It has nothing to do with painting over burn marks after the British set the house on fire in 1814, although you may have heard that story. The answer has to do with the walls themselves, which are made of sandstone. Sandstone is soft and porous and it is easy for water to seep into it. When this moisture then freezes and later thaws — over and over — the stone will eventually crack and crumble. To prevent the water from getting into the stone, the building was covered with a lime-based whitewash in 1798, two years before the first president moved in. Later, for reasons not known, workman replaced the thin whitewash with paint. It could be that the paint was thicker and did not wear away like the whitewash did. The house was given the nickname "The White House" as early as 1812. It was Theodore Roosevelt who made the name official, which he did in 1901.