Ashlars of the Parliament building and Citadella fortress made of three porous Miocene limestones, a fine-grained limestone, a medium-grained oolitic limestone and a coarse-grained bioclastic limestone, were studied and compared with quarry blocks of the same lithologies. The commonest weathering forms are white (thin and thick) and black (laminar and framboidal) crusts. To assess the processes of crust formation and detachment, descriptions of lithologies and associated weathering features were combined with micro-drilling, pore-size distribution and ultrasonic pulse velocity tests. Microbiological and textural analyses were also performed. The micro-drilling resistance measurements and ultrasonic pulse velocities clearly document the presence of crusts and the degradation of underlying fine- and medium-grained limestones. A textural change, with calcite recrystallization, is also marked by pore occlusion and reduction of microporosity in the crust zone. Crust detachment is initiated by the opening up of microfissures that develop below the cemented crust zones. Fine-grained limestone appears to be less durable than the coarse-grained variety and more prone to rapid crust formation and detachment. Ashlars from where the crusts were removed have lower micro-drilling resistance compared to quarry stones. Microbiological activity appears to play an insignificant role in crust formation and removal. Indeed, the combined effect of air pollution and related gypsum crystallization and more probably freeze-thaw weathering activity lead to crust detachment with rates strongly controlled by the texture and porosity of the limestone substrate.