From the photos, it looks very nicely done....What scale is this? "O"?
I personally am not a big fan of removable roofs...I like looking into a structure from the scale human p.o.v. Most all the lift-off roof work I have seen was somewhat ill fitting, or didn't age well over time (ie warped, or broke in some way". But then that's just my own wierd opinion....I'm a guy that likes "basics" ...and not all kinds of "gimmicky" things on models. (gimmicky not meant in derogatory manner towards your idea.)
If you do go with a removeable roof, your biggest challenge will be to work around the stack, and keep it fittig/sitting correctly all around, over the long haul.
I guess you could always make only a portion/section of the roof removeable, and make it so that it blends on with the fixed section....this way the stack could remain/be located in the fixed section. and you would avoid the problem of how to remove the roof around it.
Somewhat of example of what I am suggesting was done by Chuck Doan on his Locomotive Repair shed Dio.

Marc