Setting up a room under a roof

Hi,

Bit of a tricky situation. I have been trying to model a small server room in an existing loft space. (quick revit model attached to show the plan with roof removed).

I am having some issues when setting up the roof level. I have modelled the room as 0.5m, as that will be the lowest wall height and elevated the edge accordingly by 0.6m. However, when checking the 3D drawing in Hevacomp I seem to get walls which extend way past my highest point of the room. Is this just a graphical issue or something I've done wrong? Happy to link the model for you to have a look at. 

Also on a separate note: when setting up the roof and filling in vertical sections - do they assume the material of the wall beneath them, or do they assume the material of the roof?

Many thanks and best regards

Seb