The transition between bound and unbound surfactant bilayers, controlled by the strength of attractive van der Waals interactions, is described in a simple Flory-type picture. All the main features of the transition previously described only by functional renormalization group methods are recovered, including 1) second-order nature of the transition; 2) estimate of the critical strength of van der Waals interactions; 3) dependence of tricritical point on bilayer stiffness. Our Flory picture also reproduces known exponents for unbinding and wetting transitions in d=2 dimensions