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CHIMNEY INSPECTION DIAGNOSIS REPAIR
BACKDRAFTING HEATING EQUIPMENT
CARBON MONOXIDE - CO
CHIMNEY COMPONENT DEFINITIONS
CHIMNEY FIRE ACTION / PREVENTION
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
FLAME COLOR, BLUE vs YELLOW COMBUSTION
HEATING SYSTEM INSPECTION
HOME HEATING SAFETY
ODORS GASES SMELLS, DIAGNOSIS & CURE
SAFETY RECALLS CHIMNEYS VENTS HEATERS
STAIN DIAGNOSIS on BUILDING EXTERIORS
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
Chimney caps & crowns: this article describes the types of covers & terminations found at the top of chimneys and flues. We define chimney rain cap, chimney cap, chimney crown, and chimney pot, giving photo-examples of each of these components. We illustrate common chimney cap & crown types, choices, & defects, and we cite pertinent chimney top cap / crown building codes & standards for fire and other safety concerns.
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Some of us are confused about what to call the topmost components of a chimney. For clarity in this article, unless we state otherwise here is what we mean by "rain cap", "chimney cap", "chimney crown" and "chimney pot".
Definition of Chimney Rain Cap
A chimney rain cap is a rain cover on top of a chimney flue designed to keep out rain (which can damage the flue or appliances it vents) and intended to reduce downdrafts in the chimney in windy conditions.
Some chimney rain caps may be supported atop a masonry chimney in a position to shelter the chimney flue, and may not only cover the chimney flue opening but may also project out beyond the entire chimney top (red arrow, photo at left).
This example is still a chimney rain cap.
[Click to enlarge any image]
On many clay flue tile lined chimneys the rain cap will mount directly on to and will cover only the chimney flue itself (photo at left).
A retrofit chimney rain cap is shown in our photo. This rain cap is designed to fit over the top of a standard sized clay chimney flue tile and is held in place by four threaded bolts that press against the sides of the flue tile.
Other chimney rain caps for clay tile flues mount by friction by insertion into the interior of the top of the flue tile.
Watch out: over tightening the securing bolts of this chimney rain cap can break the flue tile, while leaving them too lose risks that the whole rain cap assembly blows away during high winds.
Definition of Chimney Cap = Chimney Crown = Mortar Cap
Masonry Chimney Caps: on a masonry chimney the chimney cap is a pre-cast concrete or poured in place concrete seal around the flue tile (on a modern masonry chimney). In our photo (at left) the chimney cap, also called a mortar cap, is the gray concrete visible around the projecting flue tiles at the top of the chimney.
This particular chimney cap is defective: too thin, cracked, leaky, missing an expansion joint at the flue tile, and lacking a drip edge projection over the chimney top.
The Masonry Institute of America calls this chimney top surface seal around the flues the chimney cap. We're following their terminology.
Others call this area the "chimney crown" in an effort to avoid confusion between the chimney cap (red arrow) and the chimney rain cap (blue arrow). To add confusion "chimney crown" is used by others to refer to decorative chimney tops or pots (described below).
Watch out: leakage into a chimney through a defective chimney cap leads to damage to the chimney structure, chimney flue, and leads to interior leaks as well as unsafe conditions.
Our photo of a metal chimney cap and rain-cap combined in a custom-fabricated design (at left), adds more confusion to chimney top terminology. This chimney was observed atop a hotel near Bar Harbor, Maine.
Definition of Chimney Pot
As we illustrate in more detail below at Decorative Chimney Pots, the term "chimney pot" is used to describe a decorative rain cap assembly on the top of a chimney, covering its flue and in some cases covering both the flue itself and the entire chimney top, including the chimney flue and chimney cap/crown.
Typically the chimney pot also adds height to the top of the chimney flue.
Our chimney pot photo (left) illustrates a retrofit or add-on chimney pot that increases the effective chimney height of a concrete block chimney, possibly aiding in solving a chimney draft problem.
Also see CHIMNEY HEIGHT EXTENSIONS.
The Chimney Rain Cap Has A Different Job From the Chimney Cap/Crown
The purpose of the chimney cap/crown [chimney crown] is to close off the space between the flue liner and chimney wall, to shed water clear of the chimney and generally prevent moisture entry.
The chimney rain rain cap has the job of keeping rain and wind down-drafts out of the chimney.
