A numerical study was conducted with the objective of developing an alternative to the existing reduced properties method for calculating the resistance in bending or axial loading of fire exposed solid timber members in wall and floor assemblies. The alternative method, called reduced cross-section method, uses zero-strength layers instead of modification factors for fire to take into account the reduction of strength and stiffness properties of the residual cross-section. Simple expressions were derived for the calculation of zero-strength layers by fitting to the existing method.