A method of installing a flooring structure is disclosed. The method includes providing an acoustic insulating layer over a sub-floor layer. The method also includes providing an underlayment layer over the acoustic insulating layer. The provided underlayment layer includes gypsum concrete having a water-resistant additive. Also disclosed is a flooring structure. The flooring structure includes an acoustic insulating layer over a sub-floor layer. The flooring structure also includes an underlayment layer having a first side and a second side opposite the first side. The underlayment layer includes gypsum concrete having a water-resistant additive. Additionally disclosed is a gypsum concrete flooring underlayment product. The product includes a mixture of alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate. The product also includes a silane-based powder intermixed with the mixture of alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate.