Z-index and placeholder

I'm using Jquery UI sortable, and I'm using the placeholder option.
My css look like that :
.holdercss { border: 1px dashed #666;z-index:1;}
If I don't use z-index, I can't see the dashes :s
But now, when I'm dragging the div, the dashes are on my div.
I want the placeholder to stay behind my div during a drag, but I want
to see the dashes.
Thank you