When must trimmer and header rafters be doubled in conventional light-frame construction?

Trimmer and header rafters in conventional light-frame construction must be doubled when the span exceeds 4 feet. This requirement is based on structural integrity and load distribution principles. When the span of these rafters is greater than 4 feet, a single rafter may not provide sufficient strength to support the loads imposed upon it, such as snow, wind, and live loads from the roof.

Doubling the rafters helps to increase their load-carrying capacity and improves overall stability. By using two rafters side by side, the structural system can adequately distribute the loads, reducing the chances of sagging or failure over time. This practice follows the guidelines laid out in building codes and structural engineering standards, ensuring safety and durability in construction.

In contrast, spans that do not exceed this threshold can often be supported by a single rafter, as the loads are within the acceptable limits for those dimensions, making doubling unnecessary.