Ferrous Metal Rooftop Elements and Upkeep – Part X

In recent articles we talked about delamination of roof membranes where attached to oxidizing ferrous metals.  Today we look closely at a few real examples from flat or low-slope roofs out in the field.

Rusty metal, single-ply, or modern membranes can lead to delamination when they are attached to metal at penetrations or flashings due to a chemical process known as oxidation. Oxidation occurs when iron molecules in the metal react with oxygen (O2) and water molecules (H2O) in the presence of moisture. 

grease duct remains
An exposed portion of a grease duct remains after the remainder of the duct between the roof penetration and the external fan have been relocated, yet it was not entirely removed from the roof as the relocation of the fan was not coincident with roof repairs.

This process, of rusting, happens on a chemical level by the formation of Iron Oxide (Rust).  Iron is abundantly available in most places on earth, it’s present right in the dirt outside, in most locations. but that particular form of iron is iron oxide. iron oxide actually goes through a pretty elaborate process of industrial manufacturing to become either iron ore or or steel as is commonly used in the production of most consumer or building construction type materials.

The formation of iron oxide, or rust, is a new compound that is less stable and more porous compared to the original metal. This compound has a tendency to flake off or become brittle over time. 

rust covered duct
The exposed portion of the duct is covered in rust at the chamfered or tapered edge of the reducer.  Iron molecules (Fe) present in the metal surface come into contact with oxygen and moisture. The iron reacts with oxygen to form iron oxide (Fe2O3), commonly known as rust. The chemical reaction can be simplified as follows: 4Fe + 3O2 + 6H2O → 4Fe2O3 · 6H2O

Rust is a brittle and less cohesive material compared to the original metal. When roofing membranes or sealants are attached to rusted metal surfaces, they may adhere initially. However, over time, the rusted metal’s decreased cohesion and adhesion properties can lead to a separation between the roofing material and the metal. 

tempered air ducts
In contrast to the majority of tempered air ducts, for heating or cooling, this rooftop duct built with welded seams, offering a much better seal than almost any joint sealant.

Delamination refers to the separation of layers, and in the context of roofing, it means the roofing membrane detaches from the rusted metal surface. The chemical composition of rust makes it less adhesive, causing the roofing material to lose its bond with the metal. 

black elastomeric sealant
A black elastomeric sealant is applied to the top of the EPDM base flashing at a duct penetration, yet oxidation of the substrate duct wall threatens delamination.

When delamination occurs, it creates gaps or openings between the roofing material and the metal surface. These gaps can allow water to penetrate beneath the roofing membrane, leading to leaks and water damage in the roofing system.

stands of exposed exhaust duct
The legs or stands of an exposed exhaust duct on a low-slope roof are set in pitch pockets, a better methodology than gun-applied or overlaid sealants.
pitch pocket
A pitch pocket is the proper, or approved, method of sealing around a pipe penetration, but in this case the sealant is deteriorated from excessive exposure to UV rays.

To prevent delamination due to rusting, it’s important to maintain the integrity of the metal surface by addressing rust and corrosion issues promptly. Protective coatings, regular inspections, and maintenance can help extend the lifespan of the metal and prevent the chemical process of oxidation from causing adhesion and cohesion failures in roofing systems.

a worm drive
A worm drive clamp secures a EMPD roof boot to a steel pipe penetrating through a EPDM roof membrane.

Routine maintenance such as the type needed as shown in the areas of the photos below where there is rusting near the connection point between a roof membrane boot and a substrate penetrating pipe, can be the difference between a properly functioning roof and a leaking roof  In this type of circumstance, the rusting metal must be chemically treated, for chemical conversion, or physically removed with abrasion or mechanical means.

worm drive clamp
A worm drive clamp secures a EMPD roof boot to a steel pipe penetrating through a EPDM roof membrane.

Smart and proactive replacement, construction, upkeep and maintenance of low slope roof and mansard roof systems requires an enthusiastic interest and understanding of historical methodologies, 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, contact us or fill out the webform below and drop us a line.  We will be in touch if we can help.

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