I use Structures plus on my servern theres some triangle ceiling pieces in that mod that work really well with this one http://thestorytellerrp.com/thread-94.html theres pictures of what I was able to do with them

you would snap a fence foundation to the front of your middle foundation, then go off that at an angle. you need normal foundations under the angled fence foundations to place them, but you can remove them after as they will all snap to that first fence foundation that isnt angled. And that probably doesnt make sense but its a bit hard to explain in text.

Cheers guys, Rose, didn't think of using the S+ triangles even though we have that mod on our server. Derp! All sorted now though cheers.