Rooftop Chimney Crowns and Flaunching

Last week, we looked at some of the concerns and issues related to waterproofing around modern chimneys where modern chimneys have been installed on historic buildings. It’s not very common, but from time to time we’ll see that type of condition. Waterproofing, counter flashing, and base flashing are extremely important. As well the components of the chimney such as the crown, flue and the connection between the crown and the flue, pointing at the brick areas of the chimney and at the flaunching in connection between the flaunching and the crown and the existence of capping and a terracotta flue cannot be taken for granted.

The outline from Part I, from last week follows below for reference:

  1. Identifying Differences
  2. Reasons for Replacement
  3. Waterproofing Challenges
  4. Integration and Facilitation

The bold areas of the outline below will be in focus today as we discuss chimney capping and spark arrestors. 

Rooftop Chimney Anatomy

  1. Capping
  2. Spark Arrestor
  3. Flue
  4. Crowning
  5. Flaunching
  6. Brick and Mortar
  7. Counterflashing
  8. Base Flashing

This past week we took a close look at the spark arrestor which acts as a barrier, trapping hot particles within the chimney before they can be carried away by the draft and potentially start a fire.  Spark arrestors were particularly important when wood-burning fireplaces were the primary source of heat in these historic buildings. As the wood burned, sparks and embers could be carried up the chimney flue and out into the open air, posing a significant fire hazard. The spark arrestor’s mesh or screen design allowed the smoke and exhaust gases to exit while containing the potentially dangerous burning particles.  While many of the interior fireplaces in these historic buildings have been converted or removed over time, the chimney flues often remain in place and serve as exhaust vents for modern HVAC systems, boilers, or hot water heaters. Even without the presence of wood-burning fires, the spark arrestor continues to play a vital role in preventing unwanted debris or small animals from entering the chimney and potentially causing blockages or gaining access to the building’s interior.

A diagram of the overall chimney and chimney anatomy and components follows below, from this past week’s blog.

modern chimneys

The mesh or screen design of the spark arrestor allows the free flow of exhaust gases while acting as a barrier against rodents, birds, or other small creatures that might attempt to nest or seek shelter within the chimney. This not only protects the building’s occupants but also ensures the proper functioning of the HVAC or heating systems by preventing obstructions in the exhaust pathway.

Chimney Flues

The spark arrestor is typically attached to the top of the chimney flue, protruding slightly above the chimney crown or capping. This particular placement allows it to effectively capture any sparks or embers before they can escape into the open air, while also allowing for unobstructed exhaust flow.  

The primary function of a chimney flue is to provide a vertical passage for the safe and efficient exhaust of smoke, gases, and hot air from a building’s interior to the outside atmosphere. In historic brick buildings of Washington, D.C, chimney flues were commonly constructed using terracotta clay tiles.  Like kiln fired clay brick, terracotta is a durable and heat-resistant material well-suited for this purpose.

Terracotta clay tiles were one the preferred choice for chimney flue construction due to their ability to withstand high temperatures and resist the corrosive effects of combustion byproducts. These tiles were carefully vertically and concentrically stacked forming a passage that extended from the fireplace or heating source at the building’s base to the chimney termination above the roof.

The chimney flue’s construction involved integrating it within the surrounding building assembly, typically consisting of brick masonry walls. As the flue rose vertically through the building, it was encased within these brick chimney walls, which provided a degree of structural support and protection for the flue assembly.

While a terracotta chimney flue itself is not a direct component of a flat or low-slope roof system, its condition and detailing are crucial from a waterproofing perspective. Improper maintenance or inadequate construction of the chimney’s exposed elements can lead to water infiltration, potentially causing damage within the building’s interior.  One of the primary concerns is the condition of the chimney crown or cap. If this protective covering is not properly assembled or becomes damaged over time, water can seep through and travel downwards along the chimney flue. Similarly, if the exposed brick masonry of the chimney stack is not well-maintained, with cracked or deteriorated mortar joints, water can find its way into the assembly and eventually reach the interior of the building.

The diagram above from last week is really helpful to understand the components at the top of the chimney and how those components are assembled to allow water to passively shed away from the top of the chimney and not enter into the top of the chimney and down into the shaft and or flew of the chimney and cause damage. But particularly the next diagram below shows a better view of the internal components on the inside of the chimney.

parts of chimney

It’s important to note that while the terracotta tiles within the chimney flue are designed to prevent smoke and gasses from escaping laterally, they are not necessarily waterproof in the downward direction. These tiles are typically assembled in a hub-and-spigot manner, with the spigot end of one tile fitting concentrically into the hub end of the next, creating a passage for smoke to travel upwards. However, this configuration may not effectively prevent water from traveling downwards along the flue if it gains entry from the top.

Proper waterproofing detailing at the chimney crown and regular maintenance of the exposed masonry are crucial to mitigating the risk of water infiltration through the chimney flue. Failure to address these issues can lead to water damage within the building, potentially causing deterioration of interior finishes, structural components, or even creating an environment conducive to mold growth.

Chimney Crowns

Chimney caps generally, through the attachment of a spark arrestor are attached to that spark arrestor and therein attached to the top of the chimney flue, allowing space for air to pass out of the chimney.  The chimney caps roll though is to prevent water from running directly down from above the terracotta chimney flue and into that interstitial space of the middle of the chimney.

Often, we’ll see chimney caps in a variety of different materials and forms. One of the most common modern methods of Chimney cab fabrication is to make the Chimney Cap integral with the spark arrestor and made of a light gauge metal. This is one of the most effective methods but historic methods also created capping without a spark arrestor. Some of these types of examples are abundant in historic chimneys tapped with Gothic arches at the top of the chimney.

The picture below is an example of a field-assembled masonry cap.

field assembled masonry cap

By comparison though, metal chimney caps will generally deflect water to the outer edge of the cap and then run from the crown to the launching and then over the sides of the chimney.

Regularly inspecting and maintaining the chimney crown is vital for preserving the integrity of historic masonry chimneys. Over time, exposure to the elements can cause cracks, chips, or other forms of damage to the capping, compromising its waterproofing capabilities. Timely repairs or replacement of deteriorated capping can prevent more extensive and costly damage to the chimney’s interior structure.

The chimney crown is a sloped or angled structure that sits directly atop the chimney stack, typically constructed from concrete, stone, or other durable materials. Its primary purpose is to shed water and prevent moisture from seeping into the chimney’s interior masonry.

Without a properly designed and installed chimney crown, water and precipitation can easily accumulate on the flat top surface of the chimney stack. Over time, this standing water can saturate the bricks and mortar, leading to significant deterioration and potential structural damage.

The sloped design of the chimney crown allows water to naturally run off the sides of the chimney, preventing it from pooling or seeping into the masonry. This simple yet effective feature helps to extend the lifespan of the chimney and minimize the need for costly repairs or rebuilding.

Additionally, the chimney crown often, dimensions to the projecting flaunching, extends slightly beyond the edges of the chimney stack, creating an overhang that further protects the masonry from direct exposure to rainfall and snowfall. This overhang acts as a shield, directing water away from the vulnerable edges and joints of the brickwork.  By effectively shedding water and preventing moisture intrusion, the chimney crown helps to mitigate the effects of freeze-thaw cycles, ensuring the structural integrity of the chimney is maintained throughout the harsh winter months.

Proper maintenance and periodic inspection of the chimney crown are essential to ensure its effectiveness. Cracks, deterioration, or improper installation can compromise its ability to shed water, potentially leading to moisture-related damage within the chimney’s masonry.

We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com.

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