How to Install a Cased Opening

Doors & Openings Skill Level: Beginner (Level 1) · Estimated Time: 30-60 Minutes

What This Guide Covers #

A cased opening is an open passage with no door, hinges or door hardware. A wood jamb lines the inside, and casing trims both faces of the wall to dress up the opening. This guide covers that wood-jamb method in new construction. A finished drywall wrap is a different finish choice; check the plans and builder's opening notes before starting.

Build a jamb that is level, plumb, straight and square, with clean joints. Casing comes afterward. The basic mitered-casing method is summarized here and taught in full in How to Case One Side of a 6'8" Door. Decorative head trim is a separate style.

Before You Begin #

• Confirm the framing and drywall are ready, the specified opening size and finish are clear, and the jamb stock and casing profile match the house.
• Flooring can be installed, or you must know its finished elevation. The jamb legs need to reach below the future finished floor surface. Establish that floor line before cutting the legs.
• Check nearby door jamb heads and the builder's layout notes. Establish the intended jamb-head elevation rather than assuming the rough-opening header is the finished height.

Tools and Materials #

• Miter saw; table saw when ripping custom jamb stock; tape measure; pencil; utility knife; small adjustable combination square; and a 6-foot level.
• Jamb installation: 15-gauge angled finish nailer and 2-3/4-inch nails.
• Jamb assembly: 1/4-inch crown stapler and 1-1/4-inch-long staples.
• Casing stage: 18-gauge brad nailer, 1-3/4-inch brads, wood glue, wood putty, sanding supplies and a nail set for proud fasteners.
• Compressor and hose for pneumatic tools. Battery tools do not need an air supply.
• Shims; actual 3/4-inch-thick jamb stock such as pine or poplar, or purpose-made jamb stock; and the approved casing profile for both faces.

Cased Opening Installation Process #

Step 1. Measure and Prepare the Jamb #

Measure the rough-opening width at the top, middle and bottom, its height at both sides, and the wall thickness around the opening. Keep the rough opening, jamb outside dimensions and finished clear opening separate. The rough opening is commonly about 2 inches wider and 1-2 inches taller than the finished opening; these are approximate planning checks for jamb stock and shim space, not framing tolerances. Resolve a clearance or layout problem with the lead before proceeding.

Use one common jamb depth for all three pieces, slightly wider than the widest wall measurement. Standard stock is commonly 4-9/16 inches deep for a 2x4 wall with 1/2-inch drywall on both faces, or 6-9/16 inches for the corresponding 2x6 wall. Check the actual wall. Custom-ripped stock gets rounded edges; match the approved jamb profile.

Step 2. Assemble the Jamb on the Floor #

Cut the two legs and the head to the required assembly size and head elevation. With purpose-made jambs, the head seats in the top rabbets of the side legs. Lay out all three pieces on the floor, fit the joints cleanly and square the assembly. Staple the assembly together with the 1/4-inch crown stapler and 1-1/4-inch-long staples before setting it in the opening.

Step 3. Set, Shim and Fasten the Assembly #

Set the assembled jamb into the opening at the established elevation. Align its head with nearby door jamb heads where the layout calls for it. Check the head for level, both legs for plumb and straightness, and the opening for square. Maintain the common jamb depth across the wall.

Use five shim points on each side, ten total. At each point, adjust the shims, check the jamb with the 6-foot level, then nail through the jamb and shims into the framing with the 15-gauge angled finish nailer and 2-3/4-inch nails. Shimming and nailing are one operation at each point; do not shim everything and hope it stays put while you nail later. Recheck as you work so fastening does not bow the jamb.

The header may need a shim; it is not an automatic extra shim point. Dave normally butterfly-nails the header. If you have not been shown that fastening method, have the lead demonstrate it before fastening; this guide does not define its nail angles or placement.

Step 4. Case Both Faces #

Use the basic mitered-casing method on both faces: a consistent 3/16-inch reveal, tight glued miters and casing seated cleanly against the jamb. Use 18-gauge 1-3/4-inch brads. Putty and sand the wood joints as part of our carpentry work. Follow How to Case One Side of a 6'8" Door for the full procedure, then repeat it on the other face.

For paint-grade work, a slightly proud jamb lets the casing angle back toward thinner drywall areas; the painter handles the outside caulk line. Do not make sanding or planing every jamb edge flush with uneven drywall the normal installation method. Stain-grade fitting needs its own job-specific detail.

Cased Opening Quality Check #

The jamb is level, plumb, straight and square, its head aligns with the intended layout, and its joints are clean. The legs reach the correct floor elevation, the jamb is supported and fastened, and neither face is recessed behind the drywall. Both faces have an even 3/16-inch casing reveal and tight, glued, puttied and sanded miters. Set proud fasteners, clean off glue, collect cutoffs and sweep. Nail-hole filling, outside caulking, painting and staining belong to the painter.

Troubleshooting #

The jamb bows when it is nailed #

Recheck the affected point with the level. Adjust its shim support and fastening so the jamb stays straight. A fastener pulling an unsupported jamb toward the framing can distort the opening.

The wall thickness varies #

Check the widest wall measurement against the common jamb depth. A slightly proud jamb is intentional for paint-grade work. A recessed jamb needs its sizing or fit corrected before casing; do not force the casing to hide it.

The head or floor alignment is wrong #

Recheck the intended jamb-head elevation and the finished floor line before cutting or fastening further. Do not use the rough-opening header as the finished clear-height reference.

The casing reveal or miters look wrong #

Check the casing layout and fit, then the jamb itself. An uneven reveal is not proof that the whole jamb must be removed. Correct the cause and follow the linked casing procedure for joint fitting and finishing.

Related Guides #

• How to Case One Side of a 6'8" Door
• 6'8" cased-opening layout notes
• Which Material and Profile Goes Where

About the reference photos: These show related openings, not a complete basic cased-opening installation. Focus only on the jamb or central opening described in each caption. Barn-door and bypass-door hardware, transoms and decorative heads are outside this lesson.

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Related Opening Photos #

These related openings illustrate shared jamb work. Read each caption for the part relevant to this lesson.

Showing 4 of 4 photos

Barn-door opening: wood jamb preparation is shared; the barn-door hardware is outside this lesson.

Bypass-door opening: the wood jamb is a shared prerequisite; door installation is outside this lesson.

Focus on the central opening only; the transoms and decorative head are outside this basic lesson.

Focus on the central opening; surrounding transoms and built-ins are outside this basic lesson.

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