How can I shell this polysurface and make it solid

Hi everyone;
I have been trying to use the surface as roo of my building. The shell command either using offset surface command doesn’t make it solid. Is there any other methods which I can use to have solid end product?
I will also attach the file thank you for your help.
Regards.Problem.3dm (2.0 MB)

What is your definition of solid? Rhino doesn’t work in solids, it works in Breps.

I mean when I use shell command it tells me that " the shell result is not solid" and also it gives errors when I try to boolean differance on the shelled object.

A solid in Rhino is a closed polysurface with no “naked” edges and no non-manifold edges. Use ShowEdges with the Naked edges option selected to find naked edges. The closed shape has naked edges.

Shell needs a solid (closed polysurface without naked edges) as the input.

Hello - your top surface is suspect to me - is it meant to be a smooth undulating surface? The perimeter object is also not very clean - I would go back to the original clean base curve and extrude that to get the sidewalls, this one:

Then, make a clean top surface (I lofted through the long isocurves)

Trim them to each other:


Join, Cap, and Shell:

In short - make your surfaces clean and as simple as possible.