What hollow metal is
A hollow metal door is built from formed steel face sheets over an internal frame and core, rather than being a solid slab of metal, which is where the name comes from. The faces are joined so the door is rigid and the edges are closed, and the inside is filled with a core material that adds stiffness, insulation and, in rated doors, fire resistance.
A hollow metal frame is steel sheet bent into the profile that lines the opening and carries the door. The door and frame are usually specified together as an assembly, because they are prepped to match: the frame is cut for the hinges and strike, and the door is prepped for the lock and closer, so the hardware fits when they meet on site.
Gauges: lighter vs heavier steel
Steel thickness in this trade is described by gauge, where a lower gauge number means thicker, heavier steel. Doors and frames come in a range of gauges, and the choice is driven by how hard the opening will be used and how much security it needs.
Lighter-gauge doors and frames are economical and fine for ordinary interior openings with normal traffic. Heavier-gauge steel is specified where the opening takes abuse or needs to resist force: exterior doors, high-traffic corridors, security openings and institutional settings like schools and detention. Specs state the gauge required for each opening, and matching it is part of pricing the job correctly.
- Lower gauge number = thicker, heavier steel
- Lighter gauge: economical, ordinary interior traffic
- Heavier gauge: exterior, high-traffic, security and institutional use
- The spec sets the required gauge per opening
Where hollow metal is used
Hollow metal shows up wherever durability, security or fire resistance matter. Common homes for it include commercial buildings, schools and universities, hospitals and healthcare, government and institutional facilities, and industrial sites. It is the default for exterior openings because steel stands up to weather and force better than most alternatives.
It is also the common choice for fire-rated openings. Steel doors and frames are readily built and listed to carry a fire rating, so when an opening sits in a fire-rated wall, hollow metal is often what fills it. Fire-rated hollow metal is a topic in its own right, covered in a separate guide.
Hollow metal vs wood and aluminum
Against wood doors, hollow metal wins on durability, security and fire performance, and it handles exterior and wet or heavy-use conditions that wood does not. Wood doors are often chosen where appearance and a warmer finish matter, such as offices and public-facing interiors, and they can be quieter and easier on the eye.
Against aluminum, the comparison is usually about purpose. Aluminum-framed entrances and storefront are built around glass for entrances and display, where visibility and a light look are the point. Hollow metal is the choice for solid, secure and rated openings. Many buildings use all three: aluminum at the glazed entrance, hollow metal on exterior and rated doors, and wood on nicer interior openings.
Standard sizes
Hollow metal doors and frames are made in standard widths, heights and thicknesses that line up with common opening sizes, which keeps most ordinary openings on predictable stock dimensions. Non-standard and oversized openings can be built, but they cost more and take longer, so it is worth catching them early when pricing a job.
Because a typical job mixes standard and special sizes across many openings, keeping sizes, gauges and prep straight per opening is a real part of the work. eMullion models doors and frames at the opening level so a distributor can price and track them against a manufacturer catalog instead of by hand.
