Originally there was a block wall here but as part of our side extension the wall was moved back to create a hallway to new rooms. I want to cut this flush to the floor and then run a half round trim along the length of the base rail to give it a nice finish. Is this holding the tread in at all or is it OK to cut?
probably would be best to have a better overview of the surrounding walls, how the staircase is linked in to the rest of the floor and wall and overall structure...
If it were mine, I'd be happy to cut it off. I might anticipate having to fix that top step back down to the string if it decides to pop up, but as it is, it looks pretty awful. Trim the string off flush to the top tread, and come back to us if there's a problem. If you don't feel that bold, trim it off to the height of the top of your balustrade channel moulding.
True, but the string ought to be fixed to the wall under that top step, and the vertical load of the staircase and users goes to the floor below.....
i would personally be happier with say 44mm [2x1] thick line along the top tread and neaten to that that will give you around 66mm hook iff you include tread thickness but then what do you do with the string cap to take the spindles and handrail assuming one is being fitted ?? without the hook any slack in the system can turn into the string dropping open side until everything tightens up ??