Common between 2 faces

Hi,

I need to check if two faces have a common area. Currently, I explore the faces of my solid commoning each in turn with the test face and use

TopoDS_Shape s = c.Shape();
GProp_GProps System;
GProp_GProps* gprops = new GProp_GProps();
BRepGProp::SurfaceProperties(s, *gprops);
gprops->Mass();

to test if mass > 0. However, the result is frequently zero when the two faces overlap. My question is do I have to set a Cascade precision variable in order to do a common between two faces?

Many thanks,

D.Bramall