The AutoCAD® Architecture Roof object has a setting called Thickness that is not as straight forward as it might seem. The Thickness setting distance actually means different things depending on whether you are utilizing Plumb or Straight edge cuts.
When a roof is created or highlighted, the Properties palette contains a Dimension category that specifies both the Thickness and Edge Cut settings. The following illustration shows a 4” Thickness with the two Edge Cut choices of Square and Plumb.
When using the Square setting for Edge Cut, the Thickness dimension is the distance from the interior face of the roof to the exterior face of the roof perpendicular to the roof line. (As an example, a 2×4 would have a 3-1/2” Thickness setting.)
When using the Plumb setting for Edge Cut, the Thickness dimension is the distance from the interior face of the roof and straight up (vertical) to the exterior face of the roof. (As an example, a 2×4 would have a different Thickness setting depending on the roof pitch.)
The following illustration shows how each of the 2 different Edge Cut settings affects the Roof object when a 4” Thickness is specified.
Since the actual Thickness needs to vary depending on the roof pitch in order to get the desired perpendicular roof thickness on Plumb edges cuts, I have created a PDF file which illustrates the required Thickness to specify to get the desired actual thickness per roof pitch.
Roof Thickness for Different Roof Pitches
Note that Roof objects are a single component and the Thickness setting is for the complete Roof thickness including any materials that you desire to be included, such as rafters, sheathing, and shingles. Roof Slab objects will give you the capability of specifying multiple separate materials.
The above information works for many (if not all) versions of AutoCAD® Architecture and Architectural Desktop®. The terminology is slightly different in the older/earlier versions, but the process is still applicable.
Applying Technology to Architecture 
Thanks for putting that up Doug. It annoyed me that back when I did convert to slabs, all the thicknesses would change. Just wanted to let you know that I have finally put up on youtube my series of tips on the Roof Object. How to add vertices, overlap, underhang, cut holes for dormers, tips on editing, errors etc, and want to let as many know about what this secretive tool can actually do. Hope you can share it around as I think it’s very valuable info for anyone doing residential work.
youtube playlist https://www.youtube.com/playlist?list=PLD9BF5D8FF70CD3DF&feature=view_all
Blog page on RO http://ausaca.blogspot.com.au/p/roof-object-video-links.html