Reservoir, or water-collecting roofs present greater thermal inertia than inverted flat roofs due to the mass of water they contain. This feature gives them better thermal performance and leads to greater stability in the indoor air temperature Ti and the wall surface temperatures. In the summer, they can dampen the effect of solar radiation and regulate external thermal loads thanks to their greater effusivity and thermal capacity. This research compares the thermal behavior of the roofs of two buildings located in Alicante on the Spanish Mediterranean coast: a loft flat in the city center and a water-covered roof in the Museum of the University of Alicante (MUA). Values for effusivity, diffusivity, thermal capacity, decrement factor, time lag and internal, as well as external thermal admittance were obtained. After monitoring both roofs during 2014, behavior simulations were performed in Design Builder using 6 different scenarios reflecting different combinations in both buildings of water-covered, inverted and conventional roofs and marble or terrazzo paving. The water-covered roof led to a higher decrement factor and time lag, as well as to a reduction of annual energy demands between 8.86% and 9.03%.