@peter. I'm not sure what problem you're facing now but the procedure is pretty simple:

-build the walls in the shape you want for the roof (one panel, two angled panels or even 3 with the middle one flat, as a roof window);
-insert the window in the walls at the right position and change its size accordingly;
-choose the color/texture for the inside and outside (roof tile texture) walls.

-select it and choose 'copy as new furniture' from tools menu (assuming you installed the "copy as new furniture' plugin http://www.sweethome3d.com/pluginsUserGuide.pdf );
-after you paste the new object and delete the old one, 'export to OBJ format' from the 3D window;

Now you should be able to import it as new furniture and rotate it as you need.