Stair Stringer Types: Cut, Closed, Mono, and Dropped Explained
By RedX Apps Team · Published 2026-07-24 · Editorial policy
What Are the Types of Stair Stringers?
There are four common stair stringer types: cut (open) stringers with a sawtooth profile the treads sit on, closed (housed) stringers where treads fit between solid boards, mono stringers - a single central beam carrying the treads - and dropped stringers, set lower so treads and risers wrap the structure. Most stairs use one of the first two; mono and dropped constructions trade simplicity for a specific look.
The choice is structural and visual at once: cut stringers show the classic stepped profile, closed stringers show one clean diagonal line, mono stringers make the treads appear to float, and dropped stringers hide the carriage entirely behind finish material. All four carry the same code numbers for the walking surface - the stringer type changes how the load gets down, not what the treads must do.
| Type | How it works | Best for |
|---|---|---|
| Cut (open) stringer | Rise and run notches sawn in a sawtooth pattern; treads sit on the level cuts | Most framed stairs: decks, basements, garages - fastest to build and easiest to tread |
| Closed (housed) stringer | Solid boards at the sides; treads and risers fit between them on cleats or routed housings | Finished interiors and weather-exposed stairs - sheds water, hides end grain, reads clean |
| Mono stringer | One central beam with tread brackets; treads cantilever both sides | Floating and modern open stairs - maximum visual lightness, engineered connections |
| Dropped stringer | Carriage set below the finished line so treads and risers wrap over it | Stairs finished in hardwood or stone where no framing may show |
Cut Stringers: The Default, and Their One Weakness
A cut stringer starts as 2x12 stock with the rise and run notched out in a sawtooth; it is the fastest stringer to build and the easiest to tread because the treads simply land on the level cuts and screw down.
The weakness is the throat - the solid wood left below the notch corners. Cutting the sawtooth removes more than half the board, so at least 5 inches of continuous material must remain or the stringer needs reinforcement. That is also why cut stringers space at 16 inches on center or tighter under 2x treads: each one is carrying on a fraction of the board it started as.
Closed and Housed Stringers: Clean Lines, More Joinery
A closed stringer stays a full-depth board - the treads and risers attach between the stringers on cleats, in dadoes, or in tapered routed housings that get wedged and glued in traditional work. Nothing is notched away, so the stringer keeps its full structural depth.
The gains are real: a clean diagonal line instead of a sawtooth, no exposed end grain drinking water on exterior stairs, and stiffer sides for mounting balustrades. The cost is joinery time - every tread lands in a fitted pocket rather than on an open cut. Outdoors, closed stringers with cleats are the quiet upgrade most deck stairs never get.
Mono Stringers: How Floating Stairs Are Built
A mono stringer (also called a single or center stringer) is one beam running up the middle of the flight, with brackets or saddles carrying each tread so it cantilevers to both sides. It is the construction behind most floating staircases - the treads appear unsupported because the support is hidden beneath their centerline.
Everything the other types share across two or three stringers concentrates in one member here, so mono stringers are commonly steel or an engineered timber section, and the tread-to-beam connections are the critical detail. All the walking-surface rules still apply, and open risers must still pass the 4-inch sphere limit where guards are required.
Dropped Stringers: Framing That Disappears
A dropped stringer is set below the finished stair line so the treads and risers - usually hardwood, stone, or a finished skin - wrap over the rough carriage. The stringer does the structural work; the finish material defines the walking surface.
The layout point that catches people: the drop must be planned from the start, because the finished rise and run are measured at the walking surface, not the framing. Design the finished stair first, then drop the carriage by the finish thickness - the same discipline as dropping any stringer for tread thickness, applied to the whole assembly.
Which Stringer Type Should You Use?
Match the stringer to the job: cut stringers for framed utility and deck stairs where speed and treadability win, closed stringers where the side of the stair shows or the weather does, mono stringers when the design brief says floating, and dropped stringers when the finish material is the stair.
In the stair builder you set the stringer type per design - cut, closed, mono, or dropped - and the blueprint, cut list, and elevations redraw for the construction you chose. Comparing two stringer types for the same flight takes about a minute, which is a cheap way to settle an argument with yourself before buying lumber.
Frequently Asked Questions
What are the four types of stair stringers?
Cut (open) stringers with a sawtooth profile the treads rest on; closed (housed) stringers where treads fit between full-depth boards; mono stringers - one central beam carrying cantilevered treads; and dropped stringers, set low so finish treads and risers wrap over the framing. Cut and closed cover most residential stairs; mono builds floating stairs; dropped hides framing under finish material.
What is the difference between open and closed stringers?
An open (cut) stringer is notched in a sawtooth, so the tread ends and the stepped profile are visible from the side. A closed stringer stays a full-depth board and the treads die into it, showing one clean diagonal line. Structurally, the closed stringer keeps all of its depth while the cut stringer keeps only the throat below the notches - which is why cut stringers need at least 5 inches of solid material remaining.
What is a mono stringer staircase?
A staircase carried by a single central beam, with brackets under each tread so the treads cantilever to both sides and appear to float. Mono stringers are usually steel or engineered timber because one member carries the entire flight. It is the standard construction for floating stairs - if you have seen open treads with no visible side support, you were probably looking at a mono stringer.
What is a dropped stringer?
A stringer set below the finished stair line so that finish treads and risers - hardwood, stone, or a built-up skin - wrap over the rough carriage. The framing carries the load while the finish defines the walking surface. The key is planning the drop up front: finished rise and run are measured at the surface people walk on, so the carriage is designed from the finished stair minus the finish thickness.
How many stringers does a staircase need?
Cut stringers under 2x wood treads: no more than 16 inches on center - three for a 36-inch stair, four at 48 inches. Closed stringers work in pairs at the sides, with a middle carriage added on wide flights. A mono stringer is by definition one, doing all the work through engineered connections. Composite treads usually require tighter support spacing than wood; follow the tread manufacturer's span rating.
Do all stringer types follow the same stair code?
Yes - the IRC's stair rules regulate the walking surface, not the carriage: risers 7-3/4 inches maximum, treads 10 inches minimum, 3/8 inch maximum variation, headroom, handrails, and guards apply identically to cut, closed, mono, and dropped construction. The stringer type changes the engineering underneath - a mono stringer's connections deserve real scrutiny - but no type escapes the geometry rules.
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Pick a Stringer Type and See the Whole Stair
Design the flight once and switch between cut, closed, mono, and dropped stringers - the blueprint, cut list, and elevations redraw for whichever construction you choose.