Metal Pan Stair Treads: Design and Installation Basics

If you have ever walked down a concrete-filled staircase in a commercial building and wondered what holds the treads together, the answer is usually a set of metal pan treads. These folded-steel pans act as both the formwork and the structural shell for concrete-filled stairs, which is why so many commercial stairways feel rock solid and stay quiet under decades of foot traffic. They are also moving into residential work, especially basement stairs, outdoor stairs, and modern homes where the owner wants a poured-concrete look on a steel frame.

This guide covers pan sizing and gauges, the connection to the stringers, the concrete fill, corrosion protection, and the installation sequence.

What Metal Pan Treads Actually Are

A metal pan tread is a shallow tray formed by folding sheet steel into a U-shaped or box-shaped pan, roughly 11 inches deep by the width of the stair, with sides tall enough to contain a few inches of concrete. A separate riser pan often closes the vertical gap between treads, giving a closed-riser stair that blocks light, dust, and sound.

The pan does two jobs at once: the folded sides and the stringer connection carry the load, and the pan is the form you pour concrete into, so no separate formwork is needed. Once the concrete cures, pan and concrete work as a composite tread, with tile, stone, terrazzo, carpet, or honed concrete as the finished surface. Most pans come from a steel fabricator as part of a complete stair assembly, shop-welded to channel or plate stringers; loose pans are also sold for retrofit work, where each pan is field-welded or bolted to an existing steel frame.

Pan Dimensions, Steel Gauges, and Code Limits

Code geometry fixes your dimensions. Under IRC R311.7.5, stair treads must be at least 10 inches deep measured nosing to nosing, risers may not exceed 7-3/4 inches, and the greatest riser or tread in any flight may not vary from the smallest by more than 3/8 inch. A common pan design uses an 11-inch-deep pan with a 1-inch nosing projection, which yields a 10-inch effective tread depth that satisfies the minimum.

Gauge choice depends on the clear span between stringers. For residential stairs with stringers spaced up to about 36 inches apart, 14-gauge steel (about 0.075 inch thick) is the usual pick. For wider stairs or commercial loading, step up to 12-gauge (about 0.105 inch) so the pan floor does not flex or oil-can under wet concrete. When in doubt, have the fabricator confirm the gauge against the span and design load.

How Pans Connect to the Stringers

The pan-to-stringer connection is the structural heart of the stair. In shop-fabricated stairs, pans are fillet-welded along both ends to channel or plate stringers, with continuous or closely spaced stitch welds so each pan transfers its full shear into the stringer.

For site-built and retrofit work there are three options. Field welding reproduces the shop detail but needs a qualified welder and fire precautions. Bolted connections through the pan side walls to clip angles work where welding is not practical and make leveling easier. A hybrid, tack-weld to set the pan then add bolted angles, gives the adjustability of bolts with the rigidity of welds.

Whatever method you use, set every pan level in both directions before final fastening, and verify riser uniformity against the 3/8-inch IRC tolerance before anything is locked in permanently.

Concrete Fill and Finish Options

With the pans set and connected, the concrete goes in. Fill each pan to within about 1/4 inch of the top of the pan walls so the finish layer can sit flush, which usually means 2 to 3 inches of fill. The pan itself provides the reinforcement for normal residential spans; wide pans sometimes get light wire mesh to control shrinkage cracking.

Estimating the fill volume is straightforward: multiply one pan's volume (footprint times fill depth) by the number of treads and add a waste factor for spillage. You can run the same math through our concrete calculator before the truck or mixer shows up.

Finish choice is up to the project. Tile and stone thin-set directly to the cured concrete once it is ground flat. Terrazzo and polished concrete turn the fill itself into the wearing surface. Whatever you pick, finish the concrete true and level, because every tread finish inherits the flatness of the fill. If the stair will see wet boots or rain, choose a finish with real traction; our guide to non-slip stair treatments covers the options that work over concrete.

