Described herein are methods for installing field-assembled flooring systems wherein the underlayment is a hybrid design that includes a combination of structural board and cementitious product. The hybrid design is configured to reduce or eliminate the curing time requirement after pouring the cementitious product (e.g., gypsum concrete). The field-assembled flooring systems can reduce or eliminate the chances of the onset of mold due to high moisture levels by removing the cementitious product from the prone areas and replacing it with structural boards. The structural boards (e.g., cellulose fiberboards) can be installed in non-critical areas such as underneath cabinets, around the perimeter of the floor, under bathtubs, in non-walk-in closets, anywhere sheet rock reaches the floor, or the like.