How should the span of each rafter be measured?

Measuring the span of each rafter along the horizontal projection provides the most accurate representation of the distance that the rafter spans between supports. This method focuses on the horizontal distance from the top of one wall to the top of another wall, indicating how much load the rafter must support without accounting for any angles or slopes that may exist due to the rafter's incline.

This type of measurement is crucial as it directly relates to structural support and load distribution in the roofing system. Using the horizontal projection ensures that the calculations for roofing materials, the design of the structure, and the overall integrity of the roof are based on accurate horizontal distances rather than angles or lengths along the slope of the rafters, which could lead to overestimating or underestimating the span and, consequently, the structural requirements.

The other methods of measurement would not be appropriate for determining the span for engineering calculations and inspections. For example, measuring along vertical projections would not represent the actual load-bearing capability, while measuring by the length of the rafters could also lead to inaccuracies due to the slope's effect on the overall span. Similarly, measuring from one end to another without considering any projections would neglect significant structural aspects necessary for a proper analysis and understanding of the rafter