We report on miniature sensors for structural health monitoring. We embed these sensors within building materials to detect the onset of microcracks in the elastic region of the deformation curve prior to the occurrence of plastic deformation. The sensors consist of thin composite lms of polydimethylsiloxane and graphite nanoparticles that conduct via tunnelling percolation. The high sensitivity of this transport regime to minute deformation produces large piezoresistance from which we infer the locally applied strain. The responses of sensors embedded in concrete, mortar and sand were measured under uniaxial strain and compared. This approach meets a need for in-situ monitoring of structural health with large scale arrays of minimally invasive sensors.