Intro
A soffit (also called a bulkhead) is an architectural element that's often utilized to fill the space between the top of wall-mounted cabinets and the ceiling. Although the space it encloses is mostly empty, it's a great place to run wiring, heat ducts, and other utilities. Over sinks, a soffit often houses a canister light.
Although it's possible to build a soffit with a depth that matches the cabinets, that design route multiplies the degree of difficulty for the cabinet installer. Making the soffit 1-1/2 inches deeper than the cabinets disguises minor cabinet alignment discrepancies and also permits the installation of a small molding strip.
When designing a soffit, allow for an overhang at the exposed ends of soffit runs. Also be sure to allow for the thickness of the drywall on the front and bottom. You don't need to overengineer the soffit because it's not a structural feature.
Take care to build the soffit level and square; it will help simplify installation of the cabinets.