Time to put some flesh (well plasterboard) on them bones
Batons were fixed to the whole underside of the stairs to provide a level surface for the plasterboard to be screwed to. On the sloping section, I simply tacked a piece of 1x2 inch in, notched with a chisel at each angle where the riser meets the tread see diagram at side. Similar batons were added to the underside of the stair 'string' running up the walls, making sure to achieve an even depth across the batons.
Batons were also added to the old ceiling to allow a false ceiling to be hung to hide wiring and create sufficient depth for spotlights.
With all the batons in place plasterboard was cut half a centimetre undersize it saves trimming and making a furry edge when you find the walls aren't precisely even and screwed on with 32mm drywall screws.
Having attempted plastering before without using corner beads, I decided it was definitely worth a try! The beads were screwed to all the angles on the exposed treads and risers and to the top of the vertical back board. This board was cut short, so it stops on the cross member supporting the cistern. The idea is to put a removable panel here for service. After the application of some scrim (jointing) tape over the joins in the plasterboard, the room was ready to be PVA'd and skimmed.
How to baton the underside of stairs
