An open flue is an open pipe
Stand on a roof and look down an uncapped chimney and the situation explains itself. There is a straight, dry, sheltered vertical shaft running into the middle of a house, and nothing at all covering the entrance.
Every year we pull nesting material out of chimneys in this county in every month of the calendar. Birds and roof rats here never get a winter off, so there is no closed season on the problem the way there is in colder places. An uncapped flue is not a risk of being occupied. It is a matter of when.
Water is the expensive part
Animals are the visible nuisance. Water is what actually costs money.
Rain falling straight down an open flue lands on the damper and the smoke shelf. That is why so many dampers in Los Angeles County are rusted solid, and it is why the smoke chamber above the firebox is so often damp and stained. Add a marine layer that keeps the masonry from drying out for weeks at a stretch, and the mortar joints inside the chimney stay wet.
By the time it shows up as a damp patch on the chimney breast in the living room, the water has already been working on the inside of the stack for years.
Embers, and why it matters more on a hillside
The third job a cap does is stop what comes up as well as what goes down. A spark arrestor screen catches embers at the top of the flue instead of letting them leave on the wind.
In the hillside and canyon parts of the county that is a requirement rather than a nicety, and on the tight lots in cities like Bellflower and Paramount it matters for a more mundane reason: the top of your flue may be a few feet from a neighbour's roof. A great many older chimneys either never had an arrestor or have one that rusted through years ago.
Why the cap you have may be doing nothing
A cap that looks present from the street is not necessarily working. The standard galvanised cap fitted on most houses here has a limited life, and near the water in Long Beach, Torrance and Palos Verdes it is considerably shorter than most people expect.
What happens is the mesh corrodes through while the frame survives. Now animals get in freely, and the remaining frame holds leaf litter and debris on top of the flue like a shelf. That is worse than no cap at all. The only way to know which you have is to look at it from the roof.
We fit stainless as a matter of course. It costs more once and considerably less over twenty years.
The crown underneath
The cap and the crown get confused constantly, so it is worth separating them. The crown is the mortar or concrete slab across the top of the masonry, and its job is to shed water off the brickwork. The cap is the metal hood and mesh over the flue opening itself. You need both, and they fail independently.
A proper crown is sloped, reinforced, and has an overhang with a drip edge that throws water clear of the brick. What most houses in this county actually have is a flat trowelled smear of ordinary mortar, sitting flush with the brick face. Those crack within a decade and then funnel water directly into the top of the stack.
There is no sense capping a flue that sits in a crown about to fail, which is why we look at both together.
If your chimney is a stucco box
In the newer tracts across Cerritos, La Mirada and Carson, a great many chimneys are not masonry at all. They are factory-built metal chimneys inside a framed and stuccoed chase, and instead of a mortar crown they have a sheet metal chase cover on top.
The galvanised covers fitted as standard in the seventies and eighties rust, pond water and leak straight down inside the chase, where it rots framing out of sight. If there is an unexplained water stain on a ceiling next to a stucco chimney, that cover is the first place to look. Stainless replacements with a proper drip edge and cross break solve it.
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Frequently Asked Questions
A stainless cap, properly sized and mechanically fixed, will normally outlast the roof it sits on. A galvanised one near the coast may last only a few years. The difference in fitted cost is small and the difference in service life is not.
Almost certainly because it was pressed onto the flue tile and is held by friction alone. That is adequate on a sheltered street and inadequate on an exposed ridge in Signal Hill or Palos Verdes, where wind loading on a tall stack is substantial. Caps in those locations need mechanically fixing to the tile or the crown.
Yes, arguably more than if you did. An unused flue still collects rain, still fills with nesting material, and still lets water reach the damper and smoke shelf. Nothing about the chimney stops needing protection because you stopped lighting fires in it.