Have you ever been told a roof is “self-supporting” as if it somehow defies gravity? We have walked in many old homes and have been amazed how the roof can still stand despite the lack of braces as is now required by today’s Codes. Well in most cases if you put a string line on any part of those old roofs, you will see a good amount of sag. When roofs are not braced, they project a horizontal force on the walls that support them.
If there is enough tensile strength in the ceiling members and their connections to the walls to restrain this horizontal force, then the roof can for the most part behave like a three dimensional plate structure that can hold itself up with the help of the ceiling framing acting as its tension tie. The photo below depicts a failure in the ceiling from being able to resist this horizontal force of the rafters and as a result the wall rotated and bent out of plumb about 3″ at the top. More roof braces and or a better rafter tie connection would have prevented this from happening.