Some rain caps also include a screen that functions as a spark arrestor to reduce the chances of a spark exiting the flue to land on and set afire a nearby roof surface or other materials.
The rain cap or hood shown in the sketch at left is charged with preventing sparks from leaving the chimney - a fire safety measure.
Sketch courtesy Carson Dunlop Associates.
The chimney "cap" [or crown] should slope away from the flue at a good rate of about 3-inches per foot.
The chimney cap should not be bonded to the flue liner or top of the chimney in order to allow for thermal expansion of the liner. The space between the cap and the flue liner must be closed with a flexible sealant.
Mortar chimney "caps" are prone to cracks and allow water to drain over the face of the chimney masonry leading to spalling, loss of mortar and leakage to the interior spaces. Corrosion at the chimney base cleanout doors are common to those types of chimneys.
Check the chimney top for damaged masonry (or rusted metal), a missing cap, damaged, cracked, or missing top seal or crown on the top of a masonry flue, and here, an important discovery (at least in some jurisdictions) is whether or not the chimney is single wythe or thicker masonry and whether or not the chimney has (or perhaps needs) a chimney liner.
Chimney cap/crown drip edge
The Brick Institute of America (BIA) recommends chimney caps of pre cast or cast-in- place concrete a minimum of 2-inch thick with a projection of 2 1/2-inch beyond the face of the masonry surround so that water shed from the top will not run down the face of the brick.
The concrete chimney crown in our photo (above) is also referred to in many texts as the "chimney cap" but in this usage, "chimney cap" refers to the capping seal on the top of a masonry or certain other chimneys - a seal that surrounds the chimney flue but does not cover it.
In Carson dunlop Associates' sketch at left the chimney crown looks recently installed and does not drain past the chimney sides. There has been water damage covered up with painted metal on the chimney side facing us.This chimney needs some safety inspection and probably new caps on the flues.
As Carson Dunlop Associates' sketch shows, a good drip edge at the chimney top cap helps reduce water and frost damage to the chimney sides and structure. Watch out particularly for flat or even in-sloping metal caps on wood-framed chimney chases built around metal chimneys - these are often a source of hidden leaks into the structure and potentially dangerous rust or corrosion damage to fireplace inserts and flues as well as damage to heating equipment.
Chimney crown/cap damage or missing drip edge
Carson Dunlop's sketch (left) show some details of good chimney cap construction. The object of these details is to avoid water and frost damage to the flue or to the chimney itself.
Chimney cap history: if a chimney has spent part of its life with no rain cap installed, or if the masonry cap is poorly constructed, there is extra risk of water damage to the flue interior.
In a masonry chimney damage may appear as frost cracking of the upper flue liners or masonry.
In any chimney, there may also be water damage to the heating appliances being vented by that chimney, such as rust, formation of corrosive condensate, or creosote.
Shown here is the concrete seal around the top of a chimney, sealing the upper chimney surface around the projecting chimney flue (clay flue tiles in this photo) in order to close off the space between the flue liner and chimney wall, to shed water clear of the chimney and generally prevent moisture entry.
Complete omission of chimney cap / crown / top seal
This chimney has been "re-lined" (maybe) using a rust-prone metal flue of unknown but highly suspect condition, passed through a single wythe brick masonry chimney with no chimney cap / crown / mortar seal to keep water and weather out of the chimney.
This is a poor chimney installation subject to leaks, damage, unsafe conditions.
Crowded chimney tops, flues, & rain caps
This chimney jams seven flues into one structure. The chimney rain caps are jammed together and the flues are so close that it was impossible to retrofit an add-on rain cap onto all of the flues. A better solution would have been a single rain cap covering all of the flues.
But this chimney top, located atop a New York building in the Hudson Valley, has other problems too:
Water & Frost-Damaged Chimney Tops - cracked, spalling masonry
Damaged chimney caps / crowns / mortar caps are shown in our two photographs at left.
In both of these photographs you can also see that water (and in a freezing climate, frost) have damaged the brick masonry of the chimney itself.
Watch out for Sealed-off Chimney Tops & Flues
As we illustrate in our photo at left, there are two concerns at this stone-faced masonry chimney.
1. The smaller chimney flue, typically venting heating equipment, has received a retrofit rain cap that may not permit adequate draft - note that the cap top is quite close to the top of the clay chimney flue tile upper surface.
