index to assess the solar performance of buildings in hot climates. in order to assess the solar performance of a building an index is developed by comparing the mean solar energy over the envelope during spring and summer with the corresponding energy of a dwelling reference. the mean daily solar energy during those two seasons was calculated averaging the solar energy corresponding to the summer solstice and the equinox, due to symmetry of the solar energy between equinoxes around the summer solstice. during that period occurs the maximum heating, mainly in places with hot climate. the index takes values between zero and one when the assessed building has a better performance than the reference; the index is negative when its solar performance retrogresses. the performances of several kinds of buildings were evaluated by means of the index: long buildings, like those used for primary schools and farms in mexico, towers and big stores. for each case several kind of roofs were tested: flat, tilted, gabbled and barrel vault. the azimuth of long building is the main factor affecting its solar performance, which is significantly improved by using a barrel roof aligned east to west. the buildings with huge roofs like those found in big stores get a light improvement when they are properly oriented and markedly when the flat roof is changed by the barrel roof. the solar performance of towers is almost invariant when the roofs were changed. the better azimuth for buildings with square plant is toward the north for any kind of roof used.