The surface of the screed must be designed in such a way that the levelling or smoothing compound is securely bonded to the subfloor and can withstand the thrusting of the floorcovering when walked upon.

In practice, a chisel or steel nail are the only tools available to the contractor for testing the stability of the firmness of the subfloor's surface. A "grid tear" test has proven to be useful in such a case. This is carried out by running a steel nail horizontally and vertically over the screed surface. The distance of the edges should be as small as possible (see diagram). The appearance of the edges (for example, jagged or clean) provides a hint about the surface firmness of the screed. A "tear" device guarantees a constant pressure when scratching the screed.

Another method of checking the firmness of the subfloor is to determine the adhesive pull strength of the flooring according to DIN 1048. According to the latest information, the following standard values should be achieved for a medium adhesive pull strength:

Insufficient hardness: