Whether you are milling for yourself or for a customer, you should make proper stacking a priority. Many people who run portable sawmills skip this step and they and their customers wind up with expensive firewood. Light colored woods, such as sycamore and maple will soon discolor, and decay and insect damage are possible for any species. At best, the stack will not dry. There are two basic techniques for stacking wood to air dry. The European system is preferred by craftsmen and custom woodworkers. The boards are stacked in sequence, with 1” thick spacers (called “stickers”) between the boards to allow air to pass through. The European drying method makes it possible to bookmatch pieces and keep wood with similar color and grain patterns together. The slab goes on the top of the stack to shed water. It helps to put straps around the stack keep it from warping, and to make it easy to move the entire log with a fork lift or front end loader. The disadvantage is that it takes more space in the mill yard and in the kiln, and it takes longer to stack.
The more conventional way to stack wood for air drying is to build the stack in layers, with a 1/2” to 1” gap between the boards, and 1” thick stickers between the layers to allow air flow. This is a more efficient use of space, but the boards tend to get out of sequence. All the boards of a given row need to be the same thickness to keep the rows even.
Either method requires a foundation. The stack needs to be at least 6” above the ground. I generally cut 6” square cants out of the worst logs for this purpose, since they will be in ground contact, and probably not usable for lumber by the time you are through with them. For the conventional (square) stacking, 4’ lengths work well, but it could vary, depending on your needs. If you will be stacking the lumber directly from the mill, cut the foundation first, so you don’t have to handle the lumber twice. The foundation cants should be about 20” apart for 1” thick lumber—any further apart, and the boards will sag a bit. I put 1” thick stickers on top of the cants just for a little more air flow under the stack.
All boards on a layer need to be the same thickness so that the next layer will lie flat. Try to have a 1/2” to 1” space between them for air flow all around the boards. I generally set the worst boards (usually from the center of the log) aside until I have a half-dozen, then put them back on the mill and saw the stack to pieces 1” thick. It makes a lot of stickers in a hurry! Part of the art of air drying lumber is keeping stickers perfectly lined up so the weight of the stack transfers straight down to the cants. Build the stacks as high as you can reach, or whatever your loader can lift safely. Top the stack off with one last row of stickers, then roofing tin, or a layer of scrap boards to shed rain and snow. I don’t use tarps, because they can trap moisture.
Then comes the really hard part… wait. The rule of thumb is 1 year per inch of thickness, but that depends on the species, your climate, and whether you plan to kiln dry it. You can kiln dry wood at any point, but the drier the wood, the less time it will spend in the kiln, and the less it will cost you. 20% moisture content is a good target for putting wood into a kiln. A good moisture meter comes in handy.