Rooftop Chimney Spark Arrestors Part 1

The chimney shown in the picture below is similar to a lot of rooftop chimneys, but the one in the picture below is missing some important elements.  That chimney is missing a cap and crown.  It could be argued that the crown is slightly less important than the cap and we could be convinced to agree, in some cases, but theses elements should work together.   This chimney is also missing flaunching. This particular rooftop chimney was likely never built with flaunching, and we know that because unlike a crown which can deteriorate, flaunching generally lasts for the life of a chimney.   In other words, within the lifespan of a brick chimney it should be expected that the crown will need to be replaced at some point,  roughly mid lifespan. By comparison though flaunching should last for the entire lifespan of the chimney. In this case, it makes sense, based on the photograph, that the flaunching was never built, even when the chimney was originally built. 

When you look even closer, you can see that at the very top of the chimney, at least the top flue tile is missing. In a typical historic chimney like this, built on a Capitol Hill brick rowhome, we would expect to see a terracotta flue tile passing through the top of the chimney and extending about 4″ above the crown. Here though, in this case, in this picture you can look a little bit down into the chimney and see that there is no terracotta flue at the top of the chimney.

So, in total this chimney is missing three important parts, in order of highest priority to lowest.:

  1. Cap
  2. Flue tile
  3. Crown
  4. Flaunching

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The audience of our blog, inclusive of our customers and readers of our website, are interested In building construction best practices. Some may argue that the omitted flue tile is most important missing element.  We believe, however, that the chimney cap is even more important than the top flue tile. The flue tile protects the inside of the chimney, at least at the top above the roof line.  We think that is very important but we also recognize that the chimney cap will not only keep water out of the inside of the brick chimney, but it will also keep water out of the chimney foundation, a location where water will pool and collect.    Pulling or ponding water in an otherwise unprotected area of masonry will lead to extensive damage over time.  As that area of masonry becomes hydrated without a way to relieve, disperse, or dissipate the collected water, that water will slowly break down the brick and mortar of the foundation, particularly in cases of historic low temperature kiln fired brick and lime mortar.

In the picture below, you can see an overview of several rooftops in Washington DC. At every one of these rooftops there are variations and differences which make each of them unique. But a commonality or consistent element between all of them is that without exception every roof has mechanical penetrations and chimneys which exist on that roof to conduct the functional and system workings of the building.

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Rooftop chimneys are ubiquitous, found on almost every single Capital Hill, Dupont Circle, and Georgetown row home rooftop.  We continue this discussion and examination of rooftop chimneys in future articles.

Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building. 

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, fill out the webform below and drop us a line.  We will be in touch if we can help.

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