Installing a roof cap without a tail-piece
 

I've got a 10" roof cap that I need to install but it does have a tail piece to put down the hole to attach my duct work to for my range hood. I was wondering if it was intended to have the duct work extend up threw the roof into the cap? I'm just trying to make sure i'm on the right track.

It is important that you have no leaks into the attic space.

No way to tell unless we see what it is you have now.
Most often there's a large metal flange at the base of the pairt that gets attached to the roof under the shingles leaving the bottom part of it on top of the shingles below so any water that runs in can get out and the line gets attached inside the attic.