public static void CreateDisplacementAndPath(Document doc, View view)
{
    // Find roof
    FilteredElementCollector fec = new FilteredElementCollector(doc);
    fec.OfClass(typeof(RoofBase));
    RoofBase roof = fec.FirstElement() as RoofBase;

    // Get a geometric reference for the path
    Reference edgeRef = GetHorizontalEdgeReference(roof);

    using (Transaction t = new Transaction(doc, "CreateDisplacementAndPath"))
    {
        t.Start();
        // Create a new top level DisplacementElement
        DisplacementElement dispElem = DisplacementElement.Create(doc, new ElementId[] { roof.Id }, new XYZ(10, 0, 20), view, null);

        // Create the path associated to the element
        DisplacementPath.Create(doc, dispElem, edgeRef, 0.5);
        t.Commit();
    }
}

private static Reference GetHorizontalEdgeReference(Element elem)
{
    //Find target edge from lower face of roof
    Options options = new Options();
    options.ComputeReferences = true;

    GeometryElement geomElem = elem.get_Geometry(options);

    foreach (var geomObj in geomElem)
    {
        if (geomObj is Solid)
        {
            Solid solid = geomObj as Solid;
            var faces = solid.Faces;

            foreach (Face face in faces)
            {
                BoundingBoxUV box = face.GetBoundingBox();
                UV midpoint = (box.Min + box.Max) / 2.0;
                if (face.ComputeNormal(midpoint).Normalize().Z < -0.1) // Downward facing, this is good enough
                {
                    var edgeLoops = face.EdgeLoops;
                    foreach (EdgeArray edgeArray in edgeLoops)
                    {
                        foreach (Edge edge in edgeArray)
                        {
                            // horizontal?
                            if (Math.Abs(edge.AsCurve().ComputeDerivatives(0.0, true).BasisX.DotProduct(XYZ.BasisZ)) - 1 <= 0.00001)
                            {
                                return edge.Reference;
                            }
                        }
                    }
                }
            }
        }
    }

    return null;
}

Public Shared Sub CreateDisplacementAndPath(doc As Document, view As View)
    ' Find roof
    Dim fec As New FilteredElementCollector(doc)
    fec.OfClass(GetType(RoofBase))
    Dim roof As RoofBase = TryCast(fec.FirstElement(), RoofBase)

    ' Get a geometric reference for the path
    Dim edgeRef As Reference = GetHorizontalEdgeReference(roof)

    Using t As New Transaction(doc, "CreateDisplacementAndPath")
        t.Start()
        ' Create a new top level DisplacementElement
        Dim dispElem As DisplacementElement = DisplacementElement.Create(doc, New ElementId() {roof.Id}, New XYZ(10, 0, 20), view, Nothing)

        ' Create the path associated to the element
        DisplacementPath.Create(doc, dispElem, edgeRef, 0.5)
        t.Commit()
    End Using
End Sub

Private Shared Function GetHorizontalEdgeReference(elem As Element) As Reference
    'Find target edge from lower face of roof
    Dim options As New Options()
    options.ComputeReferences = True

    Dim geomElem As GeometryElement = elem.Geometry(options)

    For Each geomObj As GeometryObject In geomElem
        If TypeOf geomObj Is Solid Then
            Dim solid As Solid = TryCast(geomObj, Solid)
            Dim faces = solid.Faces

            For Each face As Face In faces
                Dim box As BoundingBoxUV = face.GetBoundingBox()
                Dim midpoint As UV = (box.Min + box.Max) / 2.0
                If face.ComputeNormal(midpoint).Normalize().Z < -0.1 Then
                    ' Downward facing, this is good enough
                    Dim edgeLoops = face.EdgeLoops
                    For Each edgeArray As EdgeArray In edgeLoops
                        For Each edge As Edge In edgeArray
                            ' horizontal?
                            If Math.Abs(edge.AsCurve().ComputeDerivatives(0.0, True).BasisX.DotProduct(XYZ.BasisZ)) - 1 <= 0.00001 Then
                                Return edge.Reference
                            End If
                        Next
                    Next
                End If
            Next
        End If
    Next

    Return Nothing
End Function