A rough opening is the framed hole in a wall that a door or window unit sits inside. Getting the size right is critical. Too small and the unit won't fit; too large and you lose the structural support the framing is meant to provide. The dimensions depend on the unit size, the wall framing style, and whether the wall is load-bearing.
For most pre-hung door and window units, the rough opening is the unit's nominal size plus a small amount of clearance for shimming and leveling. The standard rule is to add 2 inches to the width and 2 to 2.5 inches to the height of the actual door or window unit size. For a 32-inch pre-hung door, the rough opening width is 34 inches (32 + 2). For a 6-foot-8-inch door, the rough opening height is 82.5 to 83 inches (80 + 2 to 3 for the threshold, shimming, and top clearance). Always confirm with the unit's installation instructions. Manufacturers specify their required rough openings, and these instructions override the general rule.
A rough opening in a wood-framed wall consists of several parts working together.
King studs are full-height studs that run from the bottom plate to the top plate on each side of the opening. They provide the primary structural support. Trimmer studs (also called jack studs) are shorter studs that run from the bottom plate up to the underside of the header. They carry the header load down to the floor. Cripple studs are short studs above the header (filling the space between the header and the top plate) and below the sill plate in window openings. The rough opening width is measured between the inner faces of the two trimmer studs.
A header is a horizontal beam spanning the top of the opening that carries the load from above down to the trimmer studs. Header sizing depends on the span width and whether the wall is load-bearing. For non-load-bearing walls, a single 2x4 laid flat (or even no header in some framing styles) may be acceptable. For load-bearing walls, a properly sized built-up header (two 2x10s or 2x12s sandwiching a layer of 1/2-inch plywood, for example) is required. Consult your local building code or a structural engineer for load-bearing header sizing on openings wider than 4 feet.
Opening a load-bearing wall is a more involved project than cutting into a partition wall, but the framing principles are the same. You just can't skip the temporary support step. Before cutting into any wall, determine whether it's load-bearing. Generally, walls running perpendicular to floor and ceiling joists carry load; walls running parallel to joists often don't. Walls on an exterior perimeter are almost always load-bearing. If you're unsure, check the framing direction in your attic or basement. When in doubt, treat the wall as load-bearing. Install temporary support walls on both sides of the opening before removing any studs, keep them in place until the header is fully installed, then remove them only after the new framing is complete and properly nailed.
If you're replacing a unit in an existing opening, measure the rough opening, not the old unit. Pull back any trim or casing and measure the clear opening width between the trimmer studs and the clear height from the subfloor to the underside of the header. Measure in at least three places (both sides and the middle for width; both corners and the center for height) to check for square and level. A diagonal measurement from corner to corner tests for square: if both diagonals are equal, the opening is square. If they differ by more than 1/4 inch, the opening is racked and will require extra shimming or squaring before the new unit goes in.
The most frequent error is cutting the rough opening to the finished unit size rather than the required rough opening size, leaving no room to shim and level the unit. The second most common mistake is undersizing the header on a load-bearing wall. This leads to sagging, sticking doors and windows, and eventually cracked drywall above the opening. Always verify the header size with your local code or a span table before framing. A third error is forgetting to account for the floor covering thickness when setting door rough opening heights. If you're installing hardwood over subfloor after the door is in, the finished floor will raise the threshold height by 3/4 inch or more, which can interfere with door clearance.
