Rooftop Air Exhaust Systems – PART I

Last week, started a series on flat roof problems associated with ferrous metal non-roof components.  These are elements that are not actually part of the roof itself. The roof itself will include elements such as the field membrane, the elements of terminations and even elements of flanges or prefabricated rubber type materials to fit around penetrations. These items are actually different and separate from the rooftop itself yet they are installed on the rooftop and often have elements of mounting or connection which go through the roof itself. 

Today, we will talk about air exhaust systems, and in the next article HVAC systems, on rooftops and some of the potential problems associated with those elements on roofs and methods of preventing leakage and required upkeep.

The outline of this series of articles follows:

    • Common types of non-roof components found or flat roofs
      • Access Systems 
      • Historic Brick chimneys
  • Air Exhaust systems
    • HVAC systems
    • Electrical power distribution
    • Low Volt and communication wiring and satellite / antennas
    • PV Panels and mounting structures
    • Guardrails and fall protection systems
  • Roofing problems caused by oxidation of ferrous metals
    • Delamination 
    • Staining and bondability
    • Structural component failure
    • Leakage through metals
  • Methods of Repair and sustainability 
    • Standard Coatings
    • Substrate preparation
    • Advanced re-coatings
    • Special roofing provisions

Air Exhaust systems

A variety of different mechanical systems on the inside of a building can use air exhaust penetrations and ducts through the rooftop. Some of the most common types of these mechanical systems are laundry dryer exhaust, boiler or heating equipment exhaust, and mechanical air vents such as bathroom fans.   Technically, we could also lump air intake systems into this category however they are relatively uncommon on the flat roofs in Washington DC. One of the most common example of a air intake system into a historic building in Washington DC could be a tankless water heater combustion air intake.  

The picture below shows a air exhaust from a bathroom fan. These types of bathroom fans are  required in almost every bathroom without a window.   It’s almost a little bit disguised, but if you look closely you can see that here in this low slope roof, the previous contractor used a shingle roof boot, for residential construction. That type of penetration flashing is not allowed in low slope or flat roofs because it does connect to the membrane in a overlaid or concentrically overlapped / interlaced way in a flat roof. Those type of roof boots work fine for many asphalt or slate shingle installations, but they do not work with low slope or flat roofs.  

You can also see that the galvanized duct termination is significantly rusted. This particular type of metal is an electrogalvanized, not a hot dipped, zinc galvanized ferrous metal. For this reason, the top upper surface of the sheet metal has completely corroded and the zinc covering end is now rusted significantly.

In recent months, this area of the roof had leaked and our company was called to check the roof and look for the source of the leak. The source, in this case was apparent, and a previous contractor had also already tried to fix it by smearing on asphalt mastic, but it did not work.  Asphaltic mastic, uncovered, is highly susceptible from exposure to ultraviolet rays and in this case has already deteriorated to the point that it is leaking again.

rooftop air exhaust

The International Building Code (IBC) does have specific requirements regarding bathroom exhaust fans. These requirements are typically related to the need for mechanical ventilation, especially in bathrooms that do not have operable windows.

The IBC specifies factors such as the minimum required ventilation rates for bathrooms, which are measured in cubic feet per minute (CFM). These ventilation rates are based on factors like the size of the bathroom and the number of fixtures (sinks, toilets, showers) within it. When a bathroom does not have a window that can be opened to provide natural ventilation, the IBC often mandates the installation of an exhaust fan to meet the required ventilation rates.

These exhaust fans help remove excess moisture from the bathroom, reducing the risk of mold growth and associated health issues. They also contribute to the overall indoor air quality and comfort of the living space.

Often, old and otherwise abandoned, discontinued from use, chimneys are used as passageways that run all the way from the basement or ground level of the building, up through the rooftop of a building. The chimney of the building was likely built at the time of initial construction of the building, but in modern times the use of these chimneys is often changed to be used for a passageway for exhaust air ducts through the building, to the exterior, through the roof.

air exhaust on roof

Although these elements are technically separate from the roof system itself, these elements are still important and integral as part of the roof system because their upkeep and maintenance is important to ensure and safeguard against water leakage from the outside of the building.  When you buy s new roof, for example, none of these items are included, but if those items leak, you have a leak that will appear to be originating in the actual roof.

The modern ductwork that runs through the historic chimney now relies on the condition of the cementitious chimney crown to direct water away from the pipe penetration and keep water out of that chimney. There is a crack at this particular chimney crown and that crack must be repaired to keep water out of the chimney.


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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