Trees are round and boards are square. Herein lies the problem with lumber. How your board will behave during it's life depends greatly on where the board came from in the tree and how the annual rings are oriented in the final cut of wood.
Trees are cut up in two typical ways:
- Flat sawn
- Quarter sawn.
Flat sawn, also called live sawn, slab sawn and plain sawn, is where the tree is sliced into a stack of boards. This is the most common method for boards to be cut as it results in the least amount of waste. Since more wood is recovered from the tree, this method of sawing wood is quite popular. The top and bottom cuts will have arcs in the end grain that look like rainbows and will lead to the formation of cups as they dry, leading to warpage as the wood dries
As you move further down the stack, you will have more diagonal rings on the outsides of the boards and more arcs towards the center. The outsides of the boards will resist warping while the middle will cup.
The middle cut of the tree will have vertical rings from the bark to the heart, then vertical rings on the other side until you reach the bark on the other side. These boards will not warp, the the heart will check.
The boards below the heart will exhibit the same grain patterns in reverse until you reach the bottom plank.
As you can imagine, these boards will warp over their life and can result in problems on a boat. For these reasons, the use of flat sawn wood is discouraged on a boat.
The alternative way to cut the tree is called quarter sawn, where the tree is cut into quarters and every board that comes out of the tree has diagonal grain. This will yield the best lumber from the tree, but up to sixty percent of the tree is lost to waste.Since most of the tree is lost to saw dust, this method of sawing lumber is unpopular with lumber mills and quarter sawn lumber is hard to find. If you are lucky enough to find quarter sawn lumber, you will feel like your luck has turned the moment you see the price tag!
The alternative is to buy flat sawn boards and make it quarter sawn. If you get a board that is near the top of bottom plank from the tree, rip it in half. Now you have two thinner planks that no longer have arcs, but instead diagonal rings that run less than 45 degrees but still diagonal.
The closer the board was to the heart of the tree, the better the grain orientation will be. As you approach the heart, the rings will be closer to 45 degrees and resemble quarter sawn lumber. This method works well for situations where you don't need wide boards, as you are taking your wide boards and ripping them in half. A 2x12 becomes two 2x6's. While the loss of width is apparent, most boat timbers don't need that much width. If you do need the width, the ripped boards can be joined back together with drifts or dowels to reinforce them from sheer forces while changing the grain orientation to reduce the risk of warpage. Instead of cupping, which is what flat sawn wood does, your quarter sawn lumber will stay relatively dimensionally stable. This means that your lumber wont warp when it gets wet and dries again, which is the life of any wood on a boat!
When shopping for lumber, try to find flat sawn boards that you can rip into quarters sawn lumber. This will save yourself the cost of buying premium quarter sawn lumber allowing you to purchase more wood with your money. It takes a bit more effort to get your lumber ready, but the end result is quality lumber while paying the price of economy boards.