Use the Modify Form command to extend walls up to a roof.
The Modify Form command is unique in that you must select the element you want to extend the form to before you select the element you want to extend.
In this example, we will extend walls up to a roof.
Select the Modify Form command and set the options to Modify the Height of a Form and the Method – To Form Or Shape
Left-click on the Roof. It will highlight, then Accept it with another Left-click
*** At this point, you must Right-click (Reset) to get the command ready to select the walls you want to extend.
Select (Left-click) the wall (or multiple walls) you want to extend up to the roof
As you select each wall, it will be extended to the roof.