In framing a house, the top plate is a horizontal lumber that ties all the studs in a wall together. It's the second piece cut when creating a wall, after the sill plate, which sits on top of a foundation slab or crawl space. The top plate also supports joists for additional floors, along with rafters and trusses in the roof.
Depending on the style of frame, top plates can either be doubled up (as shown here) or single. For load-bearing walls, the code requires a double top plate. However, for interior non-load-bearing walls and for a limited number of exterior walls, a single plate can be used if the wall studs stack directly on the top of the plate.
A single top plate is often recommended as a cost-effective alternative to a double top plate in framing a home. However, a single plate is less stable than a double, and it doesn't provide the same strength as a double.
Another concern is that a single top plate wastes lumber, since it blocks cavity space that could be filled with insulation. It also increases thermal bridging. For this reason, the trend in framing is to use double top plates when possible, and to use single plates only when necessary for structural reasons, such as where plumbing or vents need to be run through the wall.
As framing methods have changed over the years, so too have code provisions regarding single top plates. The current code, IRC section R602.3.2, includes new provisions relating to single top plates that differ by seismic design category and spacing between braced wall lines.