Contract estimates question

[Alex Popescu]

Hi all, my contractor is installing a new drywall ceiling and the joists are very crooked - more crooked than he anticipated (and I agree with him). Now he asking for more money. Can he charge more than the estimate that he gave me when the job started. The joists were visible then too. Should he have not take that into consideration? Can he ask me for more money? Thank you for any reply in advance.