STAIRIX

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Mono stringer collection

Tube SteelMono StringerStairs

Q: How much does a steel tube stringer weigh?

An HSS 12x6x1/2 in weighs 55.7 lb per foot, so a 17 ft spine for a 10 ft floor is about 950 lb before brackets. That is why we often splice it for brownstones and elevators.

One closed tube. Nothing twists.

A tube steel stair stringer is one closed rectangular steel tube, typically HSS 12×6×½″, running under the center of every tread. A closed tube resists twisting hundreds of times better than an I-beam, so the treads stay level when you step on the edge. Engineered, permitted and installed in NYC, Long Island and New Jersey, from $28k in NJconfirm.

Size my stringer →Why a tube

PE-stamped engineering
ASTM A500 structural tube
Bolted on site, no welding

Tube steel mono stringer: a matte black rectangular steel tube rising under solid oak treads, each tread on a welded V bracket

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What it is

A hollow steel spine. Solid as a beam.

Every STAIRIX mono stringer is built on structural steel tube, called HSS (hollow structural section). Its four walls form a closed box. When someone stands on the end of a tread, the tread tries to twist the spine, and a closed box resists that twist far better than any open shape of the same weight. That is why the treads of a good mono stringer stay level and quiet.

½″Steel wall, HSS 12×6

55.7 lbPer foot of spine

≈ 400×Stiffer in twist than an I-beam

A tube steel stringer cut open to show its hollow rectangular section and half-inch steel wall under oak treads

HSS 12×6×½″, cut open · four ½″ walls, closed corners, ASTM A500 Grade C

Why a closed tube.

Pick a spine and put 300 lb on the outer edge of a 42″ tread, the load the building code asks every tread to carry. Watch how far the edge drops when the spine twists.

Section + tread · HSS 12×6×½″Drawn to scale

Spine

Twist stiffness J: 222 in⁴
Tread edge drops: 0.003″
Weight: 55.7 lb/ft

Closed tube: the edge moves less than the thickness of a sheet of paper. This is the STAIRIX reference spine.

16′ spine, ends held against twist, 300 lb at the edge of a 42″ tread. Simplified elastic estimate to compare shapes; a licensed PE checks your stair.

Built to be looked at.

On a mono stringer the steel is on show. Every plate, weld and bolt is drawn before it is cut.

A bare tube steel stringer with its row of welded V brackets, before the treads go on — **The spine, before the treads**One tube, fifteen V brackets, welded and ground in the shop.

Close-up of a V-shaped steel bracket welded to the top of the steel tube, with its seat plate under a tread — **The V bracket**Narrow at the tube, wide under the tread. Welds ground smooth.

The foot of the tube steel stringer on a three-quarter-inch base plate with four anchors — **The foot**¾″ base plate, four anchors into the floor.

The top of the tube steel stringer bolted to the edge of the upper floor with a header plate — **The top connection**A ¾″ header plate, through-bolted into the floor framing.

Tube steel stringer cut open to show the half-inch wall — **Inside the tube**Room for the LED wiring, hidden end to end.

One tube, nothing to hide.Engineered in New Jersey, installed across NYC, Long Island and New Jersey. Structural tube, shop welds, bolted on site.

What size tube for your stair?

The spine size follows its length. Pick your floor-to-floor height and where the stair goes, and see the tube our engineer typically starts from.

Elevation · 10′ floor to floorLive

Floor to floor

Location

Delivery

Risers: 16 × 7½″
Tube length: ≈ 17′-0″
Typical section: HSS 12×6×½″
Weight of tube: ≈ 950 lb
Pieces: 1
Twist stiffness: 222 in⁴

At this length the 12″ tube keeps the stair above 8–10 Hz, so it does not bounce under quick steps.

Sloped length of spine	Typical tube	Weight	Twist stiffness J	Where we use it
up to 10′	HSS 6×4×¼″	15.6 lb/ft	23 in⁴	Short flights, half-levels, lofts
10′–12′	HSS 8×4×⅜″	27.5 lb/ft	48 in⁴	Split levels, 6–7′ rises
12′–14′	HSS 10×4×⅜″	32.6 lb/ft	65 in⁴	8′ floor to floor
14′–16′	HSS 10×6×⅜″	37.7 lb/ft	136 in⁴	9′ floor to floor
16′–22′	HSS 12×6×½″	55.7 lb/ft	222 in⁴	10–12′ floor to floor · our reference spine, cambered over 18′
over 22′	Add a landing or a post. One flight may rise at most 12′-0″ in NYC and 12′-7″ in NJ.

