Charring of timber structural elements in fire is one of the most fundamental phenomena that affect the fire resistance of these elements. For an accurate and safe design of structural fire resistance, it is important to consider charring of timber in natural fire exposures, since determining charring for standard fire exposure, which is a common practice, is outdated and in some cases unsafe, due to the fact that some natural fires can be much more severe. Currently, the prescriptive approach and simplified design methods fail to give information about charring of timber elements exposed to natural fire and thus, a performance-based design is needed. Therefore, this paper presents an upgrade and extension of a recently developed heat-mass-pyrolysis model named PYCIF. Originally, PYCIF model was developed only for standard fire conditions. In the present paper, several studies and analyses are performed to extend model application to natural fire conditions. Firstly, the sensitivity study is performed, where the impact of model parameters on the charring development is investigated. It is discovered, that the kinetic parameters for the reaction rate of the active cellulose production, namely activation energy E1 and pre-exponential factor A1, are the most influential. In the next analyses the model calibration for small-scale cone calorimeter tests and large-scale natural fire tests of cross-laminated timber (CLT) floor system is performed. A robust nature of the model is identified since minor parameter calibration is required for an accurate prediction of the charring depth and temperatures in timber elements exposed to various fire conditions. Furthermore, a strong connection between the heating rate of fire and kinetic parameters is discovered. In cases of faster heating rate, the kinetic parameters govern slower reaction rate of active cellulose.