# Brep surface is not planar \[SOLVED\] **URL:** \ **Category:** Grasshopper\ **Tags:** solved\ **Created:** [19 January 2026 14:58 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579 "2026-01-19T14:58:40Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![netsonicyxf](https://avatars.discourse-cdn.com/v4/letter/n/f14d63/32.png) [@netsonicyxf](https://femdesign.discourse.group/u/netsonicyxf)\ **Post date:** [19 January 2026 14:58 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/1 "2026-01-19T14:58:40Z")
The grasshopper says it’s planar, but FEM-Design says it’s not. ![image](https://europe1.discourse-cdn.com/flex017/uploads/femdesign_api/original/1X/4c351b5c788d88ab8c5d1539c686fd20e1c3d9b9.png) [planar surface.gh](https://femdesign.discourse.group/uploads/short-url/gIe4rZK7x3MixBQOo1iUfaFQF3e.gh) (10.3 KB) ---
**Author:** ![MarcoPelle](https://dub1.discourse-cdn.com/flex017/user_avatar/femdesign.discourse.group/marcopelle/32/6_2.png) [@MarcoPelle](https://femdesign.discourse.group/u/MarcoPelle)\ **Post date:** [19 January 2026 15:20 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/2 "2026-01-19T15:20:59Z")
ehi @netsonicyxf FEM-Design has a tight tollerance to check the planarity. It seems that your slab is not planar for a tollerance which is less than 0.0001. Can you double check if you can make the surface planar with that tollerance? ![image](https://europe1.discourse-cdn.com/flex017/uploads/femdesign_api/original/1X/8333797ddc62e4d4d0edad6beec918aba53cae3e.png) It is a well known issue that we would like to fix in one of the future release. One other option would be to split the quad surface in 2 triangles. Triangle are planar by definitions. ---
**Author:** ![netsonicyxf](https://avatars.discourse-cdn.com/v4/letter/n/f14d63/32.png) [@netsonicyxf](https://femdesign.discourse.group/u/netsonicyxf)\ **Post date:** [20 January 2026 06:11 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/3 "2026-01-20T06:11:45Z")
It’s planar if the tol is 0.001mm and it’s not planar if the tol is 0.0001mm. Sap2000 can handle this type of surfaces, Is it possivle for FEM-Design to handl it in the near future? ---
**Author:** ![MarcoPelle](https://dub1.discourse-cdn.com/flex017/user_avatar/femdesign.discourse.group/marcopelle/32/6_2.png) [@MarcoPelle](https://femdesign.discourse.group/u/MarcoPelle)\ **Post date:** [20 January 2026 06:53 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/4 "2026-01-20T06:53:54Z")
Absolutely. The idea is to albe able to have a less strict tolerance when checking planarity. Is your geometry made in rhino or it comes from an import process from a different software? ---
**Author:** ![netsonicyxf](https://avatars.discourse-cdn.com/v4/letter/n/f14d63/32.png) [@netsonicyxf](https://femdesign.discourse.group/u/netsonicyxf)\ **Post date:** [20 January 2026 07:06 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/5 "2026-01-20T07:06:38Z")
The geometries are created in Rhino and meshed via a Rhino plugin called “Griddle”, which is a professional mesher for FEM analysis. ---
**Author:** ![netsonicyxf](https://avatars.discourse-cdn.com/v4/letter/n/f14d63/32.png) [@netsonicyxf](https://femdesign.discourse.group/u/netsonicyxf)\ **Post date:** [23 January 2026 05:38 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/6 "2026-01-23T05:38:53Z")
These new surfaces are planar with TOL of 0.0001m. But FEM-Design still shows they aren’t planar. ![image](https://europe1.discourse-cdn.com/flex017/uploads/femdesign_api/original/1X/b0d7200a298f83b4fa5fb6bda9b4f41cdebc9359.png) [1.gh](https://femdesign.discourse.group/uploads/short-url/gchE2MBxHmFoPaCDKInWXd8ExiI.gh) (208.3 KB) ---
**Author:** ![MarcoPelle](https://dub1.discourse-cdn.com/flex017/user_avatar/femdesign.discourse.group/marcopelle/32/6_2.png) [@MarcoPelle](https://femdesign.discourse.group/u/MarcoPelle)\ **Post date:** [23 January 2026 09:43 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/7 "2026-01-23T09:43:42Z")
ehi @netsonicyxf Beautiful structure! I did send the file to support. I need to double check with the developer. One question: Would it be an option for you to split the quad surfaces in two triangles? ---
**Author:** ![MarcoPelle](https://dub1.discourse-cdn.com/flex017/user_avatar/femdesign.discourse.group/marcopelle/32/6_2.png) [@MarcoPelle](https://femdesign.discourse.group/u/MarcoPelle)\ **Post date:** [23 January 2026 12:00 UTC](https://femdesign.discourse.group/t/brep-surface-is-not-planar-solved/579/8 "2026-01-23T12:00:55Z")
@netsonicyxf I have found out that tolerance in FEM-Design is really tight. We need to use a 0.0001 m tolerance 😥 However, for you case, you can recreate the middle step as shown in the script below. ![image](https://europe1.discourse-cdn.com/flex017/uploads/femdesign_api/original/1X/c02d411ba16cfd832330a61815051ddfc0fce525.jpeg) Let me know if it works!