Water resistant flooring underlayment

A method of installing a flooring structure, the method comprising the steps of:

• providing an acoustic insulating layer over a sub-floor layer;
• and
• providing an underlayment layer over the acoustic insulating layer, the underlayment layer comprising gypsum concrete including a mixture of alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate and having a water-resistant, silane-based additive, wherein the underlayment layer comprises the gypsum concrete including the mixture of alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate as between 90% and 95% by weight of the underlayment layer prior to providing the underlayment layer over the acoustic insulating layer, and wherein the water-resistant, silane-based additive comprises between 0.4% and 9% by weight of the underlayment layer prior to providing the underlayment layer over the acoustic insulating layer,
• wherein the provided underlayment layer is adapted to directly receive a finished floor layer without a primer or sealer coat applied to the underlayment layer.