Ultraviolet-written Long Period Fibre Gratings are demonstrated to be affective tools when measuring thermal gradients of a thermally conductive material. More specifically, variations in the thermal system of a substrate are detected through transmission spectrum responses. While it is well known that uniform temperature leads to pure wavelength shifts of the transmission spectrum minimum, observed changes in transmission loss lead to predictions of thermal gradients. Since Fourier's law of heat conduction implies that heat flux measurements of known conductive materials can be predicted with knowledge of the temperature gradient, the present work demonstrates that optical fibres may be used to develop photonic heat flux sensors. Such an advancement would have a significant impact in industries, such as building materials, in which embedded sensors are used to predict surface thermal boundary layer conditions.