The load-bearing substrate must be sufficiently clean (e.g. free of grease, mortar residues, fine particles, post-treatment agents) and level (flatness tolerances in accordance with DIN 18202, except when applying a cement composite screed to the still fresh concrete). In principle, other screeds are suitable as substrates for composite structures, but also concrete, wood or steel. The suitability or unsuitability or the cases in which the subsoil requires special measures can be found in Table 2 of DIN 18560-3, in summary the following applies:
- calcium sulphate screed and magnesia screed are unsuitable as substrates for a mastic asphalt composite screed.
- magnesia screed is unsuitable as a substrate for a cement composite screed.
- In the case of load-bearing substrates made of wood or steel, this must have sufficient bending stiffness.
- In the case of a magnesia screed on a reinforced concrete ceiling, a barrier layer is required.
- In the case of larger unevenness or cables/pipelines laid on the raw ceiling, a levelling screed is first required to cover the installations to such an extent that it can serve as a load-bearing substrate for the actual composite screed.
Source: bauwion
