These two account for most of what we build, and people routinely pick the wrong one for their opening. The difference comes down to three things.
How much steel you see
A mono stringer puts one spine down the centre of the flight and the treads reach out either side. From the front it is nearly invisible; from the side it is a single line.
A double stringer runs a stringer along each edge, so the flight is framed rather than floated. It reads heavier and more architectural, and it hides the tread ends, which some people prefer.
How wide the treads can go
This is the practical one. A single centre spine has to cantilever each tread out from the middle, so there is a limit to how far it can reach before the tread needs help.
With tread supports, our brackets protrude only about two inches either side — the smallest on the market — and they are what let a mono stringer carry extra-wide and extra-thick steps in heavier materials. Without supports, everything is inside the stringer and the width comes down.
A double stringer carries the tread from both ends, so it takes width and weight without any of that.
What it has to carry
For a family house, either works. For a commercial stair, a very wide flight, or thick stone treads, the double stringer is the more direct structure and usually the more economical way to get there.
So which one
Maximum openness, standard widths — mono stringer without tread supports.
Open look, wide or heavy treads — mono stringer with tread supports.
Wide flights, commercial traffic, framed look — double stringer.
Thinking about a staircase?
Send your opening dimensions, floor-to-floor height and a photo. We come back with architectural shop drawings, three realistic 3D renderings and a firm price.