An underlayment with improved drainage is disclosed. Particularly, the invention relates to an underlayment board having a first surface and a second surface, wherein the board further includes a top edge and a bottom edge and includes at least one channel defined in the first surface with each of the at least one channel extending from the top edge to the bottom edge to form a drainage path therebetween. More particularly, the underlayment board can include at least one funnel defined proximate at least the top edge or the bottom edge, wherein the at least one funnel is in fluid communication with a corresponding one of the at least one channel. A method of finishing an exterior of a structural substrate and a building assembly are also disclosed.