Adding a closet under a staircase Is a practical solution to your storage issues. The extra space hides your wrapping paper, winter clothing, cleaning tools or pantry. While most closet doors are between 18 and 30 inches wide, you may be forced to adjust the height of door to accommodate the angle of the stairs. Building a custom-height door using a readily available hollow-core door blank is the simplest solution to finishing your under-stair closet.
Measure the width and height on both sides of the door frame. Using a protractor, note the angle at the top of the door frame.
Subtract 5/16 inch from the door frame's height. This allows for a 1/16 inch space at the top and 1/4 inch clearance at the bottom.
Pencil the measurements onto the door blank. Check the angle of the door top with the protractor. Draw the angled cut line onto the door with a pencil and yardstick.
Cut a line along the pencil line with a utility knife. Turn the door blank over and apply painter's tape to the back side of the door, centered on the cut line. The tape will help prevent chipping and splintering along the edge. Turn the door blank back to the right side.
Cut the door on the cut line with a circular saw or a jigsaw. Clamp a straight edge, such as a piece of scrap 1-by-4 board, to the door to use as a guide for the saw. Remove the painter's tape after cutting the door.
Measure the width and length of the open space at the top of the door. Remove any foam filler inside the door to a depth of 2 inches.
Cut a piece of 1-by-2 board to fit into the hollow space. Apply wood glue and slide the board into place, aligning it with the top edge of the door. Clamp the top of the door with two C-clamps to secure the board in place until the glue dries.
Predrill three nail holes equidistant apart, through the back side of the door and into the board. Tap a 3/4 inch finishing nail into each hole, using a nail set to set the nail head just below the surface of the door.
Fill the nail holes with wood putty and allow them to dry. Sand the nail holes, top of the door and the cut edges lightly with 180-grit sandpaper until smooth. Wipe the door with a tack cloth to remove the wood dust.
Prime the door with a latex primer. Allow the primer to dry, then apply two coats of satin-finish latex paint according to the manufacturer's instructions.
Attach the hinges and doorknob according to the package directions. Hang the door in the doorframe.