Rusting Metal Causing Leaks Part II

Today we are looking at a topic of rusting and corroding metals in or around or through rooftop systems in two different parts. sections #1 – 4 were discussed in our last article earlier this week.  In today’s article below we are discussing sections #5-7 of this topic, starting with exposed rooftop ductwork.

Table of Contents:

  1. Introduction
  2. Roofing Material Characteristics
  3. Visual Signs of Metal Deterioration
  4. Substitutes to Susceptible Metals
  5. Exposed Rooftop Ductwork: A Regrettable Choice
  6. Methods of Treatment for Exterior Exposed Metals
  7. Conclusion of the Series: Metal in Flat Roofing Systems

Exposed Rooftop Ductwork: A Regrettable Choice

The term “regrettable” was specifically chosen to highlight the unfortunate decision to use exposed metal ductwork on rooftops, as many fail to grasp the limitations of interior ductwork when placed on the exterior of a building. HVAC contractors in Washington DC consistently recommend this material, leading to widespread installations across the city. However, the issues with this type of ductwork often manifest years after the initial installation, and a few years’ lifespan is insufficient. Systems like these should endure much longer, especially considering rooftop systems such as membranes are designed for several decades of use.

The problems arising from using interior-grade sheet metal on the exterior rooftop of a building involve various complications. Firstly, a crucial limitation is the thin electro-galvanization coding on the external side of the metal. Unlike hot-dip galvanization, this zinc galvanization type deteriorates and fails relatively quickly. This is significant because zinc galvanization serves as the sole waterproofing or protection for the metal against oxidation and deterioration.

rusting metal

Another major issue is the ductwork being joined in relatively small sections with metal clips connecting each section and joints between them. Due to the metal’s thin nature, it experiences significant movement with temperature fluctuations. The sealants applied around these joints deteriorate over time due to exposure to temperature fluctuations and ultraviolet rays. Compounded with thermal movement, these joints break apart, allowing water to enter the ductwork. Once water infiltrates the ductwork, the next significant problem emerges.

The interior of these ducts is lined with relatively thin insulation, having a low but necessary thermal impact. This insulation acts like a sponge, absorbing water but not allowing it to dry out quickly. Consequently, the installation becomes a breeding ground for mold and other hazardous growth. The water slowly drains and relieves itself over time, resulting in gradual deterioration of the building materials around the inside of the ductwork where it enters the building.

dutchwork

Methods of Treatment for Exterior Exposed Metals

Various metals are used in elements on rooftops, each requiring different treatments to prolong their life. One such treatment is recoating ferrous metals. The life of ferrous metals can be extended with repeated routine recoating using chemical applications to protect the metal. Often, when this metal is initially installed by HVAC contractors, little effort is made to provide additional supplementary coating for increased resistance. Once the metals rust, the rust must be stripped and cleaned to return to a proper substrate. 

methods of treatment for exposed metals

Rust-converting chemicals can also be applied when rust has not led to the separation of the core elements of steel. The preparation phase of coating applications is critical, often being done poorly or partially skipped by professional painters rushing their work. Painting work is competitively priced, leading painters to convince themselves they don’t have enough time to do it right, which is a significant mistake.

Conclusion of the Series: Metal in Flat Roofing Systems

The series concludes by presenting a picture of a historic coping cap at the front mansard demising wall between two historic rowhomes in Washington DC. The ferrous metal used in this cap not only serves as a functional cover but also features specific architectural ornamentation bent into the metal over 100 years ago. 

architectural ornamentation bent

Replacing this metal in kind would be prohibitively expensive today, as the fabrication shops of the historic nature in that time no longer exist in the same level of abundance and proliferation. Essentially, two options exist for dealing with the deterioration of something with unique historic quality that cannot be easily reproduced: reproduction at a custom manner, which is extremely expensive, or substitution with something lacking historic authenticity and character, resulting in a regrettable loss from the historic fabric of the building.

Upkeep, Care and Maintenance of Washington DC Roofs

This case study is a perfect example of why a professional roofer should be used for all types of roof installation, even things that seem as simple as a balcony.  Consulting with a roofing professional, like Dupont Roofing, familiar with the specific climate conditions and the building science of roofing in Washington DC is advisable to ensure the roof assembly is appropriately designed and constructed to manage the flow of water, ice damming, capillary action and condensation effectively.

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply contact us or complete the webform below and drop us a line and we’ll respond if we can help.

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