Sizing a Rough Opening for a Prehung Door

By Mark J. Donovan

When installing a prehung door into a wall it is important that the rough opening for the door be sized correctly. If the rough opening is too small the prehung door will not fit into it, and if it is too large there will be wide gaps that will make it difficult for the door to be hung properly.

Typically the rough opening for a prehung door should be sized 2 inches wider and 2 inches higher than the door itself. For example a two-foot, eight inch (32”) by six feet, eight inch high (80”) prehung door, would require a rough opening width of 34 inches and a height of 82 inches.

This sized rough opening will allow for the doorframe casing to be installed with about 1 inch of play on both sides, and 1 inch on both top and bottom. This spare space will allow the prehung doorframe to be leveled and plumbed so that the door swings freely.

Note that when installing the prehung doorframe shims and blocking should be used at both the hinge and door lock locations. The shims and blocking should be wedged in between the prehung doorframe and the wall studs that form the rough opening. When securing the prehung doorframe to the studs, nails should penetrate through the shims/blocking material. This will help to prevent flexing of the prehung doorframe when it is opened and closed.

Note that these guidelines are typical framing standards that are employed in the home construction business. You should always check with the manufacturer or product specifications to confirm the rough opening requirements for your particular prehung door.

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