  A layered superconductor with a full pairing energy gap can be driven into a
nodal superconducting (SC) state by inter-layer pairing when the SC state
becomes more quasi-3D. We propose that this mechanism is responsible for the
observed nodal behavior in a class of iron-based SCs. We show that the intra-
and inter-layer pairings generally compete and the gap nodes develop on one of
the {\em hole} Fermi surface pockets as they become larger in the
iron-pnictides. Our results provide a natural explanation of the c-axis gap
modulations and gap nodes observed by angle resolved photoemission
spectroscopy. Moreover, we predict that an anti-correlated c-axis gap
modulations on the hole and electron pockets should be observable in the
$S^{\pm}$-wave pairing state.
