A recent increase in structural integrity requirements has led to a higher demand for accurate heat flux and temperature readings of building materiàl substrates. In this work, simultaneous two-parameter measurements of the average temperature and the temperature difference in concrete blocks are reported. To this end, long-period gratings (LPGs) were commissioned in a boron-doped photosensitive optical fiber using a pulsed CO₂ laser. The average temperature and the temperature differences were shown to be a linear combination of the spectral shifts of the center wavelength and the changes in the variations in transmitted power $I_{c}$ minima. The sensitivity and cross-sensitivity coefficients of the sensitivity matrix are also reported.