Which is a critical calculation for determining the length of a rafter?

Determining the length of a rafter is essential in roofing and involves calculating based on the total run, which is the horizontal distance from the top of the wall to the peak of the roof. The line length constant is a factor that relates to the geometry of the roof and helps to accurately determine the slope or pitch of the rafter.

When calculating rafter lengths, the total run is often multiplied by a line length constant that corresponds to the angle of the roof. This results in the effective slope length of the rafter, which is crucial for ensuring that the rafter fits correctly within the roofing framework. Accurate calculations are vital, as improper rafter lengths can lead to structural issues and roofing failures.

The other options do not directly provide a valid method for calculating rafter lengths. For instance, simply using span and ridge beam thickness does not account for the angle or pitch of the roof, while height and depth as factors do not directly correlate to the necessary length of the rafter. Similarly, dividing the total span by a rise constant is not a standard method for deriving the length of a rafter. Thus, the first option accurately reflects a method that incorporates the necessary elements for calculating the rafter's length in line with standard roofing practices.