In this study the mechanical properties of dry set plasters are of interest. Shore C hardness of different plasters is given as a function of porosity for porosities ranging from 41 to 65 vol%. The data, and data collected from literature, show that Young’s modulus follows an empirical power law for porosities ranging from 26 to 70 vol%. Flexion strengths were measured on samples of different sizes and porosities (41.4-65 vol%). As they are size dependent, strength variation cannot be correlated to the sole porosity. Finally, analysing the results with Weibull’s theory led to the proposition of a brittleness scale for plasters. Brittleness increases with decreasing porosity.