By 1757 Roy was with his regiment in France for the Rochefort expedition and then in Germany for the Battle of Minden in 1759. His technical abilities and willingness to innovate brought him to the favourable attention of his commanders. Preparatory to the battle, the various military engineers made drawings of each step of the coming battle, with each step drawn on a different sheet of paper. The commander could then study the course of the battle before it occurred, going from one sheet to the next. Lieutenant Roy, however, made his drawings on a single sheet with coordinated and accurate overlays, so that the commander could more easily study the course of the battle by examining a single sheet of paper. The commander's comprehension was greatly facilitated, and Roy's methodology was soon adopted as an advancement in military science. Thereafter his promotion was rapid, and by the end of the war in 1763 Roy was a lieutenant colonel in the regiment and director of the engineers of the Board of Ordnance as well as being the deputy quartermaster general for Germany.