The invention is a method for making a fungal-resistant sheet of paper or
paperboard for use in building materials. The method comprises adding a
hydrophobic fungicide with a solubility in water of less that 0.3 g/L at
25.degree. C. to a paper slurry during manufacture of the paper or
paperboard, and processing the paper slurry in a paper machine to create
a sheet. Addition of a second process additive, especially a cationic
fixative, synergistically improves the efficacy of the hydrophobic
fungicide. A preferred fungicide is diiodomethyl-p-tolylsulfone.