What This Article Covers -- and Who It Is For
A knockdown (KD) hollow metal frame ships in three separate pieces -- two jambs and a head -- and must be assembled on the job site before or during wall construction. Done right, the frame goes in plumb, square, and anchor-ready on the first attempt. Done out of sequence, you end up with a twisted frame, a door that binds, or -- on a fire-rated opening -- a failed inspection.
This guide walks commercial subs, school facility crews, healthcare construction teams, and industrial maintenance personnel through the correct assembly sequence for KD steel frames, with attention to wall-type differences and fire rating requirements.
What Is a Knockdown Frame?
A knockdown frame (also called a KD frame or field-assembled frame) is a hollow metal door frame manufactured in disassembled components and shipped flat. The installer joins the head to each jamb on site using a corner clip system -- typically a bend-tab or screw-secured clip built into the frame profile. This is in contrast to a welded frame, which arrives as a single rigid unit.
KD frames are the standard choice for drywall stud walls (the C-profile series) and masonry construction (the M-profile series). They ship more economically and are easier to stage inside a building under construction. The trade-off is that assembly quality directly determines whether the frame performs correctly over its service life.
Before You Start: Storage and Inspection
Poor storage is one of the most common causes of a frame that fights you at installation. Follow these rules from the moment the frame arrives on site:
- Store KD frames on end, not laid flat. Flat storage traps moisture between pieces and causes rust and permanent warping.
- Stand frames upright on wood runners or sills, at least 4 inches off the floor.
- Stack no more than 5 units deep with 1/4-inch spacers between frames for air circulation.
- Cover with canvas or tarpaulin -- not sealed plastic, which traps humidity and accelerates corrosion.
- Inspect all three pieces on delivery. Check for shipping damage to corner clips, hinge preps, and strike preps before the job starts.
Tools and Materials to Have Ready
- Carpenter's level (the longer, the better)
- Full-size carpenter's square
- Wood spreader cut to exact door opening width from minimum 1-inch-thick lumber
- Shims
- Hammer and appropriate anchors for wall type
- #8 sheet metal screws (required at every corner on fire-rated frames -- do not skip)
- Bituminous water-resistant paint if grouting in masonry with antifreeze mortar agents
The Assembly Sequence: Corner Clips and Head Connection
The sequence below applies to standard M-series masonry KD and C-series drywall KD frames. Steps are consistent across those profiles; anchor method varies by wall type (see next section).
Step 1 -- Lay Out the Three Pieces
Place the two jambs and the head flat on the floor in their assembled orientation. Identify the hinge jamb and strike jamb from the frame label or hardware schedule before connecting anything.
Step 2 -- Engage the Corner Clips
Slide the head onto each jamb using the factory bend-tab corner clip system. The clips are built into the head profile and lock into the jamb. Apply steady pressure -- do not force or use a metal pry bar on the frame face, which can damage the finish and deform the clip.
Important -- fire-rated frames: Every KD frame that carries a fire label -- masonry or drywall -- requires a #8 sheet metal screw at each corner after the clip is engaged. This is a UL listing requirement, not a suggestion. Skipping the screw means the frame is not compliant as a labeled assembly, regardless of how solid the clip feels.
Step 3 -- Install the Spreader
Cut a wood spreader to exactly the door opening width at the head. The spreader must be nearly as wide as the frame depth so it holds the jambs at correct spacing without rocking. Notch the spreader to clear the frame stops. Install one spreader at the head and, for taller openings, a second spreader at the mid-point of the opening. Do not remove spreaders until the frame is permanently set and anchored.
On welded frames that arrive with a shipping bar: remove the shipping bar before setting the frame. Never use the shipping bar as a substitute spreader -- it is not cut to door opening width and will not hold correct spacing.
Step 4 -- Stand the Frame and Plumb, Level, Square
Stand the assembled frame in the rough opening. Use your carpenter's level and square to establish three conditions before any anchor is set:
- Level head -- check the top of the frame across its full width
- Plumb jambs -- check both jambs on face and return; shim under jambs as needed
- Square corners -- measure diagonals; they must be equal
Assuming the rough opening or masonry wall is plumb, uniform clearance from top to bottom on both the hinge and strike edges of the door should follow naturally. If a jamb is not plumb, correct it with shims before anchoring -- not after.
Anchoring by Wall Type
New Masonry Construction
Set the frame and brace it. Install anchors in the frame jambs; grout in the anchor area as each course of block is laid up. Check plumb and square continuously as the wall rises. A second spreader at mid-height helps maintain the opening dimension. Anchors in the frame head are not required for standard single openings -- but they are required on fire-listed double egress openings.
Existing Masonry
Drill four 9/16-inch holes evenly spaced in each jamb for 3/8-inch expansion shell anchors. With the three pieces assembled flat on the floor, install the #8 corner screws. Install a removable spacing bar at the base to maintain the opening width while handling. Position the frame, plumb and level, shim as required, then anchor to the wall through the expansion shells.
Steel Stud Drywall Construction
Assemble the frame and tap anchors into the frame throat. Set the spreader. Attach jambs to the floor through the floor anchor or floor extension. Install jamb studs tightly against the frame anchors and attach studs to anchors with screws or by welding. The header stud must be positioned above the frame header -- not inside it -- to leave clearance for drywall and to avoid interfering with header-mounted hardware such as a door closer or overhead stop.
Critical fire rating note: On all fire-rated drywall installations, drywall must extend at least 1/2 inch into the frame throat. This applies to both steel stud and wood stud construction and is a code-level requirement, not a finishing preference.
Wood Stud Construction
Assemble the frame, stand it in position, and anchor one jamb to the floor. Set the spreader, then plumb, level, and square before touching the second jamb. Install anchors in the frame throat. Bend anchor tabs around the studs. When nailing the top anchor to a stud, nail on one jamb first, recheck plumb and square, then complete the second jamb. Take your time -- this is the step where out-of-square frames get locked in.
Grouting and Backpainting (Masonry Applications)
When a frame will be fully grouted -- especially where antifreeze agents are present in mortar -- coat the inside of the frame with bituminous, water-resistant paint before grouting. This is the installation contractor's responsibility; it is not done at the factory. Skipping this step allows moisture to migrate from the grout into the bare metal, causing rust from the inside out -- damage that is invisible until the frame is already seriously compromised.
Final Check Before the Door Goes In
Once the frame is set, anchored, and spreaders removed, run through this pre-hang checklist:
- Head is level and jambs are plumb
- Diagonals are equal (frame is square)
- Rough opening tolerance is within the allowable range -- nominal width plus or minus 1/8 inch on each side
- Corner screws installed on all fire-rated frames
- Silencers are in place in the frame stop on the strike side (the rubber grommets that cushion the door and maintain the required 1/16-inch air gap between door face and stop)
- Hinge preps, strike prep, and any electrified hardware preps align with the hardware schedule
- Drywall extends at least 1/2 inch into the frame throat on fire-rated openings
Hardware Coordination Starts at Frame Order
The KD frame assembly sequence is also the right moment to confirm that hardware preps match what is being installed. Hinge backset, strike location, closer reinforcing, and electric strike preps all need to be verified against the hardware schedule before the frame ships -- not after it is anchored in the wall.
DoorwaysPlus.com carries hollow metal frame-compatible hardware across all major categories: hinges from McKinney, Hager, and Markar; thresholds and door bottoms from Pemko; and commercial door closers from Norton, Hager, and Corbin Russwin. If you are sourcing hardware for a KD frame project or need help confirming preps, the team at DoorwaysPlus can help you get the schedule right before anything ships.