Using Home Designer Suite 2015
Here are a couple points that I had to learn the hard way about porches:
You can use the Deck room type and place a roof over it. This facilitates the use of open railings, etc.
If the porch has a shed roof attaching to the house do this:Open Build Roof from your toolbar. Deselect Auto Rebuild Roofs.
If your house currently has a roof, delete it.
Break the house exterior wall where the porch joins the house.
Specify the house roof using the second floor only.
Select the section of house wall that is under the high end of the shed and open the roof dialog.
Select the High Shed/Gable option.
Alternately open the roof dialogs for the two end walls. Set the roof type as Gable.
Select the wall at the low side of the shed and open the roof dialog.
If there is a roof type selected, deselect it. Do not specify a type.
Set the roof pitch. It can be different than your default and other roofs on the house.
Click Build roof to see the result.