This study describes monument-mapping method for detected damages, and verify in qualitative and quantitative way the decay of stone on historical building. Proposes a methodology that would allow quick and easy way to identify during the processing of digital images five weathering forms; fractures, salts, oxides, scaling, and lichen. The aim of the present work were (i) to identify the various stones used in decorating the music school of the University of Guanajuato; (ii) to undertake a basic mineralogical and petrophysic characterizations of the less known stones; (iii) quantify the extension of the weathering and the damage degree of the music school using the monument mapping method. The monument mapping method was used to identify damages over green sandstone used in the construction and restoration of buildings of the eighteenth and nineteenth centuries in Guanajuato city. The output data were classified images showing the different kind of damages that affect the green sandstone. The analyses of these data allow obtaining maps with the size and position of damages.