Knock-Down vs Welded Steel Door Frames Explained

A knock-down frame ships in pieces and bolts or screws together on site, so it can be installed into walls that are already finished. A welded frame is one rigid, factory-assembled unit that is normally set before the walls go up. Welded is the stronger, more tamper-resistant build. Knock-down is the practical choice for most renovations.

That one fact drives most of the decision. If the drywall or masonry is already there, a welded frame usually cannot get in without demolition. If the walls are still being framed, a welded frame is the cleaner and tougher option. The rest of this guide covers strength, fire ratings, anchors, gauge, and the question most property managers actually have: do you replace the frame, or just the door?

If you are still deciding what kind of door goes in the frame, start with our hollow metal doors guide. This article stays on the frame.

What is a hollow metal door frame?

A hollow metal frame is a formed steel profile that wraps the door opening: two jambs and a head. It carries the hinges, the strike for the lock, the door stops and the weatherstripping or smoke seals, and it ties the door to the wall. "Hollow" refers to the pressed-steel section, which is open on the back and fastened to the wall construction.

The two main builds are knock-down (KD) and welded. Both can be made to the same profile and the same face dimensions, so from the corridor side they can look identical. The difference is how the corners are joined and when the frame gets into the building.

Most commercial steel doors in Calgary offices, warehouses, schools, apartment stairwells and retail back-of-house areas sit in one of these two frames. Our commercial steel doors page covers the supply and install side.

What is a knock-down frame?

A knock-down frame arrives as three separate pieces: a head and two jambs. The installer assembles them on site, joining the corners with screws, clips or integral tabs, then sets the assembled frame into the opening and anchors it to the wall.

The point of the design is access. Because the frame comes apart, it can be carried into a building, assembled in place and slipped around finished drywall. It is also easier to ship and store, and a frame that takes a hit during a project can sometimes be fixed by swapping a single piece.

The tradeoff is that the corners depend on fasteners rather than a continuous weld. A well-assembled KD frame is solid for normal use. A badly assembled one racks out of square, shows gaps at the corners and lets the door drift out of alignment.

What is a welded frame?

A welded frame is manufactured as one piece. The corners are welded and the weld is ground smooth, then the frame is primed. Nothing is assembled on site, so the corner geometry is set at the factory and stays that way.

Because it cannot be taken apart, a welded frame has to go in at a specific point in the build. In stud-wall construction it is set on the floor and the walls are framed and boarded around it. In masonry it is set first and the block is built up around the jambs. In both cases the frame is part of the wall rather than something added to it.

The result is a stiffer frame that holds the door square under repeated use, and that is harder to spread or pry at the corners. That is why welded frames are common in high-traffic doors, stairwells and openings where a door is slammed or forced many times a day.

How does installation differ between the two?

Knock-down frames are installed into walls that already exist. Welded frames are installed before the walls close around them. This is the main practical difference and it decides most renovation questions.

Knock-down installation typically goes like this:

• The frame is assembled on the floor and checked for square and diagonal.
• It is set into the rough opening, plumbed and shimmed.
• It is anchored through the jambs to the wall studs, with floor anchors where the frame meets the slab.
• Drywall is lapped or returned to the frame, then finished.

Welded installation goes like this:

• The frame is set on the slab, braced and checked for plumb before any wall is built.
• Floor anchors tie it to the slab, and spreader bars or temporary braces hold the jamb width while the walls go up.
• In masonry, wire or T-type anchors are built into the mortar joints and the cavity is commonly grouted solid around the jamb.
• Drywall or block meets the frame as the wall is completed.

Setting a welded frame in an existing wall means opening the wall up to remove the old frame and build around the new one. Most owners are better off with a knock-down frame in that situation.

Renovation or new construction: which should you pick?

For renovations, tenant improvements and replacements inside finished walls, knock-down is the usual answer. For new construction where the frame can go in before the walls, welded is usually the better choice. Both can be correct on the same project.

New construction. Welded frames suit the build sequence. The frame is set once, anchored once, and the wall is built to it. You get the stiffer frame with no field assembly.

Renovation and tenant improvement. Calgary buildings of every age get re-fit when tenants change, and the walls rarely come down for a door. A knock-down frame can be fitted around existing construction without tearing out finished work, which is why it is the standard choice here.

Masonry and block walls. Welded frames are the natural fit because the block is built around them. Retrofitting into existing block is a different job, and a frame supplier or installer will want to see the opening first.

If you are planning a larger door package for a project, our commercial door installation guide explains what typically happens on site and what to prepare.

Which frame is stronger and more secure?

A welded frame is stronger at the corners and harder to defeat, because the head and jambs are one continuous piece. A knock-down frame is strong enough for most commercial doors but relies on its fasteners and on how well it is anchored to the wall.

Strength depends on three things, and the build type is only one of them:

• Corner construction. Welded corners resist racking and prying better than mechanically joined ones.
• Steel gauge. A thicker frame resists bending and kick-in force better than a thinner one, whichever way it is built.
• Anchoring and wall fill. A frame that is poorly anchored or not grouted in masonry will flex and loosen regardless of its corners.

For a security-sensitive opening, such as a mechanical room, a back door onto a lane or a stockroom, a welded frame in a heavier gauge, set into a solid wall, is the stronger specification. For an interior office door or a corridor door that sees light abuse, a knock-down frame in standard gauge is a reasonable and common choice.

