Pick up one of those brick flakes from around your chimney’s base and look at its back: flat, clean, like it was sliced off. That’s the freeze-thaw signature, and it means the brick didn’t fail on its surface. It failed from inside.
The mechanism, start to finish
Brick is fired clay, hard, but porous. When it stays wet, from a leaking crown, failed mortar joints, or plain weather saturation without protection, water occupies the pore network. Freeze it and it expands roughly 9%, hydraulic pressure inside a rigid material. A few dozen cycles and the face, the weakest plane, shears off. The exposed core is softer and more porous than the face was, so spalling accelerates itself: each popped face drinks faster than the last.
Why DFW is a spalling machine
The fix: replace and cut off the water
Spalled brick doesn’t heal, the face is gone, and no coating rebuilds fired clay. The honest repair is two moves: replace the failed units, cut out, matched (color, size, and porosity matter, that’s the masonry craft), and relaid with compatible mortar, and stop the water source, crown, joints, or general saturation, or the neighboring bricks simply take over the failure schedule. Scattered faces = targeted replacement; widespread through-course spalling starts the repair-vs-rebuild conversation.
Flakes on the ground are the cheapest version of this diagnosis you’ll ever get. Use them.