Pre-hung doors solve a lot of problems, but they cost two to three times what a slab door plus a homemade jamb costs, and the included jamb is often made from finger-jointed pine that warps within a year. Learning how to make a door jamb from solid lumber pays off the first time you build one, both in money saved and in the finish quality you get from a custom-fit setup. I have built jambs for everything from 1820s farmhouses with wonky rough openings to new construction where the framer left me an actual square opening. The process is the same either way once you know the steps.
Materials and Tools You Need
A standard interior jamb is built from three pieces of 3/4-inch lumber: two side jambs and one head jamb. Wood choice matters more than people realize. Poplar is the go-to for paint-grade interior work because it machines cleanly and takes paint smoothly. Pick clear pine for stained jambs, or match species if the door is solid wood.
- 1×6 poplar or pine boards (long enough for the height plus 6 inches of waste)
- Door slab in your chosen size, typically 1-3/8 inches thick for interior
- 3-1/2 inch trim screws or 16-gauge finish nails
- 3 mortised hinges (3.5 inch is standard for interior)
- Mortise lockset or passage knob
- Shims (cedar or composite)
- 4-foot level, framing square, combination square
- Miter saw or sharp circular saw
- Router with a hinge mortising bit or sharp chisels
Measuring the Rough Opening
Measure the rough opening width at the top, middle, and bottom. Take the smallest measurement and use it as your reference. A typical 30-inch door needs a 32-inch rough opening. The jamb itself will be 30.5 inches wide on the inside (door width plus 1/2 inch for clearance), with each jamb leg adding 3/4 inch of thickness on each side.
Height matters too. For a standard 80-inch door, plan for an 82.5-inch jamb height. The extra accounts for clearance at the top, the threshold or floor finish, and a small reveal under the door. Confirm your floor finish height before cutting because installing over carpet versus hardwood changes the math.
Ripping the Jamb Stock to Width
Standard interior walls are 4-9/16 inches thick when framed with 2x4s and covered with 1/2-inch drywall on each side. Measure your actual wall depth because old plaster walls can be anywhere from 4-1/2 to 6 inches. Rip your 1×6 stock to match the wall thickness exactly. Too narrow leaves a gap that casing cannot hide, too wide pushes casing off the wall.
I use a table saw with a sharp 60-tooth blade for clean cuts. A track saw works just as well if you do not own a table saw. Whatever you use, set the fence carefully and verify the cut on a scrap piece before ripping all three jamb boards.
Cutting the Head Jamb Dado
The strongest jamb design uses a dado joint where the head jamb sits into a slot cut into each side jamb. This is stronger than a butt joint and aligns automatically.
- Mark the dado location 1 inch down from the top of each side jamb (this gives you the 1-inch overhang above the head).
- Measure the thickness of your head jamb stock, typically 3/4 inch.
- Use a router with a 3/4-inch straight bit and a straightedge to cut a 3/8-inch deep dado across the inside face of each side jamb.
- Test fit the head jamb. It should slide in snug but not require hammering.
If you do not have a router, a circular saw with multiple passes followed by chisel cleanup works fine. Take your time on the bottom of the dado so the head jamb sits flat.
Mortising the Hinges
Mortise the hinges before you assemble the jamb. It is far easier to work on a single side jamb laying flat on a workbench than reaching into an installed frame.
Standard interior hinge layout puts the top hinge 7 inches down from the top of the door, the bottom hinge 11 inches up from the bottom, and the middle hinge centered between them. Transfer these positions to the side jamb, accounting for a 1/16-inch reveal at the top of the door. Use a hinge template and a router with a 1/2-inch straight bit set to the hinge plate thickness, usually 1/8 inch. A chisel cleans up corners that the round bit cannot reach.
If you skip the template and freehand it, mark the mortise edges with a sharp utility knife first. The score line prevents tear-out on the cut edges.
Assembling the Jamb
Lay the two side jambs face-down on a flat surface. Slide the head jamb into the dadoes and check that the assembly is square using a framing square pressed into the inside corner. Drill two countersunk pilot holes through the top of each side jamb into the head jamb, then drive 2-1/2 inch trim screws.
Glue the dado joints with Titebond III for extra strength on doors that will see heavy use. For paint-grade work, glue is optional since the mechanical joint plus screws holds well.
Installing the Jamb in the Opening
Carry the assembled jamb into the rough opening as a single unit. Set it on a pair of 1/4-inch shims if you have flooring not yet installed, or directly on the finished floor if you do.
- Plumb the hinge side first with a 4-foot level. Shim between the jamb and the rough framing at top, middle, and bottom.
- Nail or screw through the shim points with 3-1/2 inch fasteners.
- Hang the door on the hinges temporarily to verify the reveal at the top.
- Adjust the latch-side jamb to maintain a consistent 1/8-inch reveal along the entire door edge.
- Shim and fasten the latch side once the reveal is uniform.
- Shim the head jamb if needed to prevent sagging, but do not over-tighten.
Final Steps and Finishing
Cut all shim ends flush with a sharp utility knife or a flush-cut saw. Install the strike plate on the latch side after marking the latch position with the door closed. Casing goes on last, hiding all the gap between the jamb and rough framing.
A well-built site-made jamb outlasts factory pre-hung units by decades and runs about $40 in lumber versus $120 for a comparable pre-hung interior door. The time investment is roughly 90 minutes once you have done it a few times, which makes it one of the better-paying skills any DIYer can pick up.
