ledge

In shipbuilding and heavy timber framing, a ledge is a wooden member laid transversely (athwartship) between main structural beams to provide intermediate support and prevent deflection of the deck or platform above. It carries distributed loads and ties the beams together laterally, much as a joist does in building construction, but in a marine or large-span context where the primary beams are heavier and spaced further apart.

Ledges are typically squared or rectangular in cross-section and are either bolted, fastened, or mortised into the main beam structure. Their spacing depends on the load, span of the primary beams, and the thickness of the decking or flooring they support. In wooden ships, ledges might be spaced 2 to 3 meters apart and sized proportionally to the overall hull dimensions. They run the full width of the deck area, creating a grid when viewed from above alongside the main longitudinal members.

Variants and application

In industrial floors and platforms, the equivalent member might be called a cross-beam or secondary beam. In shipbuilding specifically, ledges differ from beams in that beams are the primary load-bearing members running longitudinally along the vessel, while ledges are secondary and transverse. Some designs use partial ledges or half-ledges that span only part of the deck width. The term is less common in modern steel shipbuilding, where similar stiffening is achieved through welded frames and intercostal stiffeners.

The name comes from its function: a ledge is a projection or shelf that allows something to rest upon it. In medieval and early modern timber work, ledges were the simplest way to extend the span of heavy decking without intermediate support columns. The word has survived in maritime vocabulary even as construction methods have evolved.

Ledges are prone to rot and insect damage in wooden vessels and require regular inspection at deck edges and where they meet the frames, as water can pool in these joints. Improper fastening or movement in old timber can lead to creaking and, over time, to sagging of the deck if the ledges lose their cross-section through deterioration.