The chimney that only leaks when the wind blows
Water enters a chimney at the crown, the cap, the flashing, the mortar joints or through the face of the brick. Storm rain here arrives close to horizontal, so it finds the last two, which vertical rain never would.
Five entry points, one stain on your ceiling
A chimney leak is a diagnosis problem before it is a repair problem, because all five failures produce the same damp patch on the chimney breast. Guessing costs money: sealing the wrong surface is the most common wasted repair in this trade.
- The crown, the sloped concrete lid at the top, cracked or without an overhang
- A missing, rusted or undersized cap, letting rain straight down the flue
- Flashing where the chimney meets the roof, the joint both trades assume is the other's
- Mortar joints eroded enough to take water into the core of the stack
- The brick faces themselves, saturated by wind-driven rain
Why our storms are different
Bay Area rain does not arrive as a steady vertical soak. It arrives in concentrated winter systems with enough wind behind them to drive water sideways for hours. Masonry that sheds the same volume falling vertically will soak through under that load, particularly on the windward face. On the coast the effect is severe, which is why we specify differently in Pacifica and Half Moon Bay than we do a few miles inland.
That is the explanation for the pattern people describe most often: fine all year, leaking in January. It points the diagnosis at the brickwork and the flashing rather than the crown, and it is not something an inspection from the ground will ever find.
The white powder is telling you something
That chalky bloom on the outside of the brick is efflorescence: mineral salts carried out of the masonry by water moving through it, left behind as the water evaporates. It is harmless in itself and it is proof that water is passing through the wall. If you have it, the masonry is wet inside more often than it is dry.
Do not seal it with the wrong thing
Standard masonry sealers and paints trap moisture inside the brick. Trapped water expands as it cools and spalls the faces off, so a cheap sealing job in autumn can buy you a rebuild later. Masonry needs a vapour-permeable repellent that stops liquid water entering while still letting vapour escape, applied after the actual leak is fixed and the structure has dried.
In practice most of what we do is crown and cap work, repointing, then a breathable repellent once the water has somewhere else to go.