Deck, in ship structure, is a horizontal platform extending from one side of a vessel to the other, thereby connecting them together, and forming in addition a flooring to the persons above it, and a covering to those below it. There are, as a rule, more than one deck in ships of medium size, and in steamers of the larger class—both war and mercantile vessels—there are usually three, four, and even five decks. Modern steamships of great length with three or four decks have, as a rule, two of the decks composed of iron or steel plating in addition to the usual cross deck-beams and longitudinal wood planking. All decks, especially the upper or 'weather' deck, have a slight 'camber' or 'round-down' towards the sides of the ship, in order to dislodge water and contribute to their lateral strength. The several tiers of decks are supported one upon the other by solid iron pillars, and by the ship's transverse bulkheads.
Vessels are classed for various purposes, according to the number of their decks, as 'single-decked,' 'two-decked,' 'three-decked;' and are otherwise distinguished by terms such as 'spar-decked,' 'flush-decked,' 'awning-decked,' according to the deck which is most typically characteristic. A spar deck is always the topmost or weather-deck, and stretches flush or level from stem to stern. Below it there is the main deck, on which the heavy weights (e.g. guns in war-ships) are carried. Next comes the lower deck, and below it again, in very large ships, is the orlop deck. The space between the several tiers of decks is conventionally spoken of as 'tween or between decks. Above the main deck of two-decked, or the upper of three-decked vessels, there are frequently partial decks erected; the forecastle or top-gallant forecastle deck being at the fore end, the bridge or hurricane deck about the mid-length, and the poop or quarter deck at the aft end. An awning or shelter deck is defined by Lloyd's Register Rules as 'a light superstructure, fore and aft, on the main or upper deck proper of the vessel, intended to shelter passengers or cattle, or for the conveyance of cargo either light in its nature or limited in quantity.' Other particulars about decks, their construction, &c., will be found in the articles SHIP-BUILDING, NAVY. For deck cargoes, see PLIMSOLL.