Aerodynamic efficiency was a major design priority and it was therefore decided to use a thin wing of RAF34 section. This was not thick enough to contain spars of sufficient depth to carry the flight loads and so the wing skin would have to carry most of the loads in a "stressed-skin" construction. However, the complex curves required for aerodynamic efficiency could not be manufactured using plywood. Hagg, who also had experience as a naval architect, adapted a construction technique previously used for building lifeboats. The majority of the wing was covered using two layers of 2-inch (50 mm) wide spruce planking laid diagonally across the wing, with the outer layer laid crosswise over the inner. These strips were of variable thickness, according to the loads they carried, reducing over the span of the wing from 0.5 in (12.5 mm) at the root to 0.14 in (3.5 mm) towards the tips. It was built as a single assembly around three box-spars located at 21, 40 and 65 percent chord: there was an intermediate spruce stringer between each pair of spars to prevent buckling. The ribs were made of birch ply and spruce. The outboard 6 ft (1.5 m) were skinned with various thicknesses of ply because of the difficulty of machining spruce planking to less than 0.07 in thickness. The leading edge, forward of the front spar, was also ply covered. The centre section was reinforced with two additional layers of 0.07 in spruce. This method of construction had been made possible only by the recent development of high-strength synthetic bonding resins and its success took many in the industry by surprise.