Visible Signs of Rafter Thrust
In common rafter framing, the rafters support the weight of the roof and snow (or live load) by pushing against the ridge board (and opposing rafters) at the top and pushing outward and downward at the tops of the supporting walls. The ceiling joists not only provide vertical support for the weight of the ceiling, insulation, and other contents in the attic, but they also resist the outward horizontal thrust of the rafters at the tops of the walls. That is, the ceiling joists restrain the bottom end of the rafter and the tops of the walls from thrusting outward by forming a triangular frame with the pairs of opposing rafters.
The magnitude of the horizontal thrust depends on the weight being supported by the roof, the span of the rafters and the pitch of the roof. If the ceiling joists do not provide adequate restraint for the outward thrust, the bottom ends of the rafters thrust (or push) outward.
The outward thrusting at the bottom of the rafters causes the tops of the outside walls to lean outward,
the eave to bow outward, (See Featured photograph)
the ridge of the roof to drop and sag,
the tops of the walls to separate from the ceiling,
and the rafters separate from the ridge board,
The mechanics of this are discussed in another post, Collar Ties vs. Rafter Ties.