How to Frame a Door Under Basement Steps

Measure the height of the area where you’d like to install your door. To fit a standard-size door beneath a staircase, position it beneath the highest treads. If you’re building a closet under the stairs, you might be able to position the door parallel to the treads, beneath the topmost step.

Measure the lengths along the top and bottom of the wall or area you’re framing. Use a circular saw to cut the frame’s top and bottom plates to size from pressure-treated two-by-four stock. If your door will be set perpendicular to the stair treads alongside the case, the horizontal portion of the top plate will be shorter than the bottom plate, and you’ll also need to cut a section of top plate to angle along the staircase from the horizontal top plate down to the bottom plate.

Miter-cut each end of the angled section of top plate to create tight joints in the overall framework. The proper angle for each cut depends on the angle of your staircase and whether the angled top plate runs all the way to the bottom plate or ends at the shortest stud.

Place the top and bottom plates on the floor, length to length. If the plates are of equal size, the ends of both sides should be flush. If the top plate is shorter, the ends of only one side should align. If a portion of your top plate will be angled along the staircase, lay it out on the floor at the same angle to the bottom plate.

Measure the exact width of the prehung door unit, including its jamb casing. Use this measurement to mark the door opening layout on the bottom plate and the parallel portion of the top plate.

Mark out the door’s rough opening outside the layout markings. Depending on the door manufacturer’s instructions, the rough opening might be 1/2 inch or more beyond the width of the casing. This extra space allows you to shim the door into place and ensure it’s square in its jamb.

Mark each plate 2 inches beyond the rough opening markings. The space between these lines is where the jack studs will attach to the plates. Jack studs support the door’s jamb.

Mark out the king studs on each plate, 2 inches beyond the jack stud markings. King studs are positioned flush against jack studs and act to reinforce them.

Mark out the position of the remaining studs on each plate. Starting with the center point of the king stud, space the center point of each wall stud every 16 inches along the length of the wall. Lay out an angled top plate at the exact angle at which it will be installed to ensure accurate stud markings.

Cut the frame’s studs to size from standard two-by-four stock, using a circular saw. The studs will run from the bottom plate on the floor to the top plate attached to the staircase. Each stud installed from the bottom plate to an angled top plate will have a different length, and their top sides must be cut on an angle which matches that of the staircase.

Cut the king studs so that they’re slightly longer than the distance between the top and bottom plates.

Cut the jack studs to length. The jack studs should be equal to the height of the rough opening recommended by the door manufacturer, minus 1 1/2 inches for the bottom plate.

Nail the king studs into their designated place on each plate with 10d nails.

Attach the jack studs to their designated place on the bottom plate with 10d nails. Drive nails through the jack studs into the king studs to strengthen the doorframe.

Install the wall studs along the length of both plates. If a portion of your top plate is angled, secure the angled section to the horizontal section with 10d nails before installing the studs, then drive 10d nails through the top and bottom plates to secure the studs.

Measure and cut a rough opening header out of standard two-by-four stock. The header will fit flush between the king studs and lie flat on top of the jack studs. Position the header and nail it to the king studs with 10d nails.

Measure and cut a cripple stud out of standard two-by-four stock. The cripple stud transfers structural load to the header and is positioned between the top plate and the header, bisecting the length of the header. Position the cripple stud and nail it to the header and the top plate with 10d nails.

Set the framework into position, gently tapping it into place with a hammer. Check the frame with a 4-foot level to ensure it’s situated squarely.

Drive 16d nails through the top plate into the staircase. Use 2-inch concrete nails to secure the bottom plate to the floor. Don’t nail down the section of bottom plate within the door opening.

Cut away the bottom plate between the jack studs with a hand saw or a reciprocating saw.