If you have 3D studio max export it to there then apply an optimizer modifier…Should shrink considerably unless there is a need for all that geometry which I can’t imagine for a roof.
Edit - Never mind forget this. If this is the way you were going this would have been obvious. There would be a lot of work involved with retexturing, skinning and weight painting. Probably beyond the scope of what you are trying to do. Good luck with it however.