It is important to remember that all planes that recede in perspective meet at a vanishing point that corresponds to the horizon line. In the case of inclines and declines this is also true. Incline vanishing points and decline vanishing points correspond to a vanishing point on the horizon line. They are placed directly above or below the related vanishing point on the horizon line. Again, the best way to find the incline vanishing point. Use a horizontal and approximate the angle, then continue the line until it intersects with a vertical line above (or below) the vanishing point on the horizon line. Your incline (or decline) vanishing point will be at this intersection.
To find the center of a roof pitch, draw an x from corner to corner, then place a vertical line through the central intersection. Your incline convergence lines will intersect with this line at the peak of the roof.
Check to see if you are drawing an incline or a decline. All decline vanishing points are below the horizon line. Often times when you are looking at an incline from underneath (like a cathedral ceiling) you are looking at a decline. Roads moving up or downhill and steps can be plotted using incline/decline vanishing points.