Rusting Metal Causing Leaks Part I

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 will be discussed in today’s article and sections #5-7 will be discussed in the upcoming article later this week.

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

rusting metal


Many modern single-ply roof membranes are highly resistant to moisture and deterioration associated with exposure to moisture. In fact, they are generally highly effective at deterring damage from exposure to exterior elements. Often, these material systems can have long durations of longevity. However, a system is only as strong as its weakest link when needed to defend against the forces of nature. Here, as well, there are shortcomings and vulnerabilities where a roof system must work around other adjacent building systems. Some of the main types of building systems that rooftop membranes must conform to and/or work around are HVAC systems, electrical supply systems to equipment, plumbing vent systems, and systems of fenestration such as skylights.

Roofing Material Characteristics

Roofing material manufacturers very specifically design and create their materials to resist the environmental elements of deterioration for as long as possible. Some materials are particularly good at water and moisture resistance, but those same materials also often lack characteristics of flexibility and/or pliability. Flexibility is very important in materials exposed to ever-changing thermal environments and solar exposure. Some building materials have a significant amount of movement as affected by temperature changes. In the particular case of metals used on rooftops, it’s almost always better to use the highest quality materials with the highest levels of moisture resistance, but those materials are particularly expensive, and developers will also choose to use less expensive materials to save money.

rusted metal causing leaks

Aluminum has very high moisture resistance but over time will oxidize and deteriorate. Stainless steel, as well, has excellent corrosion resistance, but it will also oxidize over time. So, although these materials are superior to many others, they are not bulletproof; they’re not perfect. Also, while these particular material types have higher performance characteristics, not all types of metal used in ductworks and specific types of cases are available in those varieties. Some types of ductwork are only available in a market-ready type of production sense in Ferris metal, which can oxidize. Regular steel or iron, for example, can oxidize in an extreme way and very quickly. An alternative to unprotected steel and iron may be coated metals, but often coated metals will also delaminate and/or deteriorate where the finish is made to protect the metal.

Over time, zinc galvanization is a common and somewhat effective type of corrosion and deterioration resistant from exposure to moisture, but there are two main classes of galvanization. The hot-dip galvanization process requires that metals be submerged in a chemical bath, and this type of application is very customized and very expensive. By comparison, electrical galvanization is much cheaper but not nearly as effective. Most of the ductwork used in regular HVAC or air conditioning-type contexts is an electro-galvanized metal. HVAC contractors, when installing ductwork on rooftop installations, will often use this same electrical galvanized sheet metal for rooftop exposed ductwork, but it is highly ineffective over time because that ductwork will rust to the point that it leaks into the interior space of the building.

zinc galvanization

Visual Signs of Metal Deterioration

One of the main signs of water entry beyond the protective coating of metals to the point that it damages and saturates the metal or allows wood to enter into susceptible portions of the building is very intuitive and obvious. Often, you can see trails of rust stains from this type of exposure and/or deterioration. When these signs are seen, they’re normally indicative of greater problems. In the picture below, for example, where there are rust stains around the round exhaust ductwork, water is entering susceptible parts of that sheet metal because the joints of the sheet metal have not been sealed. Simple gun-applied elastomeric sealants, for example, could be applied to the joint between the cement tissues chimney crown and the sides of the sheet metal ductwork, as well as between the individual joints in the walls of the duct, where they are concentrically sleeved together to allow for shaping and movement of the duct. Sealants could be applied to prevent or deter water entry into those joints. Simple professional attention, such as routine upkeep and maintenance and professional seasonal maintenance check-ups, could prevent this type of deterioration. This deterioration is more than just an aesthetic condition because as water enters into that chimney, it’s damaging both the metal and the cement tissues elements of the chimney itself, and allowing water into even more acceptable areas of the building, such as at the framing at the roof rafter and roof deck and ceiling deck below.

Substitutes to Susceptible Metals

In some cases, alternative material types can be used instead of or in lieu of susceptible sheet metals. One example is in the picture below, where a terracotta tile flute has been used in lieu of a metal duct. Terracotta is a time-tested material that has been used by humans for many thousands of years. The material happens to be naturally resistant to ultraviolet rays and has a high resistance to water exposure. In many cases, this type of material costs more than simple sheet metal, but it’s worth much more because it intrinsically has qualities that allow for it to last much longer. 

terracotta tiles material

Another type of alternative material, as discussed above, is aluminum. In this case, two rooftop exhaust duct terminations, intended for rooftop installation specifically, have been installed on a modified roof membrane system, and this type of material, although still susceptible overall, has a much higher level of resistance and durability than ferrous metals.

aluminum material exhaust duct

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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