Requirements and optimisation of silicone water repellents are considered, based on the experimental evidence. For optimum curing on alkaline substrates, the following conditions should prevail: residual pH of the substrate is less than 11 in the months following impregnation, a high degree of branching of the polymerized resin with a methyl to silicon ratio of less than 1.5, a minimum quantity of hydroxyl groups of 0.5%, sufficient depth of penetration into the substrate. Contact angle measurements were found to be an effective method of indicating water repellency. -- AATA