Why flint is different again
Flint facing is made up of individual flint nodules set into lime or cement mortar, and the mortar joints between them are usually the weakest point. High pressure aimed at the joints rather than the flint itself can erode the mortar and loosen individual stones over time, which is a much bigger repair job than the original cleaning would have been.
Period stone has the same mortar issue
Older stonework near Canterbury's historic centre is frequently set with lime mortar rather than modern cement, and lime mortar is softer and more vulnerable to being blasted out by high pressure. Checking what a wall is actually pointed with matters before deciding how to clean it.
Soft washing is the safer default
Low-pressure soft washing with an appropriate cleaning solution lifts algae, lichen and general grime chemically, avoiding direct high-pressure contact with vulnerable mortar joints. It takes longer than blasting a modern render finish but it's the right trade-off on genuinely old stonework.
Foot traffic and pollution add up over time
Being one of the most visited historic centres in the country means Canterbury's older stonework deals with a lot of ambient grime from footfall, traffic and general city pollution on top of normal weathering, which is part of why period buildings here often need cleaning more than a similar building somewhere quieter.
When to bring in a specialist instead
If a wall shows signs of significant existing damage, loose flints, or crumbling mortar, that's a job for a heritage stonework specialist first, not a pressure wash. We'll say so honestly rather than clean over a problem that needs proper repair.