Rooftop HVAC Systems and Associated Problems- PART I

Last week on our blog, we discussed rooftop air exhausts,  in 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 and 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 top of the rooftop and often have elements of mounting or connection which go through the roof itself. 

Today, we will talk about HVAC systems, and in the next article we will discuss electrical power distribution 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

hvac systems on roof

HVAC systems

Here, in Washington DC, almost every building has some type of heating system, and most buildings have cooling systems as well. Cooling systems are not actually required by the building code, but heat systems which maintain a temperature of 68° or higher are generally required by the building code.  It’s also a minimum requirement that building’s function properly and without some sort of heating source in a building.  In our climate zone though almost all types of piping will freeze in the winter without heating. When those pipes freeze they not only become inoperable, but they also can burst from the pressure of the freezing liquids within the pipe.  So with the exception of buildings that are used only for storage of materials, almost all buildings have piping which therein requires heating systems.

Heating systems and HVAC systems for that matter can be installed in a variety of options and configurations. Often, these systems are installed on rooftops to avoid the consumption of otherwise useful space on the interior of the building. The interior square footage of a building is extremely valuable, especially in a place like Washington DC where the average prices of buildings are currently over $400 per square foot (and still rising). A mechanical unit, may take up over 15 square feet on average, that would therein have a commensurate average value somewhere around $6000. It would basically be financially counter productive to manage a property financially effectively and give up so much space to a HVAC unit when there are alternative options to install that system on the rooftop of the building. Not all types of HVAC system can be installed on rooftop.  Boilers, for example, almost always have to be installed on the inside of the building and are generally installed in The basement or lower levels of the building close to the utility entry point into the building.

Overall though, it’s very pragmatic and makes simple logical sense to install HVAC systems on a rooftop where a rooftop installation is applicable to the type of HVAC equipment. This is especially true here in Washington DC.

The picture below was taken from a roof here in Northwest Washington DC on top of a commercial building with many housing or living units. Condo buildings and apartment buildings and even rowhome buildings often have condenser units like this installed on the rooftop. These units require several elements of piping and wiring from the inside of the building such as refrigerant lines, control wiring, power wiring, and often condensate discharge piping.

condenser units on condos

This particular building has had roof problems for a long time and you can see the chaotic array of different types of HVAC equipment on the roof including high pressure stairwell fans and the individual condensor units for each interior condominium unit.

high pressure stairwell fans

The next picture shows a typical Capitol Hill rowhome roof. This particular house has a ducted system installed on the rooftop from a totalpac type of HVAC unit. This particular type of unit provides both heat and cooling. With all of the ductwork penetrations through the roof,If this system adds an extra level of complexity to sealing the roof membrane around each of those individual duct penetrations.

As well, in cases of metal ducts installed on an exposed rooftop, the ductwork self must be sealed at every joint to prevent leakage from the outside to the inside of the duct down into the building. Typical HVAC duct installations only use a mastic that is not highly resistant to exposure to ultraviolet rays. For that reason, it’s important to install an additional type of coating or sealant at each of those connections. This type of sealant and thorough thinking on the matter of leakage is usually not employed by HVAC installers.  Their focus is generally limited to the installation and functional aspects of the HVAC unit by itself, and they normally do not think about or worry about sealing the unit and associated ductwork to prevent leakage into the inside of the building. Our company can help with ceiling ductwork. It’s not a roofing type of item, but it’s a ferrous metal element, like the other elements that we are discussing in this series of articles, that must be considered and treated properly to prevent leakage in the building from the rooftop. Although, it’s not part of the roof itself.

metal ducts

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.

On Key

Related Posts

fitting of painted i-beams

Painted I-Beams or Unpainted?

Once in a while people ask us whether or not I-beams on rooftops for equipment like HVAC units or HVAC condenser units should be properly