Get all bottom faces of brep

Hi Guy,
I have one Brep as in attachment. How I can get all face bottom of this brep
Thanks
Select all bottom face of brep.3dm (151.1 KB)

image

It would be simple in Grasshopper.

I use deconstruct brep then sort by Z value of center point
But it only get for one face bottom.
In my brep have three faces

You can calculate the dot product of the face normals and a Z-vector.

The filter the faces with the negative dot product…

Hi
For this Brep I found 3 faces but Z vector not same direction

brep_face_normals.gh (40.0 KB)

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Thank @martinsiegrist so much
It work fine

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Thanks, please hit the solution button

image

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