What This Guide Covers and Who It Helps
A knocked-down (KD) steel door frame ships in three separate pieces -- two jambs and a head -- and must be assembled on site before it can be set in the wall. Get the sequence right and the frame goes in plumb, square, and ready for hardware. Get it wrong and you may not discover the problem until the door is hung and binding, or until the inspector flags a fire-rated corner with a missing fastener. This guide walks commercial installers, school facility crews, and general contractors through the correct assembly order for the three most common wall types: masonry, steel stud drywall, and wood stud construction.
What Is a Knockdown Frame?
A knockdown frame -- sometimes called a KD frame or slip-together frame -- is a hollow metal door frame manufactured with a bend-tab or clip-and-screw corner system rather than factory-welded joints. The three-piece design ships flat and assembles on the job site, making it easier to move through finished corridors and stage on floors without a freight elevator large enough for a pre-assembled unit. KD frames are standard in steel stud drywall walls and existing masonry openings across schools, retail build-outs, medical office suites, and light industrial facilities.
Before You Touch a Corner Clip: Pre-Assembly Checks
Skipping the pre-assembly checklist is where most field problems originate. Before joining any pieces, confirm the following:
- Verify the rough opening. Allowable tolerance for a Drywall Knockdown profile frame is door width plus or 2-1/4 inch per jamb and 1-1/4" for the header. A 3-foot opening needs a minimum of 38-1/4" inches of clear rough opening for the width and 85-1/4" for the height (assuming a 2" header).
- Not to be confused with Masonry Knockdown frames. These frame profiles require the rough opening to be the Overall Width x Height + 1/4". Meaning a 3'0" x 7'0" frame with a 2" header would require a Rough Opening of 40-1/4" x 86-1/4"
- Check frame storage condition. KD frames should have been stored upright on wood runners with 1/4-inch spacers between units for air circulation -- never flat on the ground, never sealed in non-vented plastic. Flat storage traps moisture and causes rust and warping before installation begins. If the frames arrived or were stored flat, inspect every piece for twist before assembling.
- Confirm fire-rating requirements. If the opening is fire-rated, locate the label and verify corner fastener requirements. On fire-rated KD frames -- Drywall -- an #8 screw or tabs bent outward are required at each corner assembly for the UL listing. This is a code-compliance gate, not optional field practice.
- Check door thickness compatibility. Frames for 2-inch and 2-1/4-inch thick doors require welded construction -- not available as KD. If the hardware schedule shows an oversized door, flag it before assembly begins.
Cut and Prepare the Spreader First
The spreader is a wood board cut from minimum 1-inch thick lumber to exactly the door opening width. It installs at the head to hold the jambs at the correct spacing and prevent bowing while the wall is built or fastened around the frame. Cut clearance notches for the frame stops so the spreader sits fully across the frame depth -- a narrow board that only spans part of the throat will allow the jambs to lean and the frame to go out of square. On KD frames, the spreader is not optional; it is the tool that maintains the dimension you just verified in the rough opening.
Assembly Sequence by Wall Type
Masonry -- New Construction
- Join the three frame pieces using the bend-tab corner clips. Drive the #8 screw at each corner if the frame is fire-rated. Do not bend corner tabs past the outside face of the frame; use the correct break-off tool rather than pliers to avoid damaging the face.
- Set the assembled frame in position and brace it plumb using a carpenter's level -- longer is better. Level the head and plumb both jambs independently.
- Install the spreader at the head. A second spreader at the mid-point of the opening is recommended on taller frames to maintain width as block courses go up.
- Install wall anchors in the frame throat and grout the frame in the anchor zones as each course of block is laid. Check plumb and square continuously as the wall progresses -- do not wait until the wall is complete to discover a drift.
- Note: anchors are not required in frame heads except on fire-listed double egress openings.
Existing Masonry -- Retrofit
- Drill four 9/16-inch diameter holes evenly spaced in each jamb for 3/8-inch expansion shell anchors.
