We model a photonic crystal slab as a Fabry-Perot resonator with two propagating Bloch waves in the periodic medium. This provides a semi-analytical recipe for the computation of photonic crystal slab modes' dispersion and quality factors. We apply for the search and study of bound states in the continuum, which exist above the light line, among the leaky modes, but nevertheless are decoupled from the continuum of propagating modes and are confined inside the periodic medium. We identify them as a set of optogeometric parameters for which the quality factor of a given photonic crystal mode goes to infinity. Also, we illustrate a simple example of the vertical symmetry breaking by adding a semi-infinite dielectric substrate, and comment on some other asymmetric configurations.