I can't find any reference to this issue here on any forum area (doesn't mean it isn't here, under some obscure/carelessly named topic), so I'm posting as new topic - moderator, please feel free to re-direct this to an existing thread.
I've recently been forced to return to the ArchiCAD stair tool because the practice I'm now with is too miserly to pay for the Cadimage Stair Tool. For those that know the difference, I'm sure you feel my pain
The default ArchiCAD 15 library contains several 'pre-defined' stairs, which is where I started (since then I've gone directly to [create stair] button to get better control, but that's another story)
The <Straight Stair 15> stair object seems fine, but on closer inspection, there is a parameter missing in the object's settings pane, which has been switched off. See my screen shot collage for a visual synopsis.
Parameter name: topTreadLevel
With my basic knowledge of GDL I simply re-saved the stair object to my embedded project library as <Straight Stair 15 fixed> and switched the parameter on again. Simple, but alas, not a fix.
When the missing parameter is set to 'Top Tread Below Floor Level' the 3D stair displays in a very odd way (2D symbol looks fine)
I ask that Graphisoft please fix this and re-issue the v.15 library as a general update.