Render the underside of an overhanging sandstone step

[Outer Bongolia]

I have a sandstone staircase from my 1st floor flat down to the garden, the top landing is about 90cm x 150cm and is positioned over the door of the downstairs flat. It's probably 100 years old and bits of the sandstone have been falling off so it's a bit of a safety hazard for the lower neighbour. I've removed all unstable pieces and steel brushed it back but it looks pretty unsightly and I'd like to render it back to a finished look. I'm not sure if a straight cement/concrete render will hold for long and wondering if a resin based compound might be more stable?

 

[RandomGrinch]

Could you bond/fix a bit of cement board to the underside instead?
It may be slightly more straightforward?

 

[Outer Bongolia]

Could you bond/fix a bit of cement board to the underside instead?
It may be slightly more straightforward?

It sounds easier that rendering with something that may not be stable but I think I'd want to put some pins in rather than a straight bond. Is cement board stable enough to not crack around, say, 8mm masonry screws?

 

[RandomGrinch]

Is cement board stable enough to not crack around, say, 8mm masonry screws?

Yes, something like a 12mm hardiebacker won't crack around screw holes, but it would be best to pre-drill and countersink the board.
Also, stainless steel screws are recommended for cement boards, due to their resistance to alkalinity.

 

[freddiemercurystwin]

Are the stairs OK if chunks are falling off?

 

[Outer Bongolia]

Are the stairs OK if chunks are falling off?

I’ve removed all loose bits and refilled cracks on the steps with mortar. They’re pretty solid. It’s just the underside of the top landing I’m concerned about. I’m thinking of removing all of the loose stuff and, so long as the block feels solid, doing a cosmetic repair to the area in the photos.