The frame is the first piece of an opening that gets installed and, in most cases, the last one anyone wants to revisit. Once a hollow metal frame is set in a masonry wall and the block is laid up around it, your options for fixing a mistake are limited and expensive. That makes getting the frame specification right — before anything ships — one of the highest-leverage decisions on a commercial door and hardware project.
This article covers what contractors and facility managers need to know about steel hollow metal frames: how they're built, what the different types are for, how to select the right gauge, and where the most common specification errors show up.
How a Hollow Metal Frame Is Built
A hollow metal frame is exactly what the name suggests — steel formed into a hollow profile, then assembled into the three-sided unit (two jambs and a head) that surrounds the door opening. The profile includes a stop (the raised edge the door closes against), a face (the visible flat surface facing the room), and a return (the portion that wraps into the wall).
The dimension that most directly affects frame selection is the throat — the inside measurement of the return, which has to match the wall thickness the frame is going into. A standard 3-5/8" stud wall with 5/8" drywall on each side gives you a total wall thickness of 4-7/8", which typically gets covered by a 4-9/16" or 5" throat frame with the drywall scribed tight. Masonry walls have their own standard dimensions depending on block size and whether there's a furring layer.
Getting the throat wrong means the frame either doesn't cover the wall edge cleanly or overhangs it, neither of which is acceptable on a finished job.
Welded vs. Knocked-Down Frames
The most fundamental distinction in hollow metal frames is how they're assembled.
A welded frame — also called a "unit frame" or "factory welded" frame — arrives on the job site as a single, fully assembled piece. The corners are welded and ground smooth at the factory, and the frame ships as a complete unit. Welded frames are dimensionally precise, structurally rigid, and the preferred choice for most commercial applications. They're also larger and heavier to ship and handle, which matters on big jobs or tight freight situations.
A knocked-down (KD) frame ships as individual components — two jambs and a head — that are assembled in the field using mechanical fasteners or slip-on connectors. KD frames are easier to ship, can be carried through finished spaces without the clearance issues of a full welded unit, and are often the practical choice for remodel or renovation work where you can't get a large welded unit through existing corridors. The trade-off is that field assembly introduces more variables — if the frame isn't squared up correctly during installation, you'll feel it every time the door closes.
A third type worth knowing: the KD wrap-around or slip-on frame, used specifically in retrofit situations where an existing frame is damaged but the opening itself is sound. These are designed to install over the existing frame profile rather than requiring a full tear-out. They're a legitimate tool for the right situation, not a workaround.
For new commercial construction, welded frames are the standard. KD frames have a place, but it's a specific one.
Masonry vs. Drywall Anchoring
Frame type and anchor method are directly determined by the wall the frame is going into. This is where a lot of specification errors originate — someone orders a frame without locking down the wall construction first.
Masonry frames use steel anchors that extend from the back of the jamb into the masonry, where they're encased in mortar or grout as the wall is built up around the frame. The frame gets set first, braced plumb and square, and the block goes up around it. This is called a "frame-first" installation, and it means any mistake in the frame order is discovered after the wall is already in progress. Masonry frames are also available in "existing wall" or "after-set" configurations for openings cut into existing masonry, which use different anchor styles suited to that condition.
Drywall/stud frames (sometimes called "stud wall" or "pressed steel" frames) use a different anchor system designed to attach to steel or wood studs before the drywall is applied. Common anchor types include the compression anchor, flex anchor, and stud/snap-in style — each suited to slightly different framing conditions. These frames are typically installed after rough framing is complete and before drywall, which gives more scheduling flexibility than masonry installations.
Specifying a masonry frame for a drywall opening — or the reverse — isn't just an inconvenience. The anchor geometry is different, the installation sequence is different, and in most cases the wrong frame simply can't be made to work without modification.
Gauge Selection
Steel frame gauge follows the same counterintuitive convention as most sheet metal: a lower gauge number means thicker steel. In commercial hollow metal, the standard range runs from 14 gauge (the heaviest common frame material, roughly 0.075") down to 20 gauge (lighter, around 0.036").
16 gauge is the commercial workhorse — it's the standard specification for most interior and exterior frames in typical commercial construction. It's rigid enough to handle normal traffic and hardware loads without excessive deflection, and it's what most projects default to unless there's a specific reason to go heavier or lighter.
14 gauge is used where additional durability or security is required — high-traffic corridors, exterior frames taking direct weather exposure, or applications where forced entry resistance is a consideration. Some security specifications require 14 gauge as a minimum on all exterior frames.
18 gauge shows up in lighter-duty interior applications, and 20 gauge is generally limited to very low-traffic interior partitions. For anything that's going to see real commercial use, 16 gauge should be considered the floor.
One thing contractors sometimes miss: gauge selection affects hardware reinforcement requirements. Heavy hardware — wide throw deadbolts, heavy-duty exit devices, electric strikes — needs adequate steel behind it. A 14 gauge frame gives you more to work with. If the hardware schedule is calling for high-security or electrified hardware, make sure the frame gauge is part of that conversation.
Fire-Rated Frame Considerations
Fire-rated frames carry a label from a recognized testing laboratory indicating the rating they've been tested to — typically 20-minute, 45-minute, 60-minute, or 90-minute ratings. The frame label has to be compatible with the door and hardware labels to form a compliant fire-rated assembly. We'll go deeper on this in Part 5 of this series, but the important point here is that fire-rated frames have specific requirements around their construction and installation that standard frames don't — and substituting one for the other isn't acceptable.
The Specification Mistakes We See Most Often
After 30+ years working commercial openings across Nebraska, a few frame specification errors come up repeatedly.
Throat dimension assumed rather than measured. Wall assemblies on drawings don't always reflect what actually gets built. A nominal 4" wall might end up at 4-3/4" by the time all the layers are accounted for. Measuring the actual wall condition — or verifying it against the architectural details — before ordering the frame is a step that gets skipped more often than it should.
Frame type ordered before wall type is confirmed. On projects where wall construction is still being coordinated, someone places the frame order early to hit lead times, and they guess on masonry vs. drywall. Sometimes they guess wrong. Confirm the wall condition before the order goes in.
Omitting special conditions. Frames at exterior openings may need weatherstripping cutouts or special sealant preparations. Fire-rated openings need labeled frames. High-security applications may have reinforcement requirements. These details have to be on the specification and confirmed in the submittal — they don't get added in the field.
Assuming standard profiles will cover non-standard walls. When walls are thicker than standard due to insulation, special construction, or renovation conditions, a custom throat frame is the right answer. It's not a significant cost difference, but it does require knowing the condition before you order.
Getting the frame right is the first step toward an opening that installs cleanly and performs over the long haul. It's also the part of the opening where mistakes are hardest to recover from.
Next up in this series: hollow metal doors — construction basics, core types, gauge selection, and how to read a door specification.
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