A 127,000-square-foot door shop in Commerce City, Colorado, a Denver suburb, was scheduled for completion in the fourth quarter of 2023, with production expected to begin shortly after the new year. The facility reflects a broader shift in how doors reach the job site: millwork plants now assemble pre-hung units in a factory and ship them ready to install, moving labor out of the field and into a controlled production environment. The same logic governs window installation, where factory-built units meet site-prepared openings and the rough opening determines whether the finished product performs.
Pre-hung doors dominate new residential construction for a simple reason: the work that used to happen in a dusty corner of the framing site, cutting jambs, boring hinges, hanging slabs, now happens on production equipment with jigs and tolerances. The crew on site shims, levels, and fastens. Understanding how those units are made, and how the opening is supposed to be framed for them, is the difference between a door that swings forever and one that binds by the first season.
What a Millwork Facility Produces
A door shop takes raw components, slabs, jambs, hinges, locksets, and weatherstripping, and turns them into a pre-hung unit: a door mounted in a jamb with hardware installed, ready to be set into an opening. The Commerce City plant was sized at 127,000 square feet, part of a millwork network that pairs door and window plants with full-line distribution warehouses in the same region.
The Pre-Hung Assembly Line
Production follows a fixed sequence, and each station holds tolerances tighter than most field work can manage:
- Jamb stock is ripped and machined to width, then dadoed for hinges.
- Hinge and latch locations are bored with CNC equipment.
- The slab is fitted and hung, with reveals set on both edges.
- Weatherstripping and threshold are applied at the last station.
- The unit is wrapped, labeled with the opening size, and staged for delivery.
The factory also controls the interface with the site. Units ship with instructions that specify rough opening preparation, flashing, and sealing, because a pre-hung door performs only as well as the opening it sits in. A unit built square in the plant can still rack, bind, and leak if the framing it lands in is out of plumb.
Quality Checks Before the Truck Leaves
Finished units get a swing test, a reveal check, and a hardware function test before wrapping. Defects found on the line cost minutes; the same defects found on site cost a return trip, a reorder, and a schedule slip. That shift of quality control from the field to the factory is the economic argument for pre-hung production in the first place.
Sizing the Rough Opening
The rough opening is the framed hole the unit slides into, and it must be larger than the door itself. For a standard pre-hung door, builders frame the opening 2 inches wider and 2.5 inches taller than the door slab, which leaves room for the jamb, shims, and leveling. Get the numbers wrong and the crew either fights the unit into a hole that is too small or builds out a gap that no casing will cover.
Measuring a Rough Opening
Measure width at three heights and height at both sides, then use the smallest dimension to check the fit. An out-of-square opening forces the door unit to rack, which jams the latch and breaks the weather seal. Builders who learn how to measure a rough opening catch framing errors before the unit is on site, not after.
The table below lists common rough opening dimensions for pre-hung interior and exterior doors with 80-inch slabs:
| Door size | Slab width | Rough opening width | Rough opening height |
|---|---|---|---|
| 2-0 (24 in) | 24 in | 26 in | 82.5 in |
| 2-6 (30 in) | 30 in | 32 in | 82.5 in |
| 2-8 (32 in) | 32 in | 34 in | 82.5 in |
| 3-0 (36 in) | 36 in | 38 in | 82.5 in |
Double doors add about 1.5 inches of center clearance so the pair can swing without binding. Heights assume an 80-inch slab; taller doors, or floors finished with thick tile or stone, push the opening height up accordingly.
Tolerances matter as much as dimensions. Aim for within 1/8 inch on width, with the floor checked for level at the threshold. A crowned floor changes the effective height and forces shims to do work they were never meant to do.
- Check width at the top, middle, and bottom of the opening.
- Check height on both sides.
- Keep the diagonal difference under 1/8 inch.
- Verify the floor is level at the threshold.
- Confirm the header and jack studs before the unit arrives.
Adding a Door Opening to an Existing Wall
Renovations often require a door where none exists. Cutting through a finished wall is a structural operation, not a demolition job: the framing above the new opening must carry the load that the removed studs once carried. Skip that step and the wall settles, the header sags, and the new door stops closing within a year.
Header Requirements
A header transfers the load over the opening. For openings up to 3 feet, a doubled 2×6 header with proper jack studs usually suffices in conventional framing, but the span, the number of floors above, and snow loads all change the answer. Code tables and engineering guidance settle the question before any saw touches the wall.
The sequence matters as much as the hardware. Adding a door opening to an existing wall starts with temporary bracing, then the header goes in before the studs come out, and only then does the opening get its new jack studs, king studs, and cripple framing. Work in the wrong order and the load path is broken, even if the finished wall looks identical.
In a load-bearing wall, the header sits on jack studs that bear on the sole plate or foundation. In a non-bearing partition, a single top plate with a flat header, or even a let-in brace, may satisfy the code, but confirming the wall type first is non-negotiable. A quick check of the ceiling joist or truss direction tells you which walls carry load.
Framing and Installing the New Opening
With the header in place, the opening is framed square and plumb, then the pre-hung unit is set. The installer shims the jamb, checks the reveals, and fastens through the jamb into the framing. The sequence below covers a standard installation:
Installation Sequence
- Set the unit in the opening and center it with even side gaps.
- Check plumb on the hinge jamb before driving any fastener.
- Shim behind the hinges and the strike plate.
- Fasten through the jamb into the framing, not just into the shims.
- Confirm the door swings freely, then set the casing.
Shimming and Fastening Details
Shims go behind the hinges and the strike plate, never in the middle of the jamb where they bow the frame. Fasteners must reach the framing, and every shim gap that receives a screw needs solid wood behind it. For an opening cut into an existing wall, the same framing and installation rules apply whether the wall is load-bearing or not, though load-bearing walls get extra scrutiny on the header connection.
After fastening, check the reveal, the gap between the door edge and the jamb, on both sides and the top. A consistent 1/8-inch reveal means the unit is square in the opening. Then check the threshold for level and the latch for a clean strike. These three checks catch the errors that show up later as sticking doors and drafty seals.
Millwork Beyond Doors: Casings, Cabinets, and Built-Ins
The same facility that produces pre-hung doors often produces the trim that finishes them. Casings, baseboards, crown molding, and cabinet components all come from the millwork shop, and ordering them together cuts waste and keeps profiles consistent across the project.
Coordinating Trim and Built-Ins
Custom cabinetry and millwork production lets builders pair door schedules with built-in storage, shelving, and window seats. When the millwork shop makes the casework and the door casings from the same material run, grain and color stay consistent from room to room, and the shop can machine both to the same tolerances.
Lead times argue for bundling. Millwork is typically fabricated in batches, so doors, jambs, and trim should be ordered together and early; a large plant running full production turns orders in weeks rather than months. Contractors who coordinate the millwork order with the framing schedule avoid the classic failure mode of a door that arrives before the opening is ready, or a casing that shows up after the painter has finished.
The last step decides how the work reads. Carpentry and trim work, baseboards, crown molding, window casings, and decorative millwork, closes the gaps between the door unit and the wall and turns a functional opening into finished interior. Done in the same pass as the door installation, it keeps joints tight and profiles aligned, which is the standard a factory-built door deserves.