To obtain the actual length of a rafter, what must be added or subtracted?

To obtain the actual length of a rafter, the correct approach involves adding one half the actual thickness of the ridge board to the theoretical length. This adjustment is necessary because the theoretical length of the rafter typically assumes that the rafter rests directly against the ridge board without accounting for its actual thickness.

When constructing the roof, the rafters need to sit flush against the ridge board, which means that if the ridge board has thickness, the theoretical measurement must be adjusted to ensure that the point where the rafter meets the ridge board is accurate. By adding half the thickness of the ridge board, one ensures that the rafter extends properly to meet the ridge without being too short, which would lead to gaps or structural issues in the roofing.

Measuring and adjusting for the ridge board thickness is essential for achieving a proper fit and maintaining the integrity of the roof's structure.