lean-to

n. 1. (Arch.) A shed or slight building placed against the wall of a larger structure and having a single-pitched roof; -- called also penthouse, and to-fall. [ 1913 Webster ]

The outer circuit was covered as a lean-to, all round this inner apartment. De Foe. [ 1913 Webster ]

2. A crude, usually temporary shelter comprising a lean-to roof braced against any convenient support, as a wall, a tree or a pole. The roof may extend all the way to the ground. [ PJC ]