Chimneys through Historic Rowhome Flat Roofs – Part II

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 bass 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

  • Capping
  • Spark Arrestor
  • Flue
  • Crowning
  • Flaunching
  • Brick and Mortar
  • Counterflashing
  • Base Flashing

The picture below shows a somewhat typical historic brick chimney with a missing flue, deteriorated crown, missing spark arrestor and missing cap. Without these critical elements, water enters into the hollow void of the chimney at each rainfall or precipitation event, increasing damage, as time goes on with successive iterations of deterioration in the historic masonry.  This deterioration can be largely prevented and abated by simply having the missing elements of the flue, crown, spark arrestor and capping rebuilt or reinstalled.

brick chimneys through historic rowhome

The chimney capping, also known as the chimney cap, is a critical component in the waterproofing system of historic masonry chimneys situated above flat roofs. The primary function of a chimney cap is to prevent water infiltration into the chimney’s interior, protecting the masonry structure from potential damage and ensuring the longevity of the chimney.

The chimney capping is typically constructed from durable and at least low-permeability materials such as concrete, stone, or specialized chimney cap products.  Often aluminum or obligation resistant sheet metals can be used. These materials are chosen for their resistance to weathering and their ability to withstand the elements over an extended period. The capping is designed with a sloped or angled surface, allowing water to easily run off and preventing it from accumulating on the top of the chimney.

One of the key features of the chimney capping is its overhang, which extends beyond the edges of the chimney stack or flue. This overhang creates a protective passive projection, shielding the vulnerable joints and mortar or cementitious materials of the crown where it meets the flue, from direct exposure to non-wind-driven rainfall or snowfall. By directing water away from these critical areas, the capping helps to prevent moisture infiltration, which can lead to deterioration of the masonry over time.  In the case of many residential row home historic buildings in Washington dc, 9×9 terracotta flues were historically installed with a typical brick chimney cavity. And in most of these cases the flue would sit within the outer perimeter of the brickwork and particularly 9×9 Terracotta flues would be covered with a cap which would allow water to drip beyond the perimeter of the flu, in most cases of relatively moderate to low wind conditions coinciding with precipitation.

In addition to the sloped surface and overhang, many chimney tops incorporate flaunching, a corbelled brick projection at the perimeter of the chimney, under the crown These design elements further enhance the capping and crown’s ability to shed water, allowing it to drip away from the chimney stack and preventing it from running down the sides of the historic brick structure, at the most common occurrences of low wind rainfall. 

A spark arrestor is a screen-like mesh, normally made of aluminum or oxidation resistant metals, installed between the chimney cap and the flue.  Its original primary function was to prevent sparks or burning embers from escaping the chimney and potentially causing fires on the roof or neighboring structures.

In the historic neighborhoods of Washington DC, where buildings are often situated in rows or in close proximity to one another, the risk of a stray spark or ember igniting a neighboring rooftop or structure is a real possibility, especially with fireplaces, wood burning stoves.   In modern times today, most old fireplaces and or wood burning stoves have been removed, abandoned, and or either inoperable or defunct. However, even where these old chimneys are no longer used by historic fireplaces, they’re often used by modern boilers or older boilers that are still in use. Even though natural gas burning boilers do not exhaust combustion air laden with hot embers or sparks, a spark arrestor is still extremely helpful and may even be considered necessary to protect the home from rodents who want to burrow into the warmer parts of the building in the winter. In addition to squirrels in the winter or other rodent type crawling animals, birds will also try to enter into chimney flue to protect themselves from predators or search for refuge.  The spark arrestor will keep most animals out of the chimney flue.

The diagram below shows a sketch of a typical historic chimney with the main components of the typical chimney anatomy labeled to show the individual parts.

chimneys through historic rowhome
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, and you can call us at (202) 840-8698 and email us at

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