A Double Hip Roof is one where you start out with one pitch and then transition to another. Some examples of this type of roof is a Mansard, Gambrel, or Shed that transitions into another roof plane.
The Mansard and Gambrel are pretty straightforward. You simply set the starting pitch, the top pitch, and the horizontal distance from the fascia that the transition occurs.
The Shed that transitions to another roof plane is a little more complicated. The reason I say this is because many people will simply use an arbitrary horizontal distance for the transition with little or no consideration for how the roof would actually be built. I see this all the time with roofs on plans that are sent to me.
Let's consider the situation illustrated here. We have a roof that springs off the interior porch wall and we want to add a lower pitch shed roof from the exterior to the existing roof. The trick is doing this without moving the existing ridge.
We could just add a shed roof in this area and call it done. The problem with that is that we end up with a funky gable configuration.
A better option would be to move the roof out to the outside wall and then change the edge to a double hip.
When we move the roof edge then of course the ridge moves as well because we are increasing the roof span. If you then change the roof edge to a Double Hip and set the appropriate pitches we still don't know what horizontal distance to set in order to get the ridge back to where it previously was. Not only that...how do you even know where the original ridge was?
What I do in situations such as this is to draw a line along the existing ridge. But there is no option to draw lines while in Roof mode.
To be able to draw a line on the ridge I use one of these methods.
I have a Custom Mode which I named "Visible Roof". Custom Modes are the 5 extra modes at the bottom of the modes list. These modes can be modified to make various items visible or not visible by using Options-->Visible Items. For Visible Roof I set the Roof Outline to be visible. If you do this in the Drawing Options this setting only applies to the current drawing. If you do this in System Options this setting will be present in any new drawings you create. To rename a custom mode go to either Drawing Options or System Options and select Custom Drawing Modes and change the name to whatever you wish. If you are using this method you can use the Line tool directly if you don't have a keyboard shortcut set for Line. Just as in the previous example use snaps to draw a line on the original ridge.
Once you have drawn a line along the ridge you can move the roof edge out to the outside wall and change the edge to a Double Hip. In this image you can see the line drawn along the old ridge line. If you do not see the line you drew then go to Visible Items and turn on Shapes visibility in roof mode.
After changing the edge to Double Hip start editing the Distance setting for the Double Hip edge. As you increase the distance the ridge moves away from the roof edge. As you decrease the distance the ridge moves toward the roof edge.
When you get the Distance correct the ridge will match up with the line you drew along the original ridge line. The result is a correctly drawn roof and you avoid that funky gable that you saw with the Shed roof.
Bill is the owner and maintainer of SoftPlanTuts.com