Could a water repellent coating hurt my Ottawa brick?

A water-repellent coating can cause damage to brick, and a National Research Council building digest (CBD-30) explains the mechanism: when a vapour-permeable but water-repellent membrane is applied to the outer face, salts are deposited behind the surface layer, and the force of crystallization can cause the skin to spall. That spalling can look very similar to frost damage, which makes it hard to tell after the fact what caused it.

A mason looking at your wall can assess its condition before anything is applied.

A few things worth asking before applying anything:

• What is the coating's vapour permeability rating, and how does that interact with how your brick was made and how it currently performs?
• Is there any existing efflorescence, spalling or failed mortar that should be addressed first?
• What does the product manufacturer say about compatibility with your brick type?
• How will the coating affect any future repointing or brick replacement?

The short answer is: ask the mason before applying anything. No coating decision should be made without someone examining the wall in person, because the wrong product on the wrong brick can create the exact problem you are trying to prevent.

If the house is designated under the Ontario Heritage Act or sits within a heritage conservation district, a coating may affect the property's heritage attributes. Check your district's heritage conservation district plan or your designation by-law first, and if you are unsure whether the coating requires approval, ask the City's heritage staff before any product goes on the wall.