The tax levied by RCW
82.08.020 shall not apply to sales of form lumber to any person engaged in the constructing, repairing, decorating, or improving of new or existing buildings or other structures under, upon or above real property of or for consumers: PROVIDED, That such lumber is used or to be used first by such person for the molding of concrete in a single such contract, project or job and is thereafter incorporated into the product of that same contract, project or job as an ingredient or component thereof.