In this case we're gonna use the Extrude Curve along Curve command to build the most basic roof volume. This tutorial allows you to do individual A-framed roofs and can be combined with other roofs to make a composite roof structure. This tutorial assumes that the base of the roof is a parallelogram, which, of course is rarely true. Reference the AutoCAD drawing of the base to get plot lines, relevant LiDAR files for heights and shape, and aerial photographs and birds' eye views from Bing Maps for an idea of overall shape for the roof.
Another alternative to drawing each individual roof is to follow this Grasshopper Tutorial.
Step 1. Begin with the volume of the block that you've Extruded from the Plan.
Step 2. First, add the profile of the end of the roof as a Curve.
(ie. A closed polyline in the plane of the gable. Heights can be measured in the method described in this tutorial.)
Step 3. Draw the Curve for your Extrusion to follow.
Step 4. Use the Extrude Curve along Curve command,
Step 5. Use the "BooleanUnion" command with both volumes selected. "MergeAllFaces" simplifies the resulting volume.
"ShowEdges" allows you to analyse the volume. This is to check that your block has "No Naked Edges", meaning that it is watertight, and, most importantly, it will print well.