Please ask Google with the words: Solarterrassendach Erneuerbare-Energien-Gesetz, then ignore all advertising and take the first result.
Alternatively:
§ 32 Abs. 1 Nr. 1 Erneuerbare-Energien-Gesetz 2012: "For electricity from installations for generating electricity from solar radiation energy, the remuneration amounts to [...], if the installation is attached to, on or at a building or another structural installation and the building or the other structural installation was primarily constructed for purposes other than the generation of electricity from solar radiation energy [...]."
§ 94 Baugesetzbuch: "(1) The essential parts of a property include the items permanently connected to the land, especially buildings, as well as the products of the property, as long as they are connected to the land. (2) The essential parts of a building include the items inserted for the construction of the building."
And the disadvantage is that there is no guaranteed 12 ct feed-in tariff. Instead, only what the market provides, which can be little to nothing or even negative.