Cleaning century brick on an Ottawa house, what's safe?

There are too few published sources on brick cleaning methods to give a general answer, so a mason looking at your specific wall is the one to say what is safe for it.

A National Research Council building digest (CBD-30) notes that applying a vapour-permeable but water-repellent membrane to the outer face of brick can cause salts to deposit behind the surface layer, and the resulting crystallization pressure can spall the skin off. CBD-30 also points out that salt spalling and frost spalling look nearly identical, so even after cleaning you may not be able to tell what caused any new damage without a mason's assessment.

There are a few things worth knowing before anyone starts work. First, the Centretown and Minto Park Heritage Conservation District Plan states plainly that sandblasting is not an appropriate method on historic masonry. If your house sits in any heritage conservation district or is individually designated, check whether the cleaning method needs heritage approval before work begins. Second, ask how the site and surrounding surfaces will be protected during the work.

When you speak with a mason, ask specifically what cleaning method they propose, why they consider it appropriate for the age and condition of your brick, and whether they have used it on brick of a similar age. Ask about any sealer they may suggest after cleaning: CBD-30's caution about vapour-permeable coatings applies there too, and a mason should be able to explain the reasoning before anything goes on the wall.

Before work starts, ask for a written contract that sets out the scope, the method, and the price. If you want confirmation of WSIB coverage, a clearance certificate is valid for up to 90 calendar days and is worth requesting.