Which factor is critical when determining rafter spans in roofing?

Design load is a critical factor when determining rafter spans in roofing because it represents the weight and forces that the roof structure must support. This includes not only the dead load, which consists of the weight of the roofing materials and structural components, but also the live load, which can include snow, rain, and dynamic forces such as wind.

The calculations of rafter spans are directly based on these design loads to ensure that the rafters are adequately sized to prevent failure under expected conditions. The span must accommodate the weight and stresses without compromising structural integrity or safety.

Considering the other options, the color of shingles does not affect the structural performance of rafters; it is mainly an aesthetic choice. Length of eaves might influence water runoff or aesthetics as well, but it doesn't factor into the structural span calculations. Type of wall construction could be relevant in terms of anchoring or supporting the rafters, but it is the design load that primarily dictates the sizing and spacing of rafters for safety and compliance with building codes.