Open the revit family with unattached voids and click create tab properties panel family category and parameters.
Slice roof revit.
Valid boundaries are highlighted.
Load the family into the project and place it.
Create a void extrusion and make to required cut in the new construction phase apply the cut and done.
Select the floor to modify.
See hd version of video tutorial here.
This should keep you existing roof in tact and show a cut in the new construction phase.
In the family category and parameters dialog select cut with voids when loaded and click ok.
If anyone is still looking for a solution you can make a roof cut into an existing roof using component model in place.
Highlight the main roof on the building model and click to select it.
You can use the add split line tool to add linear edges and to split the existing face of a roof or structural floor into smaller sub regions.
Start points and endpoints can be added anywhere on the face of.
Revit roof slope can seem complicated at first glance but by learning a few critical features you can create any roof shape you desire.
Click by face or vertical.
Place the cursor in the drawing area.
Hope this is of assistance.
You can cut openings in the faces of these elements or you can select the entire element to make a vertical cut.
Use one of the opening tools to cut a vertical or perpendicular opening in a roof floor or ceiling for example to accommodate a chimney.
Watch the status bar to be sure that you are highlighting the main roof.
Determine the length of the void ideally in 3d view at wireframe mode extend this in order to intersect the roof surface that will be cut use the cut geometry command and select first the roof than the void.
Click modify floors tabshape editing paneladd split line.
Create a sloped roof in revit step 1 draw your roof step 2 define the slope.
The pick roof wall edges tool is active so that you can pick the boundaries that compose the dormer opening.
After that you can click finish to execute the cut.