The two-dimensional problem of the diffraction of a plane hydroacoustic wave at the junction between two elastic plates is considered. One of the plates is uniform and the other is reinforced with a periodic set of uniform stiffeners. The plates are joined in such a way that together they form a plane which splits the space into two subspaces. An acoustic medium fills one of these half-spaces. It is assumed that one cannot ignore reflection from the surface of the stiffeners nor neglect their moment impedance. The conditions at the junction of the plates is not completely fixed, i.e. the general solution of the problem is investigated. A modified stationary-phase method is used to sinvestigate the solution obtained and enables the asymptotic forms of the field in the neighbourhood of particular directions to be obtained.