Gravel layers are sometimes used to improve the airborne and impact sound insulation of Cross Laminated Timber (CLT) floor constructions. The gravel increases the mass and damping of the bare CLT floor and will also improve the efficiency of a floating floor. It is generally assumed that additions like linings and floating floors do not influence the vibration damping in junctions. While this is usually a reasonable assumption, the effect of the gravel on the vibration damping in CLT junctions cannot be neglected. This paper presents laboratory measurement results for the vibration reduction index of a rigid CLT junction with and without gravel. The influence of the gravel layer on the flanking transmission paths is investigated by means of the simplified SEA model of ISO 12354. SEA simulations, incorporating the effect of the gravel layer, are validated with in situ measurements.