Methods for installing underlayment systems include the steps of laying at least one piece of a pre-formed board over at least one first portion surface area of a sub-floor. The pre-formed board is preferably fire-resistant gypsum board, which may also be water- and/or moisture-resistant. A second portion surface area of the sub-floor, which is adjacent to each first portion surface area, is designated to receive a poured underlayment after a tub is installed over the laid at least one piece of pre-formed board. In some instances, a sound mat may be laid over each first portion surface area of the sub-floor, prior to laying the at least one piece of the pre-formed board, and, then, another sound mat may be laid over the second portion surface area of the sub-floor, after laying the at least one piece of pre-formed board and prior to pouring the poured underlayment.