May 2012, a famous Hollywood actor made a memorable visit to Korea to promote his new movie. During his trip, he shared on social media his love for Korea, along with several photos. Among those picture, one in particular captivated the global audience: a stunning aerial view snapshot of rooftops in downtown Seoul. Numerous comments were left on the actor’s post with remarks like, “Are those rooftop gardens? Cool!,” or “Do Koreans have tennis courts on their rooftops?”

So why are there many green rooftops in Korea?

| Polyurethane and Chromium oxide, what make Korean rooftops green

The secret behind the green rooftops that are easily found in Korea lies in the paint. According to temperature changes, building materials expand and contract, which can result in cracking, buckling, etc. Such types of deformation can lead to water leaks, corrosion, and mold, making waterproofing essential for rooftops exposed to such elements. The waterproof paint used for this purpose is made from polyurethane, a material derived from petroleum.

Polyurethane is a polymer that contains multiple urethane monomers. It is elastic like rubber, which helps prevent micro-cracks and blocks water leaks caused by rain. It plays a crucial role in preventing concrete corrosion due to its durability, water resistance, and chemical resistance. Additionally, waterproof paint made from urethane is lightweight and does not add significant load to the building. Whether applied to rooftops or exterior walls, urethane waterproof coating acts as a “protective layer,” helping to extend the building’s lifespan.

Chromium oxide (Source: Chemical Book)

Urethane waterproofing paint is made by blending various functional materials into a viscous urethane resin, much like making a smoothie. One key ingredient in this mixture is chromium oxide, which has a dark green color. As a result, green has become the basic color for urethane paint. Although it is possible to add pigments to produce various colors of urethane paint, the additional cost has led to the widespread use of the original green. This explains why green rooftops are so common in Korea. There are some other explanations about this trend. Some said because green is one of Koreans’ most favorite colors, some believed it was because this color reduces eye strain.

Meanwhile, there is a growing global trend of “cool roof technology,” which involves painting rooftops and roofs with light-colored pigments that reflect or block solar heat, thereby lowering the surface temperature of buildings. Similarly, in Korea, the use of white paint to enhance the energy efficiency and cooling of buildings is on the rise. Cities like Seoul and Busan undertaking projects to convert green rooftops to white during the summer to reflect heat and conserve energy.

How about your place? What is the dominant color of the rooftops there?