2. The larger chimney flue, probably venting a fireplace, has been closed or capped-over using a single solid piece of stone or slate. Why would someone do this? Some common explanations for this chimney top seal include:
Watch out: we often find a temporarily capped-off fireplace flue or unused chimney flue that was left in that condition by a prior building owner. The new owner, attempting to use the chimney before its safety and condition have been determined, faces risk of being driven out of the home by smoke at a fireplace or worse, potentially fatal carbon monoxide poisoning if the flue is to be used by a heating appliance.
Also see MASONRY CHIMNEY TOP DAMAGE.
At left we illustrate another "sealed-off" masonry chimney.
Located atop the Justin Morrill Smith historic home in Strafford Vermont, this brick masonry chimney has been capped-off with a metal cover. The chimney is no longer in use and preservationists wanted to protect the chimney interior and exterior from leaks and from further frost damage.
At the chimney's left corner as well as in smaller areas below you can see evidence of severe frost related spalling of its brick surfaces.
Also see Chimney Exterior Spalling
Our photos below illustrate common sources of leaks into the wood-framed chimney chase used for factory-built metal chimneys. Poorly-supported metal covers over the wood-framed chimney chase lead to a concave or sunken top cap that leaks into the chimney assembly.
The most common defect we observe at chimney rain caps is that there is none - the rain cap was never installed, or it has been lost or blown away. Below, photographs of two types of factory built metal chimneys illustrate the loss of the chimney's rain cap. At below right you can also notice water ponding around the chimney flue.
If we want proof that people don't spend a lot of time looking at their chimney, ask a home inspector or chimney sweep how often they find that the rain cap or spark arresting chimney cap has been completely lost from a chimney.
As our photographs illustrate, a missing cap invites water damage to the chimney and the equipment it vents, draft problems (no protection against downdrafts caused by some wind conditions), and unsafe operation.
Water entering the chimney can cause enough corrosion in a metal flue that the chimney needs replacement.
Water entering the chimney also risks corroded leaky flue vent connectors, leading to draft problems and carbon monoxide poisoning risks as well as costly or dangerous damage to the heating equipment itself.
Damaged or Defective Metal Rain Caps on Chimneys
The insulated metal chimney rain cap shown at left was installed on a New York home by a Hudson Valley chimney company whose owner thought we were being picky and fussy to complain about the smashed rain cap top surface.
The rain cap top cover is secured by a wing nut screwed to a bolt that protrudes through the cap top surface. The concave surface of this damaged rain cap would guarantee water leakage down the metal chimney and into the heating appliance it serves, inviting rust damage and potentially leading to costly repairs or even unsafe equipment operation.
A "field repair" of a blob of silicone around the wing nut reduces the leakage rate. The proper repair is to remove the rain cap, restore its original domed shape, and reinstall it, or replace it with a new one.
In our photo at left, the interesting chimney has a metal rain cap on one flue and not the other. The metal chimney cap looks home-made and perhaps not functional. The un-covered chimney flue (and most likely the covered one) are comprised of hard-fired clay chimney flue tiles.
The presence of creosote tar and soot stains around the left-hand rain cap and the size of its base make us suspect that what was originally a fireplace flue was covered and possibly lined with a smaller-diameter metal flue serving a woodstove.
Watch out: without disassembly or an in-flue inspection we don't know if this chimney has been lined with a metal flue, insulated, safe, and proper, or not. Further clues will be found indoors, such as perhaps a woodstove or fireplace insert. Further in-flue inspection for chimney safety is needed.
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Related Topics, found near the top of this page suggest articles closely related to this one.
1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces.
2. Combustible materials, including framing, wood siding, flooring and trim, shall be permitted to abut the sides and hearth extensions, but not the backs, of masonry fireplaces, in accordance with FIGURE R1003.12, provided such combustible materials are a minimum of 12 inches (306 mm) from the inside surface of the nearest firebox lining.
3. Exposed combustible mantels or trim may be placed directly on the masonry fireplace front surrounding the fireplace opening provided such combustible materials shall not be placed within 6 inches (153 mm) of a fireplace opening. Combustible material within 12 inches (305 mm) of the fireplace opening shall not project more than 1/8 inch (3.2 mm) for each 1-inch (25 mm) distance from such opening.