  There is a well-known upper bound on the growth rate of the merge of two
permutation classes. Curiously, there is no known merge for which this bound is
not achieved. Using staircases of permutation classes, we provide sufficient
conditions for this upper bound to be achieved. In particular, our results
apply to all merges of principal permutation classes. We end by demonstrating
how our techniques can be used to reprove a result of B\'ona.
