Unfortunately if you want to see it as a skin with thickness in section, you will have to create a roof.
Otherwise you could try duplicating the stair, lowering it slightly, change the structure to plaster and then use Solid Element Operation to subtract the main stair from the duplicate - should work but I have actually tried it.
You will probably want to remove stringers, treads, etc. from the duplicate - leaving just the structure.
Barry.
One of the forum moderators.
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