What Is a Hollow Metal Frame? Types, Gauges and Specs for Commercial Doors

A hollow metal frame is a commercial door or window frame formed from sheet steel into a hollow profile, usually galvannealed steel in 14, 16 or 18 gauge. It holds the door, carries the hinge and strike reinforcements, and anchors into a masonry, steel stud or wood stud wall. Hollow metal frames are the standard frame for offices, schools, hospitals and industrial buildings because they take heavy use, accept fire labels and work with almost any commercial hardware.

Quick answer: A hollow metal frame is a steel frame, typically 16 gauge for most commercial openings and 14 gauge for heavy use, exterior or security openings, built either welded at the factory or knock-down for assembly in the field. Mills & Nebraska fabricates welded hollow metal frames in its 36,000 sq ft Orlando facility, preps them for hardware, and applies Intertek-certified fire and wind labels in house.

What is a hollow metal frame made of?

A hollow metal frame is made of cold-rolled sheet steel bent into a profile with a stop, a rabbet for the door, two faces and a throat that wraps the wall. For most interior and exterior openings the steel is galvannealed, which means carbon steel coated with an iron-zinc alloy that holds primer well. The Steel Door Institute recommends galvannealed A40 or A60 steel over spangled galvanized G-series steel for doors and frames, with A60 preferred for rust resistance, and stainless steel for corrosive or washdown areas.

"Hollow" describes the profile, not a weak part. The inside of the frame is open so it can wrap the wall and hold anchors, hinge reinforcements and strike reinforcements welded to the inside face.

Welded vs knock-down hollow metal frames: which do you need?

A welded frame arrives as one rigid piece with the head and jambs welded at the factory, while a knock-down (KD) frame ships in three pieces and is assembled at the opening.

• Standard welded frames are set before the wall is built or as the masonry goes up. They are the most rigid option and the usual choice for masonry walls, high-traffic openings and frames with transoms or sidelites.
• Knock-down frames slip over a finished drywall partition, which makes them practical for tenant improvements, renovations and fast interior work.

Both types can carry fire labels. The Steel Door Institute notes that most manufacturers can fire rate frames shipped knocked down, so welding is often a choice about rigidity, appearance and installation rather than about the rating.

What gauge should a hollow metal frame be?

For most commercial openings Mills & Nebraska recommends 16 gauge, and 14 gauge for heavy-use, exterior, security or oversized openings. Lower numbers mean thicker steel. 18 gauge frames are used for lighter interior duty, and security frames go to 14 or 12 gauge with added reinforcement for correctional, banking and restricted spaces.

Match the frame gauge to the door it carries. A heavy door on a light frame wears hinges and strikes quickly, so check the door duty level and the hardware schedule together.

How is a hollow metal frame anchored to the wall?

The anchor depends on the wall. Masonry frames use wire or strap anchors that are built into the block, steel stud and wood stud frames use stud anchors screwed or clipped to the studs, and existing openings use expansion or bolt anchors through the jamb. Every frame also gets a floor anchor at the base of each jamb.

Grouting is a separate question. The Steel Door Institute says grout does not make a properly anchored frame sturdier, is not required for fire-rated frames, and should only be used in masonry openings, never drywall. Grout is still used where sound control or forced-entry resistance at a masonry exterior opening matters.

Can hollow metal frames be fire rated?

Yes. Labeled hollow metal frames are available for openings rated 20, 45, 60 and 90 minutes and 3 hours, with the gauge and construction set by the manufacturer's listing. The frame must carry a label for the rating of the opening, and the whole assembly is installed and maintained under NFPA 80. See fire-rated openings for how Mills & Nebraska builds and labels rated assemblies, and the fire rating guide for how ratings are assigned by wall and occupancy.

Mills & Nebraska applies Intertek-certified fire labels in house, so a frame fabricated or modified in Orlando can leave the shop labeled instead of waiting on a separate labeling step.

Hollow metal vs aluminum door frames: what is the difference?

Hollow metal frames are steel, accept a wider range of commercial hardware and offer more fire rating options, while aluminum frames are lighter and are mainly used for storefront and non-rated interior glass. The Steel Door Institute makes the same point when comparing steel and aluminum exterior openings. Choose hollow metal when the opening is fire rated, takes abuse, needs heavy hinges, exit devices or electrified hardware, or has to meet Florida wind requirements as a tested assembly.

Can a hollow metal frame hold glass?

Yes. Hollow metal frames are built as borrowed lites, sidelites, transoms and multi-lite assemblies with mullions, using the same profiles and finishes as the door frames next to them. Rated glazing has its own listing limits, so confirm the glass, the lite size and the frame rating together. For exterior openings in Florida, the frame, door and glazing are approved as one assembly; see hurricane-rated openings.

What to put on the frame schedule before you order

A complete frame schedule prevents most field problems. For each opening, list:

• Frame type (welded or knock-down) and profile, single or double rabbet.
• Wall type and thickness, which sets the jamb depth and the anchor type.
• Opening width and height, door thickness and door swing.
• Gauge and steel type (galvannealed or stainless).
• Fire rating and any wind or impact requirement.
• Hardware preps: hinge size and count, strike, closer, exit device, electric strike or power transfer.
• Lites, transoms or sidelites, with glass type.

The Steel Door Institute sets the clearance between door and frame at a maximum of 1/8 inch for single and paired steel doors, so accurate frame and door sizes on the schedule matter. Mills & Nebraska's Architectural Hardware Consultant reviews hardware preps against the schedule, and the fabrication team cuts and preps frames in house.

FAQs

What does HM frame mean?

HM frame is shorthand for hollow metal frame, the formed steel frame used for commercial doors, borrowed lites, sidelites and transoms. You will see it on door schedules and drawings next to HM door, which means a hollow metal steel door. Both are covered by the steel door and frame standards the Steel Door Institute publishes.

Are hollow metal frames structural?

No. A hollow metal frame is not designed to carry building loads, and lintels or headers above the opening carry the wall above. The frame is anchored to the wall so it can hold the door, hardware and closing forces, and in exterior Florida openings it must resist design wind pressure as part of a tested assembly.

How long do hollow metal frames last?

Properly installed and painted galvannealed frames commonly stay in service for decades, which is why they are used in schools, hospitals and public buildings. Corrosion at the base of the jambs is the usual failure point, so specify A60 galvannealed or stainless steel in wet areas and keep the bottom of the frame sealed.

Does Mills & Nebraska stock hollow metal frames?

Yes. Mills & Nebraska stocks standard hollow metal frames with Same Day lead times on stocked items from Orlando and Lakeland, fabricates custom welded frames in house, and delivers with its own fleet across Florida. Send your door schedule and the team responds within 24 business hours.

Request a quote from Mills & Nebraska with your door and frame schedule, and our Florida team will review frame types, gauges and hardware preps for every opening.