Thermal cracking of rocks is an intensively studied topic in different research areas and for various engineering problems. In this context, we present a modeling approach which describes thermal spalling of rocks with a focus on thermal spallation drilling. This drilling technology uses high thermal loads to locally destruct the surface of the rock formation. With the presented model, the operating conditions which are required to initiate spalling of rocks can be estimated. Additionally, a Spallability number is introduced allowing a categorization of rocks according to their ability to spall. The presented model is based on linear fracture mechanics and the stress intensity concept. It evaluates if a crack with a certain geometry embedded in a rock with specific properties propagates during exposure to different heat loads and external pressures. Thereby, rapid heat transfer processes are coupled with induced thermal stresses and fracture mechanics in rocks.