Any suggestions modelling the underside of this stair?

Tried to model the underside of this staircase into a coherent whole but no luck. I tried using InterpolateCrv on both sides and then loft these lines. I did it in steps but i just doesn’t look right. There has to be away to do it better, smoother. I am guessing the challenge lies in lay that the different slopes and stairs steps. I just want an believable underside
stairs - Copy.3dm (334.2 KB)

Hello - I think you can just Loft from edge to edge to edge along the bottom with a ‘Straight Sections’ style loft.

The ends are little triangular lofts…


you can grab the bottom face of each step and move them down,:

after boolean union you will have a single object that you can split with the red surface you made.
Maybe is neccesary to extend the red surfaces on each side to avoid spliting problems. You can use boolean difference as the image below

thanks! I got the edge to edge part. But not ends/corners triangular lofts. You then merge the surfaces and the extrude srf up and boolean difference?

Will try this out tomorrow! Unsure if the surface underneath will be to organic.It looks pretty good on the picture though. Great to know the method