Sandstone weathering rates are compared on two historic structures, one coastal and one inland. Weathering is accelerated at the coastal site by a factor of 1.59, although this value may underestimate the true difference. Intra-site coastal patterns show that aspect creates a threefold variation in weathering rate. Abundant micro-organism growth protects rock, whereas patchy colonisation leads to increased weathering. Weathering processes and rates cannot properly be interpreted without considering the effects of biological factors and agents, even in environments dominated by marine salts. Conservation of the built heritage requires consideration of aspect and the full range of weathering mechanisms.