· uva uvam vivendo variafit
Put a framing square on the rafter and use a level to make sure the either the vertical leg of the square is plumb or the horizontal leg is level. Then measure from the corner of the square vertically (the rise) and horizontally (the run) to the rafter. Try to maximize the dimensions that you are measuring by pulling the square away from the rafter - this will give you a more accurate reading. The angle measured from the horizon up to the rafter will be the arctangent of rise divided by the run. If you want to set your miter box the angle would 90 degrees minus the angle that you just calculated.