The most unusual feature of Tibetan architecture is that many of the houses and monasteries are built on elevated, sunny sites facing the south, and are often made out a mixture of rocks, wood, cement and earth. Little fuel is available for heat or lighting, so flat roofs are built to conserve heat, and multiple windows are constructed to let in sunlight. Walls are usually sloped inwards at 10 degrees as a precaution against frequent earthquakes in the mountainous area.