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How to build a drywall arch
Converting a plan doorway into a curved arch is a relatively straightforward project that requires some carpentry and drywall taping skills. You’ll need to frame a curved arch, bend and fasten a strip of drywall to the curved framing, and then install a flexible corner bead. None of these steps is difficult, but it’s fussy work.
First cut away the drywall inside the opening to expose the framing. Don’t worry if you break it back a few inches on the walls. Next determine the style and size of the arch (half circle, partial circle or ellipse) and make a pattern out of cardboard. Tape this up in the opening to make sure it looks OK and leaves enough headroom.
Use this template to mark out and cut two 1/2-in.-thick plywood arches. Next cut a 6-ft. 2x4 down to 2-1/2 in. wide (or 2 in. narrower than the total wall thickness).Cut one top block and two side blocks and nail them to the door framing. Center them so that the 1/2-in. plywood arches will sit flush with the existing framing. Now nail the arched plywood into place on both sides.
Cut short arch blocks and screw them between the plywood arches about every 6 in. Cut strips of 1/8-in. hardboard (such as Masonite) and nail them to the arch blocks following the curve of the plywood. Use short drywall or underlayment nails. The hardboard provides a smooth, solid backer for the drywall and eliminates creases. Run it all the way down the sides to the floor.
Next fasten the drywall over the face of the arch. Let it overhang into the archway and then cut out the curve of the arch with a drywall or keyhole saw.
Then cut a strip of drywall the same width as the total thickness of the wall. If you’re using regular 1/2-in. drywall, wet the backside of the strip to help it bend. Lay it between sawhorses so it can sag as the water soaks in. You may need to wet it several times and let it slowly bend for an hour or two. Another option is to buy 1/4-in. drywall that’s designed to bend. Look for it at drywall suppliers (search online or see “Drywall Supplies” in your yellow pages).
Start 6 in. below the curve of the arch, pushing the drywall firmly against the hardboard. Drive a pair of nails every 6 in. into your blocking around the curve. Then use straight strips to finish the sides. Apply flexible plastic corner bead (also available at drywall suppliers) to both edges. Run it all the way to the floor. Start at one end and fasten the bead with a staple gun, driving 9/16-in.-long staples every 3 inches. Keep the bead centered on the corner and tight to the drywall. This step gives the arch its final shape, so take your time.
Finally, mix up some setting-type joint compound and cover the corner bead. Apply second and third coats of joint compound, letting it dry between coats. Sand, paint and then go relax. You’ve earned it!