A decorative relief pattern is applied to one or both sides of thermoplastic slat material by rolling an embossing roller in contact with the side(s) to receive the pattern. The embossing roller has an inverse relief pattern which is an inverse of the relief pattern applied to the slat material. A transfer roller applies an ink layer to the embossing roller, with the resulting thickness of the ink layer varying according to features of the inverse relief pattern. Depressed areas of the inverse relief pattern receive relatively little ink. An air stream from an air knife is applied to the ink layer, with ink being preferentially cleaned from relatively smooth areas of the inverse relief pattern. Then, a portion of the ink remaining on the embossing roller is transferred to the slat material along with the formation of the embossed pattern in the slat material.