Final answer:
To distinguish between roof sheathing sag and rafter sag from a house's exterior, observe the pattern of the sagging—roof sheathing sag appears as dips between rafters, while rafter sag results in a uniform droop along the rafters' length.
Step-by-step explanation:
To tell the difference between roof sheathing sag and rafter sag from the exterior of a house, one can look for distinct visual clues. Roof sheathing sag is typically characterized by a dipping or a sagging appearance in the roof deck between the rafters. This sagging will either run parallel to the rafters if it's sheathing sag, or it could follow a wavy pattern over several rafters if the issue is more widespread. On the other hand, rafter sag is noticed when the rafters themselves are bending. This often results in a more uniform droop along the length of the roof where the rafters are located. The pitch of the roof can also help in analysis; as in the architecture of long houses, sharply pitched roofs were designed to prevent snow accumulation, which can also cause sagging when the weight becomes too great for the roof structure.