A Landau-Ginsburg-Devonshire-type theory is used to describe the mechanical substrate effect on equilibrium states and phase transitions in SrTiO3 epitaxial thin films. The misfit strain-temperature phase diagram of SrTiO3 films is developed taking into account the existence of two coupled instabilities (antiferrodistortive and ferroelectric) in this crystal. It is shown that SrTiO3 films remain paraelectric down to 0 K only in a narrow range of small negative misfit strains between -2 x 10(-3) and -2.2 x 10(-4). Outside this "paraelectric gap," he 2D clamping and straining of the film by the substrate leads to the appearance of ferroelectricity in SrTiO3 films. The temperature of the ferroelectric transition increases rapidly outside the aforementioned misfit strain range.