Contractor and Facility Manager Education Series #3 — Hollow Metal Doors: Construction, Core Types, and Specification

Walk through almost any commercial building in Nebraska and the door you pass through is probably a hollow metal door. Schools, hospitals, office buildings, warehouses, government facilities — hollow metal is the commercial standard for good reason. It's durable, fire-ratable, cost-effective, and available in a wide range of configurations to meet nearly any application requirement.

But "hollow metal door" covers a lot of ground. The gauge of the face sheets, the type of core, the prep work done at the factory — all of it affects how the door performs, how long it lasts, and whether it shows up ready to install or needs field modification. This article breaks down how hollow metal doors are built and what contractors and facility managers need to understand when reading a door specification.

How a Hollow Metal Door Is Built

A hollow metal door is constructed from steel face sheets — the flat panels that form the two visible surfaces of the door — welded to an internal framework that includes a channel or angle at the top, bottom, and both vertical edges (called the "stiles"). The face sheets and edge channels are welded and ground smooth, giving the door its finished profile.

The interior of that steel shell is filled with a core material, which is where most of the variation in hollow metal door construction occurs. The core determines thermal performance, fire rating capability, weight, and overall structural rigidity.

Edge construction matters too. The vertical edges of the door — the hinge stile and the lock stile — are where the hardware mounts, and they need to be reinforced appropriately for what's going on them. Standard hollow metal doors come with hardware reinforcements (also called "preps") cut and welded at the factory based on the hardware schedule. More on that in a moment.

Core Types

There are four core types you'll encounter regularly in commercial hollow metal specification:

Honeycomb core is the most common and most economical option for standard commercial applications. It uses a kraft paper or steel honeycomb fill that provides a consistent interior structure with good rigidity at a competitive cost. Honeycomb core doors are appropriate for most interior applications and a wide range of exterior uses. They're the default on many commercial projects for good reason — they perform well, they're widely available, and they keep costs manageable.

Polystyrene core replaces the honeycomb fill with polystyrene foam, which provides better thermal performance than honeycomb. These are typically specified for exterior doors where thermal efficiency matters — the polystyrene significantly reduces thermal conductivity through the door. If you're specifying exterior doors on a project with energy performance requirements, polystyrene core is worth the conversation.

Steel stiffened core uses vertical steel channels or ribs running the full height of the door interior, providing additional rigidity and impact resistance. These are used in high-abuse environments — correctional facilities, loading docks, areas with heavy cart traffic, or anywhere the door is going to take a beating. Steel stiffened doors are heavier and more expensive than honeycomb, but they hold up where a standard core wouldn't.

Temperature rise core is a fire-specific construction. These doors use a core material that limits temperature transfer through the door during a fire event — typically to a maximum of 250°F or 450°F rise above ambient on the non-fire side after 30 minutes. Temperature rise doors are required by code in certain egress path applications, particularly stairwell doors, where limiting heat buildup on the exit side matters for life safety. If a specification calls for a "temperature rise" door, this isn't a performance preference — it's a code requirement.

Gauge Selection

Just like frames, hollow metal door gauge runs counterintuitively: lower number means heavier steel. The standard range for commercial doors is 14 to 20 gauge, with 16 gauge being the commercial baseline.

16 gauge face sheets are the standard commercial specification. They provide good dent resistance and durability for typical use, and they're appropriate for the majority of interior and exterior commercial openings.

14 gauge is specified for high-traffic, high-abuse, or high-security applications. Exterior doors on busy buildings, doors subject to cart or equipment impact, or security applications where forced entry resistance is a priority are all candidates for 14 gauge. The additional steel adds weight and cost, but in the right application, it pays for itself in longevity.

18 gauge shows up in lighter-duty interior applications where traffic is light and abuse is minimal. It's an acceptable choice in the right context but shouldn't be the default on anything that's going to see real commercial use.

One specification note: door gauge and frame gauge don't have to match, but they should be coordinated. A 14 gauge door hung in a 16 gauge frame is perfectly normal — but the hardware reinforcement in both the door and frame needs to be sized appropriately for the hardware being mounted.

Door Prep and Hardware Reinforcements

"Prep" refers to the factory-cut openings and welded reinforcement plates that allow hardware to mount correctly. Every piece of hardware that attaches to a door requires a specific prep — and that prep has to be specified before the door is manufactured.

Common preps include the cylindrical lockset prep (a round hole through the face and a rectangular mortise in the edge for the latch), the mortise lockset prep (a large rectangular pocket routed into the lock stile), the closer reinforcement (a welded steel plate at the top of the door to receive the closer arm), and hinge reinforcements at each hinge location.

If a door arrives without the correct prep, it needs to be modified — either sent back or field-cut, neither of which is a good outcome. Field cutting hollow metal is possible, but it's time-consuming, it can void fire ratings if done incorrectly, and it's a cost that shows up nowhere in the original budget. The solution is simple: lock down the hardware schedule before the door order goes in, confirm the preps on the submittal, and verify before the door ships.

Handing

Handing describes which direction the door swings. A right-hand door swings away from you when the hinges are on your right as you approach from the outside (or the secure side). A left-hand door is the mirror. Reverse bevel doors swing toward you. Getting handing wrong on a door order is a real-world problem — a door with the lock stile bevel cut in the wrong direction either won't latch or will bind against the stop.

The convention can feel confusing at first, but it becomes second nature. The key is to always call handing from the outside (or the key side) of the door, and to confirm it with a simple sketch or photo during the order process when there's any ambiguity.

Reading a Door Specification

A door specification, whether it appears in the project manual or on a door schedule, typically calls out: the door size (width x height x thickness), the material and gauge, the core type, the handing, and the hardware preps required. It may also note any special construction requirements — vision lite openings, louver cutouts, special edge preparation, or fire rating.

When reviewing a door spec, the questions worth asking are: Does the gauge match the application? Is the core type appropriate for the environment? Are the preps called out on the spec consistent with the hardware schedule? Is the fire rating called for on the spec compatible with the frame and hardware that's been specified? Are there any special construction requirements that need to be confirmed before the order goes in?

We review submittals as part of our process — that's where most spec conflicts get caught before they become field problems. But the earlier those questions get asked, the less disruption they cause.

What to Watch for in the Field

A few hollow metal door issues that surface during installation are worth knowing in advance.

Warping and twist. Hollow metal doors that are stored improperly — stacked flat, exposed to moisture, or stored in conditions with significant temperature swings — can develop warp or twist before they're ever hung. Doors should be stored vertically, in a dry location, supported at the hinge and lock stiles. A door with more than 1/8" of warp across the face is going to cause problems at the stop.

Finish damage during installation. The factory primer on hollow metal doors is a shop coat — it's intended as a base for finish paint, not a finished surface. Handling damage shows up as bare metal that can rust if left exposed. Touch up primer on any damage before the door goes into service.

Hardware fit issues. If a prep location is off — even by a small amount — the hardware won't operate correctly. Minor adjustments to hinges can compensate for small alignment issues, but significant misalignment between the door prep and the hardware template means something went wrong in manufacturing and needs to be addressed before the door is finished out.

Hollow metal doors are workhorses, and when they're specified and installed correctly, they'll outlast almost anything else on the job site. The specification work upfront is what makes the installation straightforward.

Next in the series: commercial wood doors — when they're the right choice, how they're built, and what contractors need to know about handling and storage on site.

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