Corrosion Protection for Indoor and Outdoor Pans

Indoor stairs in dry buildings need only a shop primer plus field touch-up at the welds; the concrete fill shields most of the pan interior. Where the stair underside is exposed, add a topcoat so condensation cannot start rust at the weld heat-affected zones.

Outdoor pan stairs need a tougher plan. Hot-dip galvanizing after fabrication is the reliable choice because it coats every surface, including the pan interior the concrete will later cover. Field welds on galvanized pans must be re-coated with zinc-rich paint the same day, or rust rings will appear at every field connection within a year.

One outdoor detail that gets skipped too often: weep holes. Drill a small drain hole at the low corner of each pan before pouring. Without drainage, any water that finds its way under the finish sits in the pan and rots the steel from the inside, where you cannot see it until the pan face starts to blister.

Step-by-Step Installation Sequence

1. Measure field conditions: total rise floor-to-floor, total available run, and clear width. Confirm the floor structure can take the stair reactions, since a concrete-filled steel stair weighs far more than a wood stair of the same size.
2. Set and brace the stringers, plumb, parallel, and at the correct slope, before any pan goes in.
3. Dry-fit the pans from the bottom tread up to catch fabrication errors while they are still cheap to fix.
4. Level each pan in both directions, tack it in place, and check cumulative riser heights with a story pole against the 3/8-inch uniformity limit.
5. Complete the welds or bolted connections, recoating every field weld on galvanized work the same day.
6. Pour the concrete fill only after every structural connection is finished.
7. Cure per the mix design, grind high spots, and install the finish layer.

Common Mistakes That Weaken Pan Treads

The most damaging mistake is undersized steel: a thin-gauge pan spanning too far between stringers deflects under wet concrete and stays deflected, leaving a stair that feels springy and looks wavy. The second is pouring fill before the connections are complete, which makes later adjustments nearly impossible. Also common: pans set out of level, weep holes forgotten on outdoor stairs, and field welds on galvanized pans left uncoated, which is where outdoor pan stairs almost always fail first. Each of these is cheap to prevent and expensive to fix after the concrete cures.

Frequently Asked Questions

Can metal pan treads be installed on wood stringers?

Not as a standard detail. Pan treads are designed for welded or bolted steel frames; fastening steel pans to wood stringers creates a mixed-material connection that is hard to detail for the same rigidity, and concrete-filled pans demand a heavier frame than typical wood stringers provide.

How thick should the concrete fill be in a pan tread?

Fill the pan to within about 1/4 inch of the top of the pan walls so the finish layer sits flush, which usually means 2 to 3 inches of fill. Overfilling past the top edge makes the finish course thicker than planned and changes the finished tread height.

Do metal pan treads need rebar in the concrete fill?

Usually not. The folded steel pan acts as the reinforcement for normal residential spans; the concrete provides mass, a finishable surface, and impact resistance. For very wide pans or where the fabricator calls for it, a light wire mesh controls shrinkage cracking.

Can metal pan treads be used outdoors?

Yes, but only with proper corrosion protection: hot-dip galvanizing after fabrication, weep holes at each pan's low corner before pouring, and zinc-rich recoat on all field welds. An outdoor pan stair built like an indoor one will rust from the inside out.

What building code applies to metal pan stairs?

The same stair geometry rules as any other stair. Under IRC R311.7, risers max out at 7-3/4 inches, treads must be at least 10 inches deep nosing to nosing, and the largest and smallest riser or tread in a flight must be within 3/8 inch of each other. Commercial stairs fall under IBC Chapter 10.

Wrapping Up

Metal pan stair treads earn their popularity by solving two problems at once: they form the concrete and they carry the load, in a package that is rigid, quiet, and durable when detailed correctly. Get the gauge right for the span, keep every riser within tolerance, protect the steel for its exposure, and do not pour the fill until the steelwork is finished and level.