Water repellants are chemicals that are applied to a masonry building to keep water from penetrating the masonry. Early commercial versions of these treatments tended to penetrate only the very surface of the masonry. Used in combination with inappropriate Portland cement on a building designed for lime mortar, these materials cause severe and rapid damage to masonry. Water finds its way past the thin veneer of repellant and into the substrata of the masonry wall. Water trapped inside the wall dissolves salts in the masonry and causes crystallization of salts in masonry below the surface of the brick or stone. This is called Sub-Efflorescence… It is one of the two causes of rapid damage. The second major cause is freeze/thaw cycles. Again, if water is trapped in the masonry behind the water repellant, the masonry stays wet and freeze thaw causes the water to expand in the brick or stone causing spauling.
Recently, new water repellants using chemicals called Silanes and Siloxanes have come on the market. These chemicals absorb more deeply into the masonry and chemically bond to the masonry in a more long-lasting way. These materials are considered “breathable,” in the sense that they allow a percentage of the water vapor in air to pass through.
However, there are some problems with modern water repellant materials as well. The first problem is that even the best water repellants need to be reapplied by a skilled and experienced company about once every 10 years. I have yet to work on a building whose owner has shown me a log of all the things that have happened to the building before they owned it. Even a good steward of a historic building can not guarantee the good sense of those who come after them. The second problem is that it is imperative that the masonry wall not have any mortar or masonry cracks. Even a recently repointed masonry wall can develop problems without the knowledge of the owner.
I am not a purist preservationist, I do realize the need for modern technologies under certain circumstances. I would recommend using these products under limited circumstances, such as on horizontal masonry surfaces where copper or aluminum caps or flashing are aesthetically unappealing. But generally, mechanical solutions like flashing last for a 100 years or more, they are both visible and inspect-able and can be replaced by anyone.