Ref: Beijingbuzz,.....At the north end of the Temple of Heaven is the majestic Hall of Prayer for Good Harvests, an impressive three-tiered wooden structure that sits on a large, three-tiered marble plinth, 6 meters high.The hall itself is 38.2 meters high (32 meters from the plinth) and 24.2 meters in diameter. Remarkably, not a single nail was used in its construction - a feat that required a high level of craftmanship. Initially constructed in 1420 in a square design, it was reconstructed in 1545 during Emperor JiaJing's reign as a round building with a three-tier roof much as we know it today. At that time the roof was constructed with blue, yellow and green glazed tiles symbolising Heaven, Earth and the mortal world.
During Qing Emperor QianLong's reign it was reconstructed again in 1751. This time the roof was tiled only in azure colored tiles.