Look at the red and purple curves in the uploaded image.
The red curve does not close the ceiling even after ExtrudeCrv.
The red curve is an edited version of the purple curve.
I don’t know why, could you please take a look at the data
try running curve boolean to clean up the curves first, then extrude solid
Thank you for the advice.
I tried. But it wasn’t capped.
make a planar surface from your curves 1st and extrude that instead of the curves
Thank you for the advice.
I can do it.
This was due to the fact that the curve was slightly distorted and not flat.
that’ll break it every time … 
glad you got it sorted.
