This paper presents the design and performance of a novel optical fibre relative humidity (RH) sensor device, based on fibre Bragg gratings (FBGs), but specifically developed for monitoring building stone deterioration. The performance of the sensor thus created was first assessed under experimental conditions of controlled wetting and drying cycles of limestone blocks, simulating conditions 'in-the-field' with actual building stone. Positive results have been obtained and these are discussed and further verified through subsequent a series of extensive field tests, carried out on a test wall in Oxford. This was built specifically using similar methods to those used in construction in past centuries, to allow an accurate simulation of processes occurring with wetting and drying in the historic walls in the city and University of Oxford. ©2010 IEEE.
