Hi @djhg
The underlying geometry is skewed, indicating that the surface has somehow been forced into shape, eg. using JoinEdge or by lowering tolerances. If you run Untrim with KeepTrimObjects=Yes you’ll see the underlying plane is by no means flat in the Front viewport, while the trim curve used is correct/flat. You can simply use PlanarSrf on the trim curve to get the correct result, which will then offset without problems.
How the existing surface got so out of tolerance, I don’t know. If it was made in Rhino, maybe it’s worth looking into
HTH, Jakob
Thanks Jakob. My way out of this was to copy all the edges of the 2d surface and rebuild it with planarsrf. That one offset into an even slab. The initial object from which the original 2d surface came from an object which had undergone a fair amount of editing which I suppose added a problem.