How to Build a Wall Under a Soffit

Measure the wall height to the soffit base and measure the width needed to reach side walls. Construct the new partition wall of 2-by-4-inch boards formed by a top plate, sole plate and studs on 16-inch centers. Mark all surfaces along the top and bottom plate where studs will fit with a pencil mark. Cut the wall framing pieces with a circular saw and assemble framework on the floor.

Cut a door opening in the framework to fit the door you will install. Cut a header board and nail it across the top of the door opening. Don't cut the threshold opening out of the sole plate yet.

Tilt the partition into standing position under the soffit wall. Secure the wall side framing to studs or framing on adjoining side walls by using 4-inch wood screws. Ensure walls are plumb by using a 4-foot carpenter's level. Slide wood shims under the sole plate to level the bottom, if needed. Screw the partition framing to floor joists using 4-inch screws driven by a electric screwdriver.

Cut the door threshold between studs with a handsaw.Take care not to cut the subflooring. Toenail the ends of the plate you cut by inserting two 10dd nails into jack studs above the threshold, so the door will withstand slamming.

Cut 5/8-inch drywall to fit the wall on both sides. Tape seams with drywall tape, add joint compound and allow to dry. Sand the seams as smoothly as possible and repeat the joint compound process two more times. Roll flat or satin enamel paint on the wall.

Finish up wall details. Measure and cut door facing trim from 1-by-4-inch hardwood. Miter door facing corners. Cut door jamb materials and route space for door hardware to fit the sides of the opening. Cut threshold covering with a fine-toothed handsaw and screw it to floor joists with 4-inch screws.

Measure and cut molding to cover any seam between a soffit box and the separate free-standing wall. Nail the molding into the upper plate of the wall framing using paneling nails, but don't attach it to the soffit with nails or screws.

Hang the interior door and install door hardware. Nail weather-stripping around the door, if the partition wall is in an unheated building space.