Wash. Rev. Code § 61.16.020
Whenever the amount due on any mortgage is paid, the mortgagee or the mortgagee's legal representatives or assigns shall, at the request of any person interested in the property mortgaged, execute an instrument in writing referring to the mortgage by the volume and page of the record or otherwise sufficiently describing it and acknowledging satisfaction in full thereof. Said instrument shall be duly acknowledged, and upon request shall be recorded in the county wherein the mortgaged property is situated. Every instrument of writing heretofore recorded and purporting to be a satisfaction of mortgage, which sufficiently describes the mortgage which it purports to satisfy so that the same may be readily identified, and which has been duly acknowledged before an officer authorized by law to take acknowledgments or oaths, is hereby declared legal and valid, and a certified copy of the record thereof is hereby constituted prima facie evidence of such satisfaction.
RCW 61.16.020