Build a soffit as an enclosure for utilities or a mount for cabinets.
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.
Snap a chalk line parallel to the wall onto the bottom of the joists. To run a soffit parallel to the joists, you'll probably have to add nailers between the joists. If you're framing an L- or U-shape soffit, install the first part parallel to the longest wall, then square the other soffit legs to it.
Tack-nail the runner assembly to only two joists. Check for level along the length of the lower runner. Add tapered shims, if necessary, between the upper runner and joist to level the assembly and to eliminate any gaps. If possible, drive your fastener through the shim to make sure the shim stays in place. Otherwise, smear a touch of woodworker's glue or construction adhesive on both sides of the shim near its tip to secure it to the framing. Let the glue dry before you trim the shim.
Screw 1x4 horizontal supports to the top edge of the wall runner and the lower runner. You can space these about 48 inches on center. Make sure the soffit stays square. Cover the face and bottom of the soffit with drywall. Install corner bead on outside corners and tape the inside corners.