In the case of a monumental or historic building, old roof tiles, especially the original ones, should be preserved as part of the original structure. For this purpose, treatment methods for deteriorated old roof tiles to permit their reuse were investigated by the author. Satisfactory results were obtained for the treatment of the old roof tiles by impregnation with a solution of methyl tri-ethoxy silane in organic solvent (SS-101). One of the biggest advantages of this technique is that its procedure is very simple. The old roof tile is immersed in SS-101 for about two hours, and then dried in air for more than two weeks. By this treatment, the rate of water absorption in the old roof tile is extremely reduced, as for example, absorption was reduced from about 15% down to only 1% after 24 hours of immersion in water. The durability of the treated old roof tile was tested by artificially accelerated deterioration, mainly as to the rate of water absorption; these results were compared with results of tests on both untreated tiles and new ones. This test proves that the old roof tiles treated by SS-101 had a very good durability.