The prehistoric temples found on the Maltese islands, dated from 3600-2500 BC, are unique examples of truly megalithic complexes. Although the temples can still be viewed today, they are unroofed. One of the major questions that still has to be answered is: Were the temples roofed, and if so with what? The key evidence for the presence of roofs is the hypogeum temple at Hal Saflieni, found in 1902, which appears to be an imitation of the above ground temples and the discovery, at Mgarr, of a contemporary miniature model in limestone with a roof. Since then, Ceschi in 1939 and more recently Piovanelli in 1988 have proposed that the temples were roofed with Globerigerina limestone slabs. Although convincingly illustrated, neither of these "reconstructions" has been tested for real stability and strength. In this paper we describe a detailed investigation of the reconstructions of Ceschi and Piovanelli. We use computer graphics and structural engineering techniques, based on the actual measured strength of Globerigerina limestone, to show whether in fact these reconstructions are indeed valid.