Read this before installing rooftop mechanical equipment
Our company does NOT do Mechanical or HVAC work, but since we specialize in flat roofing work, we encounter and interface with rooftop mechanical equipment installations very often. A flat roof is like free real estate for a property because the flat roof can be used to store or house things like mechanical plants, chillers, and equipment which would either consume useful building space or yard space. So space can be saved by putting equipment on the rooftop, and is very valuable in cities like Washington, DC.
How Mechanical Equipment Should Be Handled On A Flat Roof
In the past, we have discussed the flat roofing considerations related to penetrations of mechanical services such as pipes, cabling, electrical wires, and control wiring as they pass through the roof membrane. Some of those articles can be seen in the links below:
Importance of Pitch Pockets at Roof Penetrations
Slip Sheets And Vibration Isolation at Mechanical Equipment
Flat rooftop, non-industrial, HVAC and mechanical equipment is a very urban type of layout, uncommon outside of the city for a few reasons:
- Outside of the city, non-industrial flat roofs are less common
- Outside of the city, yard space is less valuable, so it’s increasingly cheaper, further from the city, to just use more land to house mechanical equipment.
The picture below shows an example of an HVAC unit mounted on a rooftop with ductwork that penetrates through the rooftop membrane. This particular type of unit is sometimes referred to as a total pack, or a all-in-one type unit which has both heating and cooling capabilities, and not only provides both heating and cooling, but also includes the pressurized fan system in the same unit.
Inside of the cities, it’s common for mechanical equipment to be installed on the rooftops of flat roof buildings, even residential rowhomes. Although, this is very uncommon in the suburbs. Exposed ductwork on a rooftop is difficult to fully seal properly, for a few reasons. Typical sheet metal ductwork is not designed or manufactured for exterior installation and generally has no intrinsic design elements intended to seal joints in horizontal runs of the ductwork from water entry.
In the picture above, you can see that the horizontal top of the exposed ductwork is covered or clad with an inslative membrane. This particular material is made a bit like bubble wrap. Cellular insulation of this type can be somewhat effective from a thermal resistance perspective, yet most materials of this class generally lack ultraviolet resistance and the durability found in most roof membrane materials. Roll roofing, which is much stronger and more durable, is difficult to seal in a complex geometric context of a ductwork system. High performance and industrial liquid applied roof coatings work very well, but require knowledge and training and are generally not available to non-professional roofers. When HVAC contractors or people other than low-slope roofing contractors attempt to seal the elements out of ductwork, they normally come up short, missing the mark and installing short-ife materials which do not perform well.
Maintaining A Flat Roof System
Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of 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.
