The word ondol means "warm stone." An historic ondol floor is usually made of granite blocks, about 5 cm thick, supported by columns of brick or stone. The space between the supports provides the path for the hot flue gases. On top of the granite blocks is a clay or concrete surface for smoothness. Originally, this would have been topped with a covering made of oiled paper.

A typical Korean house in the Choson Dynasty consisted of two rooms plus a kitchen. The flue from the kitchen stove was routed under the floors of the other rooms. Often the house had one or more additional fireplaces located outdoors, at or below ground level, with flues also passing under the floor. These were just for heating (not cooking). The chimneys were located on the side of the house, opposite the fireplaces.

The photo above shows a structure at Ojukon, the Kangnung home of Choson Dynasty statesman Yi Yul Guk. The dark rectangular hole at ground level, just to the left of lower center in the photo, is the outdoor fireplace. Note the soot streak above it. To the right of the photo, the freestanding chimney (surprisingly, it is lower than the roofline) is visible.

Below is a kitchen at Ojukon, showing two ondol cooking fireplaces, and a closeup of an ondol fireplace at Songyojong (The House of 100 Rooms). This fireplace is located below ground level, literally under the floor.

After the Korean war, the South Korean government undertook an extensive reforestation program. Private cutting of wood for heating is seldom allowed. Modern Korean homes are built with heating pipes embedded in the floors, which are usually concrete covered with vinyl (carpet is seldom to be seen in Korean homes). These pipes circulate water warmed by a conventional oil or gas boiler. Today, you're not likely to see a genuine wood-fired ondol in use, except in historic settings (such as Hahoe Village, below).