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Related to jack rafter: Valley rafter
jack rafter[′jak ‚raf·tər]
A short, secondary, or simulated rafter.
Any rafter shorter than the full length of the sloping roof, such as one beginning or ending at a hip or valley. See also: Rafter
Any rafter that is shorter than the usual length of the rafters used in the same building; esp. occurs in hip roofs.