The mechanism
Water gets absorbed into masonry. When it freezes it expands by roughly nine percent. That expansion happens inside the pore structure of the brick and mortar, and it pushes the material apart from within.
One freeze does very little. Several hundred cycles do a great deal. The damage is cumulative and it accelerates, because each cycle opens the pores slightly and lets in more water for the next one.
This is why the mountain communities see far worse masonry deterioration than the valley. San Bernardino gets few freezing nights. Crestline cycles through 32°F repeatedly all winter, which is the worst possible pattern: enough freezing to do damage, not enough sustained cold to lock the moisture in place.
Why the top fails first
The crown is the sloped masonry slab capping the chimney, and its whole job is to throw water clear of the brick below. When it is built properly — cast, reinforced, with a projecting drip edge — it does that for decades.
A great many chimneys never got that. What they got instead is a thin mortar wash smeared over the top, flush with the brick, with no reinforcement and no overhang. Those crack within a few years. Once cracked, water runs into the chimney rather than off it, and the freeze-thaw cycle now has access to the whole structure.
A missing cap compounds it, adding rain directly down the flue onto the smoke shelf and damper, plus birds and nesting debris.
What the damage looks like as it progresses
- Hairline cracks across the crown, often first visible where it meets the flue tile
- White chalky efflorescence staining running down the exterior brick, which is dissolved salts carried out by moisture
- Mortar joints recessed or crumbling on the weather-facing side
- Brick faces flaking, popping, or turning up in the gutter — this is spalling, and it means units are being lost
- A visible lean or bow in the stack above the roofline
The progression is roughly in that order, and the cost of intervening rises sharply along it. Sealing a hairline-cracked crown is a modest job. Rebuilding a stack that has lost its brick faces is not.
Exposure is not uniform
One elevation of a chimney is often visibly worse than the others, and that tells you something useful. The weather-facing side takes driven rain and wind-loaded snow while the sheltered side stays comparatively dry, so joints wash out on one face years before the other.
This is at its most extreme in Rimforest, sitting on the crest of the ridge at around 5,700 feet with nothing upwind. Chimneys there take weather from the side rather than from above, and we do more cap and crown work per house there than anywhere else we cover. It also means repointing is frequently one-sided — there is no point pricing a full wrap when the sheltered face is sound.
The cheap repair that prevents the expensive one
A stainless cap with arrestor mesh, mechanically fastened, and a crown that is either resurfaced with a flexible coating or recast with proper slope and a drip edge. That combination is the highest-value work on the chimney for the money, by a wide margin.
Whether a crown can be resurfaced or needs recasting depends on the actual crack pattern and thickness. Hairline cracking in an otherwise sound crown seals well. A crown that is thin, unreinforced, or already broken through has failed structurally, and coating it hides the problem while water keeps entering. The top of a chimney is the one place a homeowner cannot inspect, so we photograph it and show you which one you have.
Want a professional to take a look?
Baker Chimneys Incorporated serves 12 communities across the San Bernardino Mountains and the valley below. Free, no-obligation estimates.
Frequently Asked Questions
Late summer or early autumn. Crown work needs temperatures above freezing while it cures, and roof-level work in snow is either unsafe or impossible. In Green Valley Lake and Running Springs the window closes earlier than most people expect.
It is the best value item on the chimney. Without one, rain falls directly down the flue onto the damper and smoke shelf, animals nest in the passage, and embers can leave the top — which in this fire environment is not a small matter. Caps cost a fraction of what the resulting water damage costs to put right.
It slows it considerably, but only after the crown, cap, flashing and joints are sound. A breathable siloxane repellent on sound masonry is genuinely worthwhile at this elevation. Applied to a chimney that is still letting water in, it traps moisture and makes matters worse.