Container Shipping: How Much Lumber Fits Inside? 40′,20′ TRUTH

Wood Moisture Content and Weight

Moisture content is defined as the weight of the water in the wood divided by the weight of the wood. This number is then multiplied by 100 to become a percentage. If the weight of the water equals the weight of the wood, the MC is 100 percent.

source: Oregon State

Lumber weight significantly impacts  lumber shipping options, material handling methods and structural design decisions. Wood has two mass elements – water weight  and raw wood fiber density. Both factors comprise the lumber’s overall weight.

The best way to accurately measure wood moisture content is with a moisture meter.

Lumber’s moisture content greatly impacts it weight. According to Professor Gene Wengert, a freshly cut red oak log contains approximately 80% water. Rather than water, the lumber industry typically refers to this as moisture content.

Water is not light. Further, water weighs 8.33LB/gallon.

Thus, the more moisture the lumber contains, the heavier the lumber board will be.

For Example, submerged maple logs pulled from lake bottoms.  Once these logs are salvaged and brought to the surface, they are extremely heavy. This is due to a close to 100% moisture content. No water loss occurs while the logs are completely underwater.

However, the logs begin losing water as soon as the are exposed to air. In fact, they air dry at their fastest rate immediately after salvage. 

Think of it like a wet shirt being laid out to air dry on a chair. At first, the shirt is so drenched water drips off the shirt onto the ground solely due to gravity.  A few hours later, the shirt is still damp but the pooled up water below it is gone. 

Accordingly, small logs commonly drop from 80% to 30% moisture content quickly from exposure to dry air. 

Typically, such air dried timber is then brought down to 8-12% moisture content through the use of a dry kiln.  While air drying down to that moisture content is possible, it takes a long time.

Does kiln drying keep lumber at a consistent moisture content ? Contrary to popular opinion, ordinary kiln drying does not set Lumber’s moisture content permanently. I have seen and measured kiln dried lumber moisture fluctuations. To sum up, over time lumber acclimates to its surrounding humidity levels.

For example, we are located in Fontana, California. Fontana is a desert climate. When we buy kiln dried alder lumber from sawmills in the Pacific North West, our Wagner moisture meter sometimes measures 10-11% when we unload. However, after a few weeks in Fontana, the average moisture content drops anywhere from 1-2%. This is without putting lumber on sticks. If we sticker the alder and expose the wood to our Santa Ana winds for a while, it may drop down another 2-4%.

Can we reduce wood weights by reducing moisture?

Yes. This is a side effect of drying wood. However, there are limitations.

Once, we had a client ask us ebonize maple wood. Basically, they wanted to substitute Maple lumber  for ultra expensive Ebony wood. 

They were not really interested in the moisture content. They just wanted it to look like Ebony. 

To do this, we sent northern hard maple into a roasting kiln at the highest temperature we could set it. I believe it was close to 500 degrees Celsius. 

The maple turned a consistent black hue, similar to Ebony in color tone. Unfortunately, the extreme temperature exposure required to achieve this dark coloration reduced the maple Lumber’s moisture content to around 2%.  This once vibrant hardwood looked like Ebony but had the pliability of Glass. Additionally, it was weak. I dropped a piece on the ground and it shattered.  It would not hold screws or accept nails without splitting.

Importance of Wood Weight

The reason why weight is important is that containers have a maximum weight capacity. Often, new shippers focus on the dimensional limits of the container.

However, overweight containers cause problems for both shippers and receivers.  Accordingly, containers have weight restrictions. For example,  in California, the maximum weight for a 40′ container is 44,000LB.

Why Overload a Container?

Overloading a container is bad stowing. It can result in a tipped freight or container sway during transit.

Sometimes it is done out of ignorance. However, more often overloading occurs when shippers try to reduce their per unit shipping cost.  Since shippers pay a flat cost for the container, the prorated cost drops with each additional shipping unit. For example, hardwood lumber exporters may add an extra bundle or two. When they do, the container rate remains constant.