About 60% of the 4000 km engineered sheet pile walls in the Netherlands are made of timber. The species most applied for this application are azobé (Lophira Alata) and treated pine (Pinus sylvestris). In order to optimise material use as much as possible the fifth percentile strength value of the system of coupled boards has been determined using a combination of experimental tests and numerical Monte Carlo simulations. The experiments consist of four point bending tests on single boards as well as on parallel boards coupled by tongue and groove. The actual fifth percentile strength value of the individual boards has been determined. The increase in strength as a result of the system effect is written as a load sharing factor on the fifth percentile strength value of the single boards. Load-sharing factors have been derived for the two  species and were found to be 1.17 for azobé and 1.33 for treated pine.