process and might end up costing you more than the wood itself unless you have the needed facilities on-site or on hand. No method is guaranteed to work, as each wood species reacts differently. The core of the idea is to add moisture content back into the wood fibers making it more flexible and then clamping it in the desired position or shape. Below are several methods for fixing warps.

  1. The basic method requires little to no equipment but also has a meager chance of success. It requires that you spray the warped wood with water, allowing the lumber to soak up the water and then clamp the lumber into the correct position to dry and achieve its equilibrium moisture content. However, the wood might snap when bent or forced into the required position, or you might run the risk of oversoaking the wood and damaging it more.
  2. Ironing is a technique where you apply a wet towel or cloth to the warped area and then put a hot clothes iron on top of the wet towel or cloth. The idea here is to use steam to soften and moisturize the wood’s fibers and then clamp it into position. Due to the size of the clothes iron, the size of the warp you can straighten is limited and would require you to do large warps in sections. But if the moisture content is lower in one region of the wood while you attempt to straighten it, you might put too much strain on it and break it off.
  3. A steam machine is a specialized piece of equipment used by professionals to fix warps or even cause bowing to create barrels or unique pieces. It works along with the same principles as ironing but with an industrial-sized steamer connected to a lattice or holed surface. The lattice is clamped to the affected part of the wood to allow for maximum exposure.

Conclusion

Sealing wood does not prevent it from warping. It can help prevent warping, but the best way to avoid wood warping is to be aware of the wood’s moisture content by measuring it and ensuring that it reaches its equilibrium moisture content before working with it.

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