there are 2 ways to solve this problem.
1st: Trim the wall with a roof. Place the roof on a different layer, working in 3d make sure the roof lines up with the under side of the stair, then go edit>trim roof>trim top. Hide the layer containing the roof and bingo.
2nd: use the soild elements operation use the wall as the target and the stair as the operator, choose subtraction with upwards extrusion and bingo again xx