Deck ( ship ) A deck is a permanent covering over a compartment or a hull of a ship . On a boat or ship , the primary or upper deck is the horizontal structure which forms the ' roof ' for the hull , which both strengthens the hull and serves as the primary working surface . Vessels often have more than one level both within the hull and in the superstructure above the primary deck which are similar to the floors of a multi - storey building , and which are also referred to as decks , as are specific compartments and decks built over specific areas of the superstructure . ( Decks for some purposes have specific names ; see below . ) Structure . The purpose of the upper or primary deck is structural , and only secondarily to provide weather - tightness , and to support people and equipment . The deck serves as the lid to the complex box girder which is the hull . It resists tension , compression , and racking forces . The deck ' s scantling is usually the same as the topsides , or might be heavier if the deck is expected to carry heavier loads ( for example a container ship ) . The deck will be reinforced around deck fittings such as the capstan , cleat s , or bollard s . On ships with more than one level , deck refers to the level itself . The actual floor surface is called the sole , the term deck refers to a structural member tying the ships frames or ribs together over the keel . In modern ships , the interior decks are usually numbered from the primary deck , which is # 1 , downward and upward . So the first deck below the primary deck will be # 2 , and the first above the primary deck will be # A2 or # S2 ( for " Above " or " Superstructure " ) . However , ships may also call decks by common names , or ( especially on cruise ships ) may invent fanciful and romantic names for a specific deck or area of that specific ship , such as the " Lido deck " of the Princess Cruises ' " Love Boat " . Equipment mounted on deck , such as the ship ' s wheel , binnacle , fife rail s , and so forth , may be collectively referred to as deck furniture . Weather decks in western designs evolved from having structures fore and aft ( forward or front and aft or The Rear of the ship mostly clear , then in the 19th century pilothouses / wheelhouses and deckhouses began to appear , eventually developing into the superstructure of modern ships . Eastern designs developed earlier , with efficient middle decks and minimalist fore and aft cabin structures across a range of designs . Common names for decks . In vessels having more than one deck there are various naming conventions , numerically , alphabetically , etc . However , there are also various common historical names and types of decks : Construction . Methods in wood . A traditional wood deck would consist of planks laid fore and aft over beams and along carlins , the seams of which are caulked and paid with tar . A yacht or other fancy boat might then have the deck canvased , with the fabric laid down in a thick layer of paint or sealant , and additional coats painted over . The wash or apron boards form the joint between the deck planking and that of the topsides , and are caulked similarly . Modern " constructed decks " are used primarily on fiberglass , composite , and cold - molded hulls . The under structure of beams and carlins is the same as above . The decking itself is usually multiple layers of marine - grade plywood , covered over with layers of fibreglass in a plastic resin such as epoxy or polyester overlapped onto the topsides of the hull . Methods in metal . Generally speaking , the method outlined for " constructed decks " is most similar to metal decks . The deck plating is laid over metal beams and carlins and tacked temporarily in place . The difficulty in metal construction is avoiding distortion of the plate while welding due to the high heat involved in the process . Welds are usually double pass , meaning each seam is welded twice , a time consuming process which may take longer than building the wood deck . But welds result in a waterproof deck which is strong and easily repairable . The deck structure is welded to the hull , making it structurally a single unit . Because a metal deck , painted to reduce corrosion , can be quite slippery , pick up heat from the sun , and be quite loud to work on , a layer of wood decking or thick non - skid paint is often applied to its surface . Methods in fiberglass . The process for building a deck in fiberglass is the same as for building a hull : a female mould is built , a layer of gel coat is sprayed in , then layers of fiberglass in resin are built up to the required deck thickness ( if the deck has a core , the outer skin layers of fiberglass and resin are laid , then the core material , and finally the inner skin layers . ) The deck is removed from the mould and usually mechanically fastened to the hull . Fiberglass decks are quite slick with their mirror - smooth surfaces , so a non - skid texture is often moulded into their surface , or non - skid pads glued down in working areas . Rules of thumb to determine the deck scantlings . The thickness of the decking affects how strong the hull is , and is directly related to how thick the skin of the hull itself is , which is of course related to how large the vessel is , the kind of work it is expected to do , and the kind of weather it may reasonably be expected to endure . While a Naval Engineer or Architect may have precise methods of determining what the scantlings should be , traditional builders used previous experiences and simpler rules - of - thumb to determine how thick the deck should be built . The numbers derived by these formulae gives a rough number for determining the average thickness of materials based on some crude hull measurements . Below the waterline the thickness should be approximately 115 % of the result , while upper topsides and decks might be reduced to 85 % of the result .