Tree roots contain a high proportion of forest biomass, but methods to describe root biomass vary greatly due to the emphasis of different studies and different excavation methods used. In this paper, total root biomass and root distribution data of 27 trees from the northern Kazakh steppe were approximated by allometric functions depending on tree height, stump diameter, root diameter class, and optionally, depth in the ground. Our regressions offer good overall approximations to the data except for the uppermost horizon and the thickest roots (which include the stump). Logarithmic standard deviations range from 0.23 to 0.39 for total root biomass and from 0.74 to 1.09 for root distribution, and R² range from 0.96 to 0.98 and from 0.76 to 0.89, respectively. Although the original goal of constructing a cumulative function in both root diameter class and depth in the ground was not attained due to unfavourable statistical conditions, bivariate interpolation can be used with our functions to overcome the limitations of the set of class boundaries used. In addition, regressions were developed which eliminate the need to measure stump diameter and maximum depth of root penetration.