What type of roof system involves a two-way joist layout for concrete?

A two-way joist layout for concrete is highly characteristic of waffle slab forming. This type of system uses a grid of ribs or joists in both directions, resulting in a flat, lightweight, and strong structural element. The specific design involves a series of columns supporting the waffle slab, which can span larger distances than traditional flat slabs or beams. This allows for efficient load distribution and can be an effective method in areas requiring significant structural capacity while also reducing concrete usage.

The other options describe different roof types that do not employ this two-way joist method. Flat roofs typically use a single slab and do not have the recessed structure of a waffle slab. Gabled roofs and shed roofs refer to specific architectural shapes and slopes, focusing on the configuration and design of the roof rather than the underlying structural support method.