a little tip that might help you
select the CW and the slab you want to snap to in plan and go to 3D window so that you only have these two things showing. then in 3D, select the CW and enter its editing mode. Now, turn off the frames and panels 'layers' and turn on the scheme layer. grab and stretch the edge you want and whilst stretching, move your mouse to hover over the snap points of the slab. as you 've only two elements showing in 3d, it should be easy to find the snappable points of the slab (you won't be able to snap to a surface, only edges and corners)
HTH