One practical note: the frame is only as secure as the strike area. A solid door in a weak frame can still be spread at the latch. If break-ins or forced entry are the concern, say so up front, because it changes what should be specified for the frame, the hinges and the strike reinforcement.

What does the frame gauge mean: 16 gauge vs 14 gauge?

Gauge is the thickness of the steel. A lower number means thicker steel. Sixteen gauge is the common frame thickness for standard commercial doors, and 14 gauge is the heavier step up for higher-use or higher-security openings.

In practical terms:

• 16 gauge suits interior and moderate-use commercial doors such as offices, corridors and washrooms.
• 14 gauge suits heavier doors, higher-traffic openings, exterior doors and places where abuse or forced entry is a real risk.

Thicker steel resists denting and bending, holds hinge screws and reinforcements better, and gives a more solid feel under repeated slamming. It also weighs more and costs more, so there is no reason to put 14 gauge on every door in a building.

The sensible approach is to match the frame to the door. A heavy exterior door hung in a light frame is a mismatch, and the frame is where the failure will show up first. Ask for the gauge and the build type in writing on any quote, so you can compare like with like. The steel vs aluminum commercial doors article explains where steel makes sense in the first place.

Do fire-rated openings need a specific frame?

Yes. A fire-rated door is rated as part of an assembly, and the frame carries its own label. The label on the frame has to match the rating the opening requires, and it has to be consistent with the door's label. A rated door in an unlabelled or lower-rated frame does not make a rated opening.

Frames for rated openings carry a permanent label, usually on the hinge jamb, that shows what the frame is listed for. When a frame is replaced, the new frame must carry the correct label for the opening, and the door, hardware and any glazing have to match the rating as well. Whether a particular frame build is acceptable for a particular rated opening, and which rating that opening needs, is decided per job against the building's drawings and the applicable code, not assumed from the frame type.

That is a question for the project's code consultant or the authority having jurisdiction, and we work to what they specify. For an overview of what ratings mean and what owners should check, read our fire-rated steel doors owner's guide.

Two habits save trouble on fire-rated frames. Never paint over the label, and never cut or drill the frame for new hardware without checking whether that is allowed for the label. Both are common ways an otherwise good opening ends up failing an inspection.

What anchors hold the frame in the wall?

Frames are held in place by anchors, and the anchor type depends on the wall. Wrong or missing anchors are one of the most common reasons a frame loosens, sags or rattles, even when the frame itself is fine.

• Stud anchors tie the jambs to steel or wood studs in a drywall partition. They are fixed inside the wall cavity, so on a new build they are set before boarding.
• Floor anchors tie the base of each jamb to the slab or subfloor, keeping the bottom of the frame from moving.
• Masonry anchors, usually wire or T-type, sit in the mortar joints of a block wall. In many installations the frame cavity is grouted solid, which stiffens the jamb and resists forced entry.
• Jamb and head fasteners secure a knock-down frame to the opening and hold the assembly square.
• Silencers are small rubber bumpers pressed into the strike jamb. They are not structural, but a missing set makes a door slam louder and wears the paint.

On an existing frame that has loosened, the usual cause is that the anchors have worked free or were never enough for the door's weight and use. Tightening or adding fixings can fix a frame that is otherwise straight and undamaged.

Do you need a new frame, or just a new door?

If the frame is straight, solid and correctly labelled, replace only the door. If the frame is bent, rusted through, out of square or carries the wrong fire label, replacing the door alone will not fix the problem and the new door will fail the same way.

Look at the frame before you order a door. These are the signs that a frame has to go:

• Corrosion at the base of the jambs. Calgary's winter salt and slush eat the bottom 6 to 12 inches of a steel frame first, especially at exterior and entry doors.
• A bent or kicked-in strike jamb. Forced entry attempts and heavy impacts often leave the latch side twisted.
• A door that will not hold alignment. Gaps that keep returning after adjustment point to a frame that has moved or racked.
• Cracked welds or open corners. The frame is no longer one rigid unit.
• Wrong or missing label on a fire-rated opening.

A frame with only surface rust, light dents or worn paint can often be repaired and repainted without replacement. Our guides on common steel door repairs and signs you need to replace a commercial steel door walk through what is fixable and what is not.

When a frame does have to be replaced in an occupied building, a knock-down frame is the one that usually fits, because it goes in without rebuilding the wall around it.

How do you choose between knock-down and welded?

Choose by the state of the wall, the use of the door and the rating of the opening. If the wall is finished, choose knock-down. If the wall is not built yet, a welded frame is usually better, particularly for heavy use, security or masonry.

A quick way to settle it:

• Walls already finished and the door is moderate use: knock-down, 16 gauge.
• Walls already finished and the door is exterior or heavy use: knock-down in a heavier gauge, with proper anchoring.
• Walls not built yet, high traffic or security matters: welded, 14 gauge or as specified.
• Masonry going up around the opening: welded, grouted as the project requires.
• Fire-rated opening: whatever the project specification says, with a label that matches the door.

Whichever build you pick, get the frame build, gauge, anchor type and label requirement written down together. Frames are cheap to specify correctly and expensive to rework after the wall is closed.

Talk to C Town Doors about frames in Calgary

C Town Doors supplies and installs hollow metal doors and frames for commercial buildings in Calgary, Airdrie, Cochrane, Chestermere and Okotoks. If you are not sure whether an existing frame can stay, send us the opening details and we will tell you what we see.

Read more about our steel doors and frames service, or contact us with the wall type, door size and any fire rating shown on the drawings. You can also call (403) 668-6686.