How to Make a Large Doorway Arch

Cut away the drywall using a utility knife until the framing is exposed on both sides and the top. If there are corner beads in the doorway, cut them away using a hacksaw.

Measure the distance of the doorway between the two side frames.

Tie a piece of string to a pin and push the pin through the center of the cardboard. Tie a pencil to the other end of the string. Pull the pencil away from the center of the cardboard until the string is fully stretched, then start drawing a circle.

Cut out the circle and divide it into two equal parts to form cardboard arches.

Tape one of the cardboard arches to the bottom of the top doorway frame to ensure there is enough headroom.

Trace the cardboard arch on two pieces of 1/2-inch plywood to get two plywood arches of the same shape and size.

Position the cardboard pieces on the top framing, one on either side, and nail them into the frame using 2-inch nails. Because the doorway is large, you may need assistance lifting the arches into place.

Cut two pieces of drywall that roughly resemble the size of the plywood arches. Place them over the arches and nail them into the arches using 2-inch nails, covering the sides of the arches completely. Cut away any excess drywall using a utility knife.

Trim the wooden blocks so that they can fit in between the plywood pieces and shape them so they run along the curve of the arch smoothly.

Screw the wooden blocks in between the plywood pieces along the bottom of the arch using drywall screws.

Bend the hardboard into the shape of the archway, position it directly beneath the arch and nail it into the wooden blocks using 2-inch nails.

Cut a strip of drywall measuring the thickness of the archway and soak it in water until it becomes flexible enough to bend to the shape of the arch.

Nail the drywall strip into the hardboard using 2-inch nails, spaced every six inches, bending it along the curve of the arch as you go along.

Apply flexible corner beads into the edges of the arch. Make sure the corner of these beads are flush with the edges of the arch and that they are pushed against the archway tightly.

Mix joint compound and apply two layers to the corner beads, covering them.

Sand the archway with sandpaper and paint it when dry.