Similarly, properly designed and carefully installed flashings between concrete window sills and copings will guard against a common source of efflorescence. The over-all design of a building to ensure that the walls are kept as dry as possible in service, such as by the use of a roof of considerable overhang and the avoidance of horizontal surfaces adjacent to masonry, unless adequately shielded from splashing and water flow into it, are features that offer protection against efflorescence. If water drains are placed against masonry they should be of ample capacity, and they should be maintained in serviceable condition. In short, the design of a building with respect to obtaining the maximum degree of durability of masonry by protecting it from excessive dampness will also be favourable to the avoidance of efflorescence. In recent years there has been developed a new class of materials called silicones, which have come into use in the building field mainly in connection with attempts to correct problems of penetration of rain into unit masonry.
Some manufacturers also advocate their use for the prevention of efflorescence on masonry walls.
When a brick or some other type of unit is treated with silicone, its surface usually becomes highly water-repellent. Formation of efflorescence on that surface is then usually suppressed because solutions of salts in the brickwork are prevented from moving to the surface to evaporate as would normally be the case. The moisture evaporates beneath the treated surface, however, and salt deposits accumulate there rather than on the surface. Localized accumulation of salts and their crystallization may cause the surface of the brick to be spalled or flaked off, so that the use of a silicone treatment to suppress efflorescence may be dangerous in some cases. This is particularly true when there are large amounts of salts in the masonry and the units are soft and porous.