Abstract: A sheet metal structure having two panels of substantially the same cross section connected to one another by a lap joint at their outer ends. Each panel has at least one major U-shaped section which has a base wall and a pair of oppositely disposed side walls. The base wall is formed with a plurality of minor U-shaped longitudinally extending ribs which form a plurality of alternately disposed minor crests and velleys. The major U-shaped sections and the minor ribs of the panels nest with one another at the lap joint. A spacer panel is provided for spacing the nesting ends from one another to facilitate the rigid securing of the ends. The spacer panel has a body portion formed with a plurality of spacer lugs at one edge thereof which project outwardly from the body portion in a spaced relationship. The lugs have a thickness substantially equal to the thickness of the panels which are to be joined.