What is Flat Sawn Lumber?
A board sawn through the heart of a log is called quarter sawn and the annular rings are 90º to 60º to the face. As the log is further sawn, the ring- to-face angle will progressively flatten. A rift sawn board has an annular ring angle to the face of 60º to 30º.
A flat sawn, or plain sawn, board has a ring-to-face angle of 30º to 0º. These are the boards sawn from the edge of the log and comprise the least stable lumber in the log. However, these boards can also be the clearest lumber cut from a particular log. The commonly recognized grain pattern is called “cathedral” or has the annular rings running in “U” shapes on the board face. Unless otherwise specified, most lumber is flat or plain sawn.