The presence of cracks is a determining factor for the deterioration of reinforced concrete structures. Laser Spot Thermography (LST) is a fast and contactless technique for cracks identification. A computational procedure to characterize the crack depth by means of LST is presented here. We propose a heuristic form of the temperature at the concrete slab surface from which we derive a relation (called beta-tool) between the thermal gap across the crack and the unknown depth of the damage. In this way, by means of a fast survey, it is possible to distinguish between shallow and deep fractures (more than 15 mm deep), for which further specific analysis will be needed. (C) 2019 Elsevier Ltd. All rights reserved.