Weights and J from AISC section properties (A500). Typical starting points, not a design: the PE sizes your tube for your tread width, guard and floor.

Tube, square, flat bar or I-beam?

Mono stringers are built from several steel shapes. Here is how they compare, honestly.

Spine shape	Looks	Twist	Best for	At STAIRIX
Rectangular tube HSS 8×4 to 12×6	Slim from the front, deep from the side	Excellent	Most stairs, any span	Standard · this page
Square tube HSS 6×6, 8×8	Chunkier, more industrial	Excellent	Short spans, lofts, a heavier look	Same price · ask for it
Flat bar fin 1–1¼″ plate	A thin blade, very minimal	Weak on its own	Short spans, or tied to a wall	Flat bar stringer →
Folded plate box laser-cut plate	Sharp edges, follows the treads	Excellent (closed)	Sculptural stairs	Zig-zag stringer →
I-beam or channel W8, C10	Industrial, flanges on show	Poor: treads tip	Double stringers, hidden framing	Not for mono stringers
Round tube or rolled pipe, bent HSS	Soft, curved	Excellent	Curved and helical stairs	Curved mono stringer →

Rectangular 12×6Reads as a slim line from the front. Depth goes where the bending is.

Square 8×8Wider from the front and heavier looking. Good for short, sturdy flights.

Tube steel mono stringer in a concrete room, oak treads and glass guard, seen from below

Connections

Every joint, drawn first.

A mono stringer is only as good as its ends. Both are moment connections, sized by our engineer.

The tube lands on a ¾″ steel plate, about 16″ × 16″, with four anchors into concrete or through-bolts into added wood framing. It can sit on the finished floor or hide under it, so the tube grows out of the floor.
The most loaded joint. It carries weight, a horizontal push and the twist from every tread, up to about 3,400 ft-lb on a long spine. A ¾″ header plate is through-bolted into doubled LVL or a steel beam at the floor opening, never hung on a joist hanger.
A 17′ tube weighs close to 1,000 lb and will not go up a brownstone stair. We cut it in the shop where the bending is small and join it on site with an internal steel sleeve and Grade 8 bolts. Once painted, the joint is a hairline.

The tube, in numbers.

What goes into a STAIRIX tube steel stringer for a typical 10′ floor-to-floor stair.

Bare tube steel stringer with welded V brackets on a warm white background

Component	Specification
Tube	HSS 12×6×½″, ASTM A500 Grade C, 50 ksi, about 17′ long
Brackets	V brackets from ¼–⅜″ plate, welded all round, seat plate 16″ × 9″
Welds	Shop welds by AWS D1.1 certified welders, visible welds ground smooth
Foot	¾″ base plate, 4 anchors ½–⅝″, scanned for rebar before drilling
Top	¾″ header plate, through-bolted. A moment connection, not a hanger
Splice	Optional internal sleeve with Grade 8 bolts, placed where moment is low
Treads	42″ × 11″ × 3¾″ solid wood, 18″ past the tube each side
Loads	40 psf, plus 300 lb on the edge of any tread
Stiffness	L/480, first frequency 8–10 Hz or more

No welding on siteEvery site connection bolts together. No fire, no fumes, no hot-work permit in your home.

Wiring insideLED wiring runs inside the tube to one driver, so no cable is ever seen.

Old floorsBrownstone joists often cannot take the top load. We add an LVL or steel header and the PE checks the path to the ground.

Finish the steel.

The tube is sandblasted, primed and powder coated in our partner shop. Pick a color, or show the steel itself.

Matte black powder-coated steel tube and V bracket — **Matte black**Fine texture · most chosen

White powder-coated steel tube and V bracket — **White**Disappears against white walls

Bronze powder-coated steel tube and V bracket — **Bronze**Warm with walnut and smoked oak

Raw steel tube under a clear coat — **Raw steel, clear coat**Mill marks and variation kept · quoted

Brushed stainless steel tube and bracket — **Brushed stainless**Coastal and outdoor · +40–60% on steel

Outdoors

Hot-dip galvanized to ASTM A123, with vent holes drilled in the tube as ASTM A385 asks, then powder coated (duplex). Stainless for salt air at the Jersey Shore or the Hamptons.

