Millipede Topostruct 3D minimal thickness Boundary Region

Hi,

I am trying to achieve a 3D topostruct optimization. I managed to get some interesting result in 2D, but when I try in 3D I don't get the desired output. 
My Boundary surface is supposed to be thin as it represents my facade. Is it too thin?

Can anyone bring me some expertise on this?

Thanks

Eliott