Timber has been a popular building material for centuries and offers significant sustainable credentials, high mechanical and durability properties. Availability, ease to use, convenience and economy have made timber the most used construction material in history but, as it is a natural material, uncertainty in its mechanical characteristics is considerably higher than man-made structural materials. National codes and engineers usually employ high factor of safety to incorporate timber strength uncertainty in design of new structures and reinforcement of existing ones. This paper presents the results of 221 bending tests carried out on unreinforced and reinforced soft- and hardwood beams (fir and oakwood) and illustrates the reinforcement effect on timber capacity and strength uncertainty. Both firwood and oakwood beams have been tested in flexure before and after the application of a composite reinforcement made of FRP (Fiber Reinforced Polymer) unidirectional sheet. The uncertainty in the strength of reinforced timber is also quantified and modelled. Test results show that the FRP reinforcement is effective for both enhancing the beam load-carrying capacity and for reducing strength uncertainties.