The problem of evaluating the effects of an explosion inside a monumental building is addressed; namely, we focus on the determination of the overpressure due to the shock wave. Three models are compared: a thermodynamic approach, based on the JWL model and two empirical approaches, based on the CONWEP method or the recommendations TM5-1300, both of them approaches proposed by USACE. In particular, a detailed procedure for the use of TM5-1300 is proposed, making use of new analytical expressions of the physical parameters of a hemispherical blast fitting the experimental data of Kingery and Bulmash. The novelty of these expressions is that they fit accurately the experimental data over the entire variation range of the scaled distance. In the paper, we show the existence of reflected waves focus, a phenomenon that alters sensibly the pressure field in case of non-typical geometries, like vaults or domes. We show that for this kind of structures the assessment of the blast effects is adequately estimated by the JWL method, while in the case of more typical, planar geometries, namely of flat walls, the assessment of the overpressure is sufficiently estimated also by TM5-1300.