How roof mounted HVAC units can cause damage.

Aqua Roofing on damage from A/C units.

No one wants to stay in a sweltering-hot building — so keeping workers and tenants comfortably cool is high on most facility managers’ lists. Although they make the climate bearable indoors, rooftop air conditioning units are among the biggest culprits when it comes to roof-related problems. 

HVAC Condensation

Condensation from HVAC units has to go somewhere, but if it discharges on the roof, it can create localized ponding, which increases the risk of damage. Constant moisture attracts airborne debris, leading to mold, vegetative growth and premature membrane deterioration. It creates unsightly stains and discoloration as well. The solution is to direct HVAC condensation discharge away from the surface of the roof, either through interior discharge lines, when feasible, or by installing condensation lines that direct the discharge to the nearest roof drain or gutter.

Awkward Installation Causing Low Spots

Commercial HVAC units weigh around 400 to 4,000 lbs, which means sometimes an awkward installation or unforeseen settling will cause a low spot around the unit. This can lead to the flashing becoming loose and/or puddling water around the unit. Having the unit regularly inspected, a few times a year at least, will prevent any long-standing puddling water problems.


Commercial HVAC units installed when the roof and building were constructed are less likely to cause problems to a flat roof. When a unit is installed otherwise, the flat roof membrane must be cut into, which can create leakage problems. HVAC technicians are not roofers, and sometimes the materials and methods they use to install the units are not proper. The sealed flashing underneath the HVAC unit is the main protection from moisture penetration getting into the cut open flat roof membrane. If this sealant or flashing comes unsealed, water can directly flow into the substructure of a flat roof; mold, rot, and leaks can become the result of unsealed flashing.

HVAC Tech Troubles

HVAC service technicians aren’t always trained to look out for the welfare of the roof. Struggling to reach under a low HVAC frame or carelessly tossing tools around can damage a roof membrane. If the tech does not report the incident, it can lead to leaking and saturation damage to underlying insulation. You can reduce the risk by having someone on the roof observing when a technician is servicing the HVAC unit. By installing walkpads around HVAC units, you can also lessen the chance of damage to the membrane in these high-traffic areas.


All penetrating objects on a flat roof are potential causes of leaks and damage, so they should be regularly inspected. The most common object to cause leakage problems is the large and heavy commercial HVAC units. Even when they’re installed well and carefully, inspecting them regularly is important to prevent any leakage, mold, or substructure damage.

If you need our help, just let us know...  -Aqua Roofing