The prediction of final deflection of timber beams at the end of service life is one of the most questionable issues in design, since creep develops due to very complicated physically mechanical time-dependent behaviour of wood influenced by a wide range of factors. The current study deals with the investigation of more significant relationships of creep in timber beams under natural environmental conditions and is aimed to define a appropriate mathematical model to describe the deformation in bending. There is a model presented in a new edition based on the known Burger body concept and has been numerically tested by estimates from experiments performed in longterm static loading of pine (Pinus Sylvestris) beams in a four point bending test. The loading regime has been stated at a level with service conditions for outdoor structures: full load for 70 days (winter season) and half load for period without snow (190 days). It has been proved that the proposed mathematical model works, and may be recognized as appropriate for bending strain prediction in timber beams with possible improvements in the future. It has been revealed from the tests that the changes of relative humidity and subsequential variations of the moisture content of the wood in longterm performance do not affect the rate of creep significantly.