# Selecting bottom faces of exploded geometry

**URL:** <https://forum.dynamobim.com/t/selecting-bottom-faces-of-exploded-geometry/110229>\
**Category:** Revit\
**Tags:** geometry, dynamo\
**Created:** [May 19, 2025, 9:36pm UTC](https://forum.dynamobim.com/t/selecting-bottom-faces-of-exploded-geometry/110229 "2025-05-19T21:36:39Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![3bslamz](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.dynamobim.com/3bslamz/32/185167_2.png) [@3bslamz](https://forum.dynamobim.com/u/3bslamz)\
**Post date:** [May 19, 2025, 9:36pm UTC](https://forum.dynamobim.com/t/selecting-bottom-faces-of-exploded-geometry/110229/1 "2025-05-19T21:36:39Z")

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Hi everyone,  
I’m trying to select the the bottom face of each box (volume) after exploding them. (these boxes (geometry) were resulted from the intersection of columns with floor elements retrieved from revit).

I’m doing so by:

- getting the center point of each face for every single box.
- extracting the z values from these center points
- find the minimum z value in a surface for each box
- finally sorting the faces according to the lowest z values.

But for some reason I can’t achieve the desired results. The final step is what I think I’m not doing properly, or maybe the whole approach is just not good, idk.

 ![Screenshot 2025-05-20 001229](https://us1.discourse-cdn.com/flex022/uploads/dynamobim/original/3X/6/4/6443919b924ca100ec3fc9d26661c0088da58e71.png)  
 ![Screenshot 2025-05-20 001316](https://us1.discourse-cdn.com/flex022/uploads/dynamobim/original/3X/e/0/e0c9d3e16fb61781b741f88249c65d6a2181a336.png)

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<div class="post-metadata">

**Author:** ![Mike.Buttery](https://avatars.discourse-cdn.com/v4/letter/m/b19c9b/32.png) [@Mike.Buttery](https://forum.dynamobim.com/u/Mike.Buttery)\
**Post date:** [May 19, 2025, 11:39pm UTC](https://forum.dynamobim.com/t/selecting-bottom-faces-of-exploded-geometry/110229/2 "2025-05-19T23:39:40Z")

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Try using the surface normal - the bottom face should have the negative Z vector (0, 0, -1)

 ![bottom_face_by_surface_normal](https://us1.discourse-cdn.com/flex022/uploads/dynamobim/original/3X/7/d/7d27acaf2fb06ea61cdfc7943fedd581b89cf45c.png)  
 ![image](https://us1.discourse-cdn.com/flex022/uploads/dynamobim/original/3X/d/2/d2fc20ea1c84843c7e2ccfc13d866da99924b12b.png)

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<div class="post-metadata">

**Author:** ![3bslamz](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.dynamobim.com/3bslamz/32/185167_2.png) [@3bslamz](https://forum.dynamobim.com/u/3bslamz)\
**Post date:** [May 20, 2025, 4:34am UTC](https://forum.dynamobim.com/t/selecting-bottom-faces-of-exploded-geometry/110229/3 "2025-05-20T04:34:39Z")

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Thanks a lot Mike, that was helpful