A Guide to Air Drying Timber
So, you have just had a tree cut into lumber and you are chomping at the bit to start using it. Well unfortunately it is not ready just yet! Air-drying lumber is a technique that has been around for as long as people have been cutting trees down. Now, with other alternative drying methods available it's a cost-effective and environmentally friendly option. All you need is a bit of time and a little bit of effort. While the process of drying seems simple ( you cut the wood, put it in a pile and wait for it to dry) over time the practice has been refined and perfected to achieve the best results. In this guide, I will give you an overview of what I understand the best method to be. I hope to provide you with the knowledge to reduce issues with your lumber and provide the best yield possible. It is important to understand that once the sawyer has finished milling, the timber is not ready just yet.
Defects and What Causes Them
When drying lumber there is a balance that needs to be achieved between too fast and too slow. Without the proper preparation work and correct techniques you will run into a range of different problems. This will increase the likelihood of defects occurring in the lumber and result in a lower yield. Some of the common defects that occur are:
Checking
Checking is where cracks develop in the wood from the timber drying too fast. Surface checking occurs when the face of the lumber dries faster than the internal. However, this may reverse in the later stages and checks can close (especially in hardwoods) when the opposite occurs. End checking occurs when moisture moves lengthways much faster than sideways. A piece of timber is essentially a bundle of straws and water passes far more easily through the ends than the sides. This results in the ends drying much faster and the stress causes the fibers to separate.
Warping
Warping occurs when different parts of the timber dries at different rates. The part of the timber dries fastest shrinks first and this causes stress throughout the timber. This stress pulls the board in different ways and causing a range of different warping. The four most common types of warping is a cup, bow, twist and crook.
Stain and Decay
This occurs when there is an inadequate amount of airflow and timber is drying too slowly, or when it is getting wet once stacked. This moisture is a breeding ground for a range of fungi, which attack the wood and degrade its integrity.
Preparing Lumber for Air Drying
Depending on your requirements, the log will be cut up either as dimensioned lumber or slabs. The easiest and most time-efficient way to prepare lumber for drying is to stack it as soon as it is milled. This saves double handling of the lumber and is the most efficient use of your time. At this time the key steps that need to occur are:
Having a clear area that can be easily accessed by a tractor or forklift to move the stacks. Alternatively, it can be stacked straight onto a trailer if it is being moved. Even better, if possible, stack the timber where it is going to be left to dry.
The milling should be uniform and even, allowing the lumber to be evenly stacked. For ease of sorting and using it in the future, it is recommended that lumber of different dimensions is not mixed.
Painting the ends of the timber. This is done in both dimensioned lumber and slabs. Throughout the drying process, moisture loss is greatest through the end grain causing an uneven moisture level within the piece of wood. This can then result in some of the defects as described above.
Having a Solid Foundation
A solid foundation is very important as this sets the standard for the process of air drying. To begin with, the ground should be as level as possible. If the base is twisted then there is more chance that the boards will warp during the drying process. Any weeds should be removed, and ideally, the timber should be dried on compacted metal or a solid surface. This will reduce the chance of the ground compacting under the weight of the timber and creating an uneven base, resulting in… yup, warping. It will also dry faster than if it is stacked over bare ground. The spacing between bearers will depend on the type of wood that has been milled. As a rule of thumb bearers should be spaced approximately 1m apart for softwoods, and 50cm apart for hardwoods. Depending on the thickness of the lumber adjust this closer as needed to reduce sagging. This will allow even distribution of the weight without constricting air flow too much.
The key to having success when air drying is as much even airflow around the timber as possible. This includes the base. Bearers should be a minimum thickness of 150mm to allow for this. The added benefit is that you are easily able to get forks under the stack to move it if required. If the timber is being dried outside, orientation of the stack should be taken into consideration where possible. If needed, the end of the stack should be facing the prevailing wind and not the side.
The Layout of the Stack
Now that you have a solid foundation, you are good to start stacking up that freshly sawn timber! The first layer will set the standard for the remainder of the stack, so make sure to take your time and get it right, it will save you from having to fix it in the future and help to give you the most usable timber at the end of this. A slight gap should be left between each piece rather than having them stacked up right next to each other. Again, this allows for air circulation around the entire piece of timber.
Once you have the first layer all sorted, next is what is referred to as filleting. This is where a divider is placed between each layer. These should be stacked in line with one another so that the weight is evenly distributed throughout the stack and bearers. Fillet sizes also play a role in the drying process. The thicker the fillet the more air can circulate each piece. Hardwoods usually use fillets between 10mm and 19mm, and softwoods 20mm to 25mm.
A few key points when building your stack:
Keep the boards placed evenly over one another. Pay attention to the stack and ensure that a lean is not developing through the stack.
Having boards of even thickness and width makes the stacking process much easier. If you have different sized boards, I would recommend starting another stack. If you have a range of lengths in a board size, these can be placed end to end to form the full length of the stack.
If you are using freshly sawn (green) fillets make sure that the stack is covered immediately. If not then it is possible for straining to occur during the drying process.
Make sure to consider the lifting capacity of your machinery. If needed, place a set of bearings after a certain number of layers and start a new stack. This will ensure you can move the stack when needed.
Protecting the Lumber
In an ideal world, you can dry your lumber in a shed or barn. However, space is a premium and unfortunately, this is not always possible. To help mitigates this a few steps should be taken:
First and foremost is ensuring that the timber is protected with a cover. This can be done with whatever you have on hand, but I recommend tin. Just remember not to place this directly on top of the stack, use some bearers to help that critical airflow. The key is ensuring that it does not leak or allow water in.
The outside of the stack will dry faster than the inside, this is because it is exposed to a high rate of airflow. To help reduce uneven air movement it out use a shade cloth or wind barrier. This will reduce the airflow and allow the lumber to dry out evenly. If possible you should also look to restack it partway through the drying process. This will move the inside boards to the outside of the stack and vise versa.
To help protect warping, especially in the top few layers, you can either strap them or place weights on top. I prefer to strap them as I believe it helps to minimize warping throughout the whole stack, as well as keep it secure for moving. As a hot tip, ratchet straps work brilliantly for this. Alternatively, weights of some form will work, just so long as they are evenly distributed over the stack.
Weather will impact the drying process as well. In the summer the lumber can dry too quickly, resulting in defects. This can be mitigated by placing the stack in a well-shaded area or using a shade cloth to achieve the same effect. In the winter drying can be very slow and this presents the opportunity for fungi to grow. As already explained, the most important thing is to ensure that water does not get onto the lumber.
Now the Waiting
And there you have it. Essentially, that is all you need to know to ensure that your lumber dries evenly and minimizes the risk of defects. Stack rotation can be done to dry the lumber evenly, even restacking it so that pieces in the middle are moved to the outside as explained above. Once the timber has dried to below 20% moisture content then it can be block stacked and moved inside. The very rough timeframe for drying is 1 year per inch (25mm) of thickness. This will vary depending on the type of wood but it will give you a rough planning figure.
While there is a fair amount of science and common sense used when determining how to dry wood, there is still a slight art to it. Just remember that lumber is a living thing, even after being cut down. That is what I believe contributes to the lovely warmth and feel of wood. In saying this, movement is natural and it will want to do both strange and wonderful things. This is all part of the process and the joy of working with lumber. Any defects that do occur does not necessarily mean the timber cannot be used. Defects can become stunning features of a project, such as splits being secured with bowtie joints!
