A plate advancement mechanism is configured to cooperate with a stack of plates consisting of alternating first and second roofing plates with alternately positioned indexing notches along an indexing edge of the plates. An indexer including at least one indexing tab that projects beneath the indexing edge of the plates reciprocates parallel to the indexing edge of the roofing plates between a first feed position and a second feed position. The indexer first feed position aligns the indexing tab with the indexing notch of the first plates and the indexer second feed position aligns said indexing tab with the indexing notch of the second plates. The indexer first feed position and said second feed position are coordinated with shuttle movement to release the lowermost roofing plate from the stack onto the shuttle by aligning the indexing tab (or tabs) with the indexing notch (or notches) of the lowermost plate