Final answer:
To determine the correct length for a common rafter, subtract half the thickness of the ridgeboard from the total rafter length. This ensures a proper fit at the peak where the rafters meet the ridgeboard.
Step-by-step explanation:
In construction, specifically when working with common rafters, it is necessary to consider the dimensions of all components to achieve the correct length. When calculating the length of a common rafter, you must subtract half the thickness of the ridgeboard. This is because the ridgeboard represents the peak where two opposing rafters meet, and each rafter will butt against half of the ridgeboard's thickness. Therefore, subtracting half the thickness ensures that the rafters fit together properly at the peak. This approach can be analogous to breaking down measurements into smaller, manageable units to better estimate or calculate lengths, as is common in various estimation techniques.