A lean-to is a type of simple structure originally added to an existing building with the rafters "leaning" against another wall. Free-standing structures open on one or more sides (colloquially referred to as lean-tos in spite of being unattached to anything) are generally used as shelters.
A lean-to addition is an appendix to an existing structure constructed to fulfill a new need. Sometimes, it covers an external staircase, as in a 15th century addition against one of the walls of the large chapter room of the cathedral of Meaux. Other uses include protecting entrances, or establishing covered markets outside existing buildings.[1]
A lean-to is originally defined as a structure in which the rafters lean against another building or wall, also referred to in prior times as a penthouse.[2] These structures characteristically have shed roofs, also referred to as "skillions", or “outshots” and “catslides” when the shed’s roof is a direct extension of a larger structure’s.
A lean-to shelter is a simplified free-standing version of a wilderness hut with three solid walls and a single- or, in the case of an Adirondack lean-to, offset-pitched gable roof. The open side is commonly oriented away from the prevailing weather. Often it is made of rough logs or unfinished wood and used for camping.
This style of lean-to is popular in Finland and Scandinavia, and known as a Finnish: laavu in Finnish, Swedish: gapskjul or Swedish: slogbod in Swedish, and Norwegian: gapahuk in Norwegian.