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 painted and finished or left unfinished in the red iron or raw iron stage. Steel has a blue steel type finish like a typical mill iron or steel patina.  Often, when shipped or bought from the factory the steel beam will be painted with a red primer.

The red primers and the factory finish do not last long and we strongly suggest that they are in fact painted.  When steel beams are left uncoated or unfinished they will rust on a roof and in the picture below you can see an example of a set of Steel beams under a condenser unit on a rooftop in Washington DC.

how painted i-beams work

HVAC contractors have a specific trade discipline and a special niche of the construction industry.  They are not necessarily expected to know about things outside of their trade, but it would help in several cases or circumstances for the HVAC systems to be installed with care, to not damage the roof system. 

In this particular example, the steel beams are set directly on top of the parapet coping of the roof.  The parapet and applied coping on top of the parapet run at an angle downward from the front side of the building towards the rear. The roof also runs in the same direction with a slight angle towards the rear end the parapet follows this angle or grade.

The lower steel beam is set on top of a leveling plate to bring the lower beam up a little bit higher, almost to completely level with the upper beam. The upper beam though is set directly on top of the aluminum coping.  It’s better practice to instead set the steel beams on top of an isolating material, preferably a material with a degree of resiliency.

fitting of painted i-beams

The contractors who installed or provided upkeep, repair, and maintenance on this roof and the structural I-beam support system for the HVAC condenser unit had painted the i-beams originally or at some time after installation. However, at this point in time the paint has been on a dress or without maintenance for several years and the paint has worn off. Paint coatings need to be maintained and reapplied every 3 to 7 years, on average.
installation of painted i-beams

As the steel beams have rusted, a trail of iron oxide deposit has run down from the steel support structure onto the low slope roof system and left staining on top of the thermostat roof membrane. The iron oxide staining, in itself, doesn’t really destroy the roof membrane, but it detracts from the roof’s inherent high reflectivity.

In the picture below, you can see that where the HVAC condenser unit Bears directly on top of the steel I beams, it is separated from the I-beams with a neoprene or butyl rubber and synthetic cork spacer or isolator. This isolating material allows for reduction of vibration. When the unit runs and it’s typical cycles, it has a compressor and a motor and a fan and fan belt system which create a lot of motion and action.  That motion and action creates a ton of vibration which can transfer down into the building. These small isolators at the corners of the units, between the condenser unit and the steel support system significantly help reduce vibration or the transfer of vibration.

why painted i-beams are important

In the closer angle of the photo below, you can see that the surface of the I-beam, where the condenser bears on top of the isolator and where the isolator makes contact with the I-beam, is covered with a relatively thin surface of iron oxide or rust. These areas need to be recoded to prevent accelerated deterioration.

function of painted i-beams

At a nearby building, here in Washington DC, a similar steel framework setup has been installed to mount that building’s condenser unit above the rooftop. Here though, the steel framing elements were never properly primed or painted and the entirety of the steel I-beam is covered with an iron oxide / rust.  Over time, without proper treatment, the iron oxide will result in more extensive rust and lead to scaling and even ultimate failure of the i-beams. Replacement and reinstallation of new i-beams is particularly complex and very costly. This additional work and expense can simply be avoided by a comparatively very inexpensive routine maintenance and upkeep and repainting and or scraping and or sanding of the rust before re-priming and repainting. Particularly in many cases rust converting primers can be very effective in stabilizing areas with surface rust.

importance of painted i-beams
We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at, and you can call us at (202) 840-8698 and email us at

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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