Floors, Roofs, and Ceilings - Autodesk Revit Architecture 2015 Essentials - page 73

Figure 3.20 Creating the arc
5. Finish the sketch. The roof springs from the arc you created, but it's
not reaching back and connecting to the roof face. This issue is easy
to resolve.
6. Select the roof extrusion, and then select the Join/Unjoin Roof tool
on the Geometry panel.
7. Hover over the rear edge of the extruded roof, as shown in the left
image of Figure 3.21.
Figure 3.21 Attaching the roof
8. Select the face of the previously created roof that you want to
connect to the extruded roof. The extruded roof now extends
back to meet the face of the other roof (Figure 3.22). If either roof
is modified, Revit Architecture will do its best to maintain this
connected relationship.