Building materials differ in their aptitude to resist bad weather and pollution. The interpretation of conventional laboratory experiments does not make it possible to choose precisely where in a building each type of material is best fit for use. A life-size experiment conducted at the C.E.B.T.P.(Centre Experimental du Batiment) affords a new basis for the appraisal of the durablilty of soft stones according to their function. The physical characteristics linked to their porous structure specify the necessary qualities in the materials to select.