Hi Luca!
Unfortunately, Autodesk killed T-splines in favor of their own Fusion 360 program. The good thing is you can continue the course because Rhino 6 has Subd's! They're hidden away, but there is a command that lets you convert a mesh to a Subd geometry. Select the roof mesh and type in the command: "SubdFromMesh" (without the quotations) in the Rhino command prompt and you'll turn the roof mesh geometry into a Subd. It's not optimal yet, because you can't edit it in Grasshopper, but at least you can model the roof shape and convert it back to a mesh or NURBS polysurface. Using either "ToNurbs" or "Mesh" commands. Be warned that these commands ("SubdFromMesh" and "ToNurbs") are very beta and don't have autocomplete in Rhino. Mcneel will implement these features when they iron out the various bugs, but for now it will work more or less as an alternative.
More info here: https://docs.google.com/document/d/1BsNLtTj8s0J-7uBuuKDRWJq6VtOLe_WlgPi-oQ7SxoI/edit
Good luck!
Arie