For some custom designs, Revit’s monolithic stair generation fails (the green arrow depicts the monolithic stair while the red arrow on the left depicts a layout for which this type of stair does not generate). It is generally possible to model the stair with an assembled stair type, but this does not depict what is required, especially the base of the stair, which cannot house reinforcing (the stair on the lower right).

This is one of the case where I prefer to use AutoCAD to model the geometry. I use surfaces to I prefer to use a partial helix to generate the bottom corner surface.

After the angle of the intended geometry is measured it is translated into the number of turns that will be required. I set my units to the eigth decimal place to make the approximation as accurate as possible. The radius of the helix is made wider than necessary as it will be trimmed by other surfaces.

This partial helix is used for one do the guides for the base surface. The sweep profiles are two lines at the base of the steps and the guides the helix and a vertical line.

Lofting a surface results in the following:

After some trimming planar surfaces have been created, the relevant surfaces are trimmed and the solid is sculpted.

When this has been done for the top corner as well then the interleading steps have their base added in.

When the solids are all added together, the stairs are finished.

After noticing the stairs are too thick, the exercise is quickly repeated and instead of building a surface through sculpturing surfaces, extruded solids (green) are sliced using the helixes (yellow).