How to Determine the Crowns on Rafters

The term rafter refers to 2-by-8 or 2-by-10 pieces of dimensional lumber that are installed in a home directly under the roof deck. The rafters are installed at an angle, and they support the OSB board that forms the roof surface. Because these boards are milled directly from the trunk of a large tree, they often are naturally curved in one direction due to the wood grain. This curve is called the "crown." The rafters should be installed with the crown facing upward to help support the weight of the building.

Instructions

- 1
  Pick a 2-by-8 piece of dimensional lumber up by one end while the other end remains on the ground.
- 2
  Hold the timber vertically, so the long dimension is vertical, and you can look down the 1 1/2-inch wide edge.
- 3
  Hold the board near your eye, and look down the edge of the board toward the end resting on the ground. You are looking to see if the board is curved slightly upward or downward.
- 4
  Flip the board over and repeat the process. In virtually every instance, the long board will be slightly curved upward toward one of the narrow edges.
- 5
  Mark the wide side of the board with a pencil, and draw an arrow toward the edge of the board that has the crown. When the rafters are installed, they should be mounted with the arrows facing upward.

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