You could try take the material away, set workplane by 3 point on the slab and and then set material again.
Or cheat a bit if that dont work, a thin solid ( 0.1-0.5 mm) on the top of roofslab and add the material to that one.
In this case it seems not possible, but many time it's easier to make normal roof's, add material, convert to roofslab and then delete the half you dont need.
Depending on your TC version, you may need to set material oposite, like bottom will be top after converting
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