Exterior floating stairs →

Any RAL color on request. All metal finishes → · Powder coat →

What it costs.

Installed prices include design, PE engineering, the steel tube, fabrication, powder coat, delivery and installation. Permit fees and co-op deposits are billed at cost.

Straight tube steel mono stringer with oak treads

Mono · straight

NJfrom $28kconfirm

NYCfrom $34k

9–10′ floor to floor, 15 treads
HSS 12×6×½″ tube, matte black
White oak, 42″ × 3¾″, natural oil
Cable or black rod guard

Tube steel mono stringer with frameless glass guard

Mono Glass · most chosen

NJ$36–48k

NYC$42–58k

Same stair and tube
Frameless laminated glass guard
Glass around the upper-floor opening
Walnut or smoked oak on request

Need a turn?

The same tube steel spine runs through every layout: L-shaped, U-shaped, winder and curved.

Each landing or 90° turn adds $3–5k
Turns need a post or a wall

Steel option	Effect	Steel option	Effect
Square tube instead of rectangular	no change	White, bronze or any RAL color	no change
Raw steel under clear coat	quoted	Brushed stainless tube	+40–60% on steel
Hot-dip galvanized + powder coat	quoted	Shop splice for tight delivery	+$0.8–1.5k

Steel is only about 5% of what a mono stringer costs: engineering, fabrication, finishing and installation make up the rest. Full cost breakdown →

Our promise.

A floating stair is a big decision. Here is what we put in writing.

Fixed price after the measureOnce we measure your space, the price is locked. No change orders for anything we should have seen.

10-year structural warrantyTube, brackets, plates and connections. 2 years on installation, 5 on the finish, and a free first-year check-up.

PE-stamped, permits handledA licensed engineer stamps every stair. We file with NYC DOB NOW or your NJ town and prepare co-op packages.

Licensed and insuredNJ Home Improvement Contractor #confirm · general liability and workers’ comp, certificates for your building.

Dates you can plan aroundFabrication in 6 weeks from approved drawings, a 2–3 day install, and a dated schedule in your contract.

Pay as it gets built50% to start engineering, 40% before delivery, 10% only when you sign off on the finished stair.

For architects & designers

Steel stringer details and sizes

HSS sizing by span, base and header connection details, tread attachment and the shop splice. We produce PE-stamped shop drawings for your approval.

Tube steel stringer with brackets, product view — The same geometry as your shop drawings.

Tube steel mono stringer rising along a concrete wall under oak treads

One closed tube.
Nothing twists.

Before we begin.

Still unsure? Send a photo of your stairwell and we'll tell you which tube fits.

What is a tube steel stair stringer?

A stair stringer made from structural steel tube (HSS), a hollow rectangular or square section with four closed walls. In a mono stringer stair, one tube runs under the center of the treads and carries all of them on welded brackets.

What size steel tube do I need for a stair stringer?

It depends on the sloped length of the spine. As a starting point: up to 10′ an HSS 6×4, 12–14′ an HSS 10×4, 14–16′ an HSS 10×6, and 16–22′ an HSS 12×6×½″. Over about 22′ add a landing. A licensed PE confirms the size for your tread width, guard and floor.

How much does a steel tube stringer weigh?

An HSS 12×6×½″ weighs 55.7 lb per foot, so a 17′ spine for a 10′ floor is about 950 lb before brackets. That is why we often splice it for brownstones and elevators.

Why not use an I-beam or a channel?

Open shapes twist easily. With 300 lb on the edge of a tread, a W8×31 I-beam can let the edge drop about an inch on a 16′ span. A closed tube of similar size moves a few thousandths of an inch. I-beams work for double stringers, not for one central spine.

Rectangular or square tube?

Rectangular tube puts the depth where the bending is and looks slimmer from the front. Square tube looks heavier and suits short, sturdy flights. Both resist twist well and cost about the same.

Can a tube steel stringer be used outdoors?

Yes, if it is hot-dip galvanized with vent holes and then powder coated, or made of stainless steel near salt water. An interior powder coat alone will rust outside.

Will the stair bounce?

Not when the tube is sized for its length. We design to L/480 and a first frequency of 8–10 Hz or more, stiffer than most floors.

How much does a tube steel mono stringer cost?

From about $28k installed in New Jersey and $34k in New York City for a straight 9–10′ stair with oak treads and a cable or rod guard. With frameless glass, about $36–48k in NJ and $42–58k in NYC.

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