Riesel ist ein bayerischer Ausdruck für Kies, der zur Herstellung von Beton oder als Schüttgut benötigt wird.
Riedel is the Bavarian expression for Kies. It is added to produce concrete.

http://www.broetzner.de/RUNDKORN_-_SAND_RIESEL_KIES.html

https://www.inn-kies.de/produkte/kies

Post-grading, but for the record.

Ballast is the name given to the gravel placed on flat roofs to protect the underlying waterproofing membrane from the sun, to guard against wind uplift, to protect against damage by flying objects (including birds which might peck at it), and to provide a degree of thermal insulation.

There may be weight/cost considerations that result in the depth of ballast being reduced. If it is too thin, walking on it could be potentially damaging to the membrane, so concrete slabs can be placed on top of the ballast to create walkways. They spread the load of a person over a sufficiently large area to prevent puncturing of the membrane.

"Ballasted roofs are inverted or warm flat roof systems where the waterproofing membrane is secured against wind uplift and protected from other exposure and mechanical damage by a layer of gravel or paving - known as ballast."