in a simple way see (image 1 ) that showing name of layer syntax that should be in your Autocad files you send it to us and sure to get your result you are looking for
Image 1 Chimney Syntax
To draw the Chimney, it must be ensured that the shape is closed curves, see (image 2).
sure if they are many Chimney in your roof you can draw them all in same layer see (image 3) example.
Curves and lines must be closed
Image 2 Chimney drawing Way
Image 3 Chimney in one floor