What spalling is
The Rowhouse Manual explains that the outside of Philadelphia brick is its most durable part, because it was exposed to the most heat when fired; the inside is much softer. When water is trapped in the brick and freezes, it expands and breaks the surface. That’s spalling. Once the hard face is gone, the soft interior wears quickly.
Common causes on Philly houses
- Hard mortar. NPS warns that mortar harder than the brick forces stress and moisture into the brick instead of the joint.
- Non-breathable paint or coatings. The Rowhouse Manual says not to paint brick with material that isn’t breathable, because trapped water freezes and spalls the brick.
- Sandblasting or acid cleaning. The Manual says both can remove the brick’s durable skin.
- Dripping air conditioners and leaks. The Manual shows brick damaged by a dripping air conditioner, and warns against holes through solid walls.
Repairing it
Individual damaged bricks can be cut out and replaced with matching brick, salvaged where possible. In a historic district, brick replacement is on the list of work the Historical Commission reviews. Angi lists brick repair at $20–$40 per square foot, and HomeGuide lists brick repair at $500–$2,500. Cost guide.
When it isn’t cosmetic
Cracks that run through bricks rather than along joints, bulging, or a wall pulling away from its neighbor are structural questions. Angi notes structural cracks can cost well over $1,000 to repair. Ask a structural engineer to look before you pay for repointing. Bowing walls & star bolts.