  Metamaterials and metasurfaces are at the pinnacle of wave propagation
engineering, yet their design has thus far been mainly focused on
deep-subwavelength periodicities, practically forming an effective medium. Such
an approach overlooks important structural degrees-of-freedom, e.g. the
interplay between the corrugation periodicity and depth and how it affects the
beam transport. Here, we present Slack Metasurfaces - weakly modulated
metal-dielectric interfaces unlocking all structural degrees-of-freedom that
affect the wave propagation. We experimentally demonstrate control over the
anisotropy of surface waves in such metasurfaces, leading to yet, unexplored,
dual stage topological transitions. We further utilize these metasurfaces to
show unique backward focusing of surface waves driven by an umklapp process -
momentum relaxation empowered by the periodic nature of the structure. Our
findings can be applied to any type of guided waves, introducing a simple and
diverse method for controlling wave propagation in artificial media.
