Modern Chimneys through Historic Rowhome Flat Roofs – Part I

It’s not entirely common but once in a while on flat roofs in Washington DC, we will see relatively modern rebuilt or new chimneys. Rooftop chimneys come through the roof just a few feet above the roof surface. Chimneys will often be built on interior demising walls of rowhomes. They exist on the inside or interior of the building, but rise up through the roof of the building. These chimneys then have to have the roof membrane terminated against the side of the chimney so that a connection is established and maintain between the roof membrane and the chimney side. New chimneys though with modern mortar and brick masonry seem a bit odd, above the rooftop of a historic building, but they are sometimes an interesting surprise we find on the roof.

The outline for this 2-part series of articles follows.

Part I

  1. Identifying Differences:
    Recognizable differences between old and new rooftop chimney
  2. Reasons for Replacement:
    Reasons for replacing or installing a new chimney
  3. Waterproofing Challenges:
    Creating a waterproof connection between a membrane and a brick chimney
  4. Integration and Facilitation:
    Tips and tricks to make the integration of a new chimney work properly

Part II

Rooftop Chimney Anatomy

  • Capping
  • Spark Arrestors
  • Crowning
  • Flaunching
  • Brick and Mortar
  • Counterflashing
  • Base Flashing
  • Decking

modern chimneysWashington D.C.’s historic buildings feature a range of historic architectural styles, ranging from Italianates, to Queen Anne’s and Wardmans. Amidst the historic structural brick walls, stone foundations, slate mansard roofs and historic BUR and standing seam roofs, the occasional presence of newer chimneys, either rebuilt or added, rising just feet above the roofline can seem slightly out of place. But these modern chimneys and retrofits serve important functions, adding mechanical passageways and or replacing deteriorated originals to preserve buildings’ utility and safety. Installing new chimneys on historic flat roofs also poses unique waterproofing challenges.

modern chimneys on flat roof

Identifying Old and New

Many original chimneys in Washington D.C. date back over a century. Their brickwork often displays the deterioration of unmitigated environmental exposure over the many decades of time, with cracks, missing sections, crumbling mortar, and noticeable leaning. By contrast, replacement chimneys should feature straight, plumb brickwork and functionally sufficient flues, terminated with proper crowns and flaunching. However, we often find, modern chimneys have been improperly built by people (not necessarily actual contractors) who don’t know how chimneys are required to be built and know even less about best practices.

identifying old and new chimneys

brick exteriors of new chimney

The brick exteriors of new chimneys also may look contemporary, with smooth or consistent faced whole, undamaged bricks. By comparison, historic chimneys were often built with what we referred to today as a common brick fools-up common bricks were made with unsifted or unfiltered substrate components which included mostly clay but also included lots of impurities such as stone and or other materials. When looking at the exposed face of a common historic brick, these impurities and or agglomerated materials show signs of inconsistencies. Another distinction is size – newer chimneys are often smaller, tailored to reduced uses like venting heaters or decorative fireplaces. Newer bricks generally look very consistent because today we have heavy hydraulic, electric, or gas powered equipment to separate materials and lift heavy loads that couldn’t be done in the same scale in historic times.

Reasons for Replacement

There are several different reasons or motivations for installing new chimneys atop historic D.C. buildings. The primary driver is deterioration of original chimneys beyond repair. Poor maintenance and water infiltration can cause severe internal and external damage. New chimneys should provide a reset, a fresh start, meaning: reliable, durable venting and prevent further decay. Unfortunately though, in reality if construction is done cheap and dirty with unknowledgeable or untrained workers, the new result may be worse than an old half deteriorated chimney.

structurally compromised chimneys

Safety is also a concern with structurally compromised chimneys at risk of collapse. We often see chimneys leaning with an angle or degree of incline worse than the Leaning Tower of Pisa. It’s very concerning because when those chimneys eventually do collapse, they can cause serious additional damage to the building and / or kill people below.

Another reason is changing needs; early chimneys accommodated multiple fireplaces for heating, whereas modern buildings may only require minimal venting. Aesthetics also occasionally play a role, as sleek new chimneys degrade the traditional elements of historic buildings’ rooflines. Most of the chimneys found on the flat roof portion of the buildings though cannot be seen from the street and sidewalks where aesthetic concerns may apply.

 chimneys on the flat roof portion

Waterproofing Challenges

Integrating modern chimneys with historic flat roofs involves a handful of waterproofing challenges. Historic membrane roofs on older buildings range from built-up and standing seam roofs. These can be rigid and irregular, complicating a watertight seal with any new penetrations. Polymer single-ply membranes, like EPDM or TPO, are becoming somewhat ubiquitous on contemporary flat roofs. Their flexibility helps significance yet still also requires careful detailing at chimney connections, as joints are vulnerable to movement and vibration.

integrated modern chimneys

Integration and Facilitation

Several approaches help integrate new chimneys through both historic and modern roof membranes.

 new chimneys through both historic and modern roof membranes

Multi-part adjustable flashing systems accommodate differing joint widths, roof angles, and chimney shapes. Mechanical fastening, in some cases, increases long-term reliability compared to adhesion alone at critical membrane connection points. One of the most common in dependable ways to connect a flexible membrane to a solid vertical surface such as a chimney sidewall would be through a termination bar. Termination bars are made from extruded aluminum, in most cases.

new chimneys replacement

In most cases, termination bars will have a double ridge with a perforation in the middle for attachment in six-inch spacing. The ridges in the otherwise flat aluminum extrusion provide rigidity because metals extruded or cut into flat strips can be vulnerable to bending or buckling forces applied in the longitudinal direction parallel to their length. The ridges strengthen the extrusion against those longitudinal stresses and forces.

connection points to a chimney

Durable, high-quality sealants at all uncovered terminating edges, such as at the connection points to a chimney or roof penetrations, provide an additional layer of defense to prevent water migration.

chimney masonry

Cricket flashings divert water around chimneys, in many cases also help in deterring ponding which leads to better practice in avoiding unnecessary water head pressure at seams. We talked about some of the principles around using crickets in a recent article.

Counterflashing overlapped by chimney masonry provides redundancy if primary seals fail. The building code specifically requires that if not using a term termination bar, if using an aluminum counter flashing, for example, the edge of the counter flash, you should be set into the chimney masonry in a reglet or raggle.

new rooftop chimneys

With careful attention to detail and allowances for roof movement, new rooftop chimneys can be successfully integrated into Washington D.C. ’s historic urban landscape. This effort helps preserve these structures for the future while retaining their architectural heritage. Chimney replacement is just one way that building owners balance preservation with evolving needs, keeping the city’s history alive.

We recommend every building owner in DC who cares about 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

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