In order to have highly flexible secondary beamlines collimation is crucial. Collimators are blocks of material placed along the beamline that let the main beam pass within the collimator aperture limits, while most of the particles beyond it interact in the material and gets absorbed or loses enough energy to be deflected away from the beam core by the magnets placed downstream. Some particles can interact at the edge of the collimators and be accepted within the main beam, creating background for the experiments downstream. The thickness of the layer contributing to this potential background is called Skin Depth. It depends on the material of the collimator, on the beam momentum as well as the acceptance of the beamline in terms of momentum band and divergence. This article presents the first studies with GEANT4 and FLUKA based simulations on the effects of such interactions at the CERN secondary beamlines.