- Assemble the three frame pieces flat on the floor. Install four #8 x 1/2-inch sheet metal screws at each corner head-to-jamb joint (required for UL fire rating on rated frames). Install a removable spacing bar at the base to maintain opening width during positioning.
- Lift the frame into the opening. Plumb and level. Shim as required under the jambs.
- Anchor the frame to the wall with expansion shell anchors. Shim behind anchors as needed to maintain plumb.
Steel Stud Drywall Wall
- Assemble the frame and tap the throat anchors into position -- or weld them in if the spec requires welded anchors.
- Square, brace, and plumb the frame. Set the spreader at the head.
- Attach jambs to the floor through the floor anchor or floor extension. Install jamb studs tightly against the frame anchors, running to both the floor runner and the ceiling runner.
- Attach studs to frame anchors with screws (drill from the back side of the stud through both stud and anchor) or weld per spec.
- The header stud must be positioned above -- not at -- the frame header to leave room for drywall and avoid interfering with header-mounted hardware.
- Fire-rated installations: drywall must extend at least 1/2 inch into the frame throat. This is a confirmed code requirement, not a manufacturer preference.
Wood Stud Wall
- Assemble the frame and stand it in position. Anchor one jamb to the floor and set the spreader. Plumb, level, and square the frame, then anchor the second jamb and brace.
- Install anchors in the frame throat.
- Install double jamb studs to floor and ceiling runners and header. Bend anchor tabs around the studs, leaving the correct clearance between the frame return and the stud for the wall material.
- Square and nail the top anchor to the stud on one jamb only. Check plumb and square, then continue nailing. Repeat for the opposite jamb.
- Anchors are not required in heads except for fire-listed double egress openings. On fire-rated assemblies, confirm drywall extends 1/2 inch into the frame.
Common Errors That Show Up After the Wall Is Closed
- Missing corner screw on a fire-rated frame. The frame looks assembled, but it will not pass inspection and the UL label is compromised.
- Spreader removed too early. Without the spreader in place while studs are being attached, jambs can bow inward and throw off the door gap before the frame is ever permanently set.
- Drywall stopping short of the frame throat. On fire-rated steel stud installations, drywall that does not penetrate at least 1/2 inch into the frame creates a gap that fails inspection and compromises the assembly rating.
- Frames stored flat before installation. Warped jambs that were stored incorrectly produce a frame that cannot be shimmed plumb -- a problem that cascades into hardware alignment, door clearance, and weatherstripping gaps.
Hardware Considerations That Start at Frame Assembly
The assembly sequence also affects what hardware can be installed later. Electric strike preps in fire-rated frames require that the drywall penetration and the electric strike listing both be confirmed before the wall closes -- two conditions that must be met simultaneously and cannot be retrofitted easily. If the hardware schedule includes electrified hardware, door position switches, or power transfer requirements, those frame preps need to be coordinated with the KD assembly -- not added after the studs are in.
At DoorwaysPlus.com, we carry hollow metal frame components alongside the full hardware complement for each opening type -- from hinges and closers to electrified strikes and door position switches. If your project includes fire-rated KD frames or openings that need to be coordinated with access control, our team can help you sequence the order so frame preps and hardware ship together.
Quick Reference: KD Frame Assembly Sequence Checklist
- Verify rough opening dimensions before assembly
- Inspect frames for warping if storage conditions are in question
- Confirm fire-rating label and corner fastener requirement (#8 screw at each corner)
- Cut spreader to exact door opening width with stop notches
- Assemble corner clips -- use break-off tool, not pliers
- Set frame, install spreader at head, brace plumb and level
- Anchor per wall type; install second spreader at mid-height on tall openings
- Verify drywall penetration of 1/2 inch into frame throat on fire-rated openings
- Do not remove spreader until frame is permanently anchored
- Coordinate electrified frame preps before wall closes