@nnnah and @Lilly_LL if the principal applicant applied to extend his work permit before it expired, he can still continue to work. That must be to extend the same kind of work permit though. He will be on implied status until CIC makes a decision on the extension of the work permit, so he will be allowed to continue working.
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=188&top=17
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=188&top=17