Ianas,
First. Only do one half of the roof at a time. I started by making the smallers half-cone.
1. Draw half-circle and close it with line tool
2. Select Mesh tool and use magic wand to create mesh from original half circle.
3. Create a node at the midpoint of the line and elevate it to create the conical shape. You should now have a mesh with a vertex, looking like half of a circus tent.
4. Drag a copy directly over the original mesh and change its elevation settings by 25 to 50 mm.
5. Usement
Solid Element Operation
with upper mesh as target and lower mesh as operator, set to
Subtraction with downward extrusion
and hit execute
6. Place the lower mesh on another layer and make it invisible.
7.Repeat for other half of roof.