The thermoremanent magnetization (TRM) acquired by the building materials during their baking can be disturbed by any later accidental or functional heating. The total magnetization being the vectorial summing of several partial ones, we show the possibility of finding the thermal history of the material by a zero field gradual demagnetizing. We support the theoretical point of view with several studies of tiles and bricks which were burnt by fires during their utilization as building materials. This thermal disturbance appears, whether these materials were in a spatially fixed position as with paving tiles in a floor or in a variable one as for a gradually collapsing roof. -- AATA