Jordan Truesdell’s answer to the question about raising ceiling joists (Q&A, 10/08) was very informative. I see a lot of houses where carpenters have raised the ties several feet to create a high ceiling or attic room, but they don’t realize that the dimension of the rafters may need to be increased. When a tie is raised above the lower ends of a pair of rafters, the bending moment (the amount of flexural force) in each rafter increases.
So, for the 4/12 roof in the example, the rafter bending moment when the tie is at the top plate is 1,440 foot-pounds. But when the tie is raised to 16 inches above the wall, the bending moment in the rafter reaches 3,200 foot-pounds. This is important, because bending moment is what governs the size of the rafters. So, for example, according to...
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