The inelastic material behaviour of thermally toughened and heat strengthened Soda-Lime-Silica glass at room temperature was investigated. A four-point bending test configuration was used. The delayed elastic deformations were measured and reveal a significant time-dependent behaviour. The possible origin such as sub-critical crack growth, ion diffusion or absorption of water molecules is discussed briefly. The aim of the study is a better understanding of the nature of glass itself, in order to use this brittle material as a structural member in the field of civil engineering according to appropriate safety requirements.