Construction materials are generally flat so we avoid surfaces that curve in two directions.
A 3 sided shape (a triangle) will always be flat, but a 4 (or more) sided shape can end up curving in two directions.
There is no easy way in Revit to create, or ensure, or test, that a surface is flat. There are various tricks, which all have limitations. Below are some of the methods that can be used to create planar surfaces.
GENERAL MASS MODELLING
A quick method is to use an extruded void to cut at the plane you want to create.
Create a solid form.
Place a Reference Plane that defines the top you want (i.e. the plane to cut solid). Give the Reference Plane a name.
Create an extruded void form hosted on the reference plane. Make it the same size or bigger than the solid form.
Cut the void from the solid form.
If the reference plane is moved the void (and cut) will move with it.
CONCEPTUAL MODELLING
In conceptual modelling (mass or adaptive generic model) create a triangular plane from lines between 3 points.
Use that plane as the host for other points.
Then create a multi-sided plane or solid from lines drawn between the all of the points.
When any of the original 3 points are moved the plane hosted points will move up and down (Z direction) to maintain a flat plane.
Make the triangular plane invisible by unticking its Visible parameter (don't delete it).
ADAPTIVE FAMILY
An adaptive Generic Model can be created where the 4 corners are placed when inserting the family. The first 3 points define the slope, the 4th point's height is set by the family so that a flat plane is created.
HOW TO CREATE A 4 POINT PLANAR ADAPTIVE FAMILY
PLANAR CONCEPTUAL MODELLING PANELS
Panels in a conceptual model divided surface that stay flat.
Similar to method to creating an adaptive family.
Video from Buildz blog: