how to align a bsp to another bsp?

as in the picture, I want to align the wall above to the selected surface of the ground.

There is a way that dragging the wall with snapping to the grid.

but the problem of that way is the ground size is not align to the grid.

any other solutions?

118213-微信截图_20161211125134.png

Happen to open this thread via search engine but seems no one reply. Let me do this although its a long time post hopefully it was solved.

Simple way: make sure the wall BSP object is above the ground, and hit key “END” and the keyboard, the wall bsp will automaticlly dropped down onto the ground and align to the ground surface.

hope this could help to anyone from search engine.