Inside damage because of roof leak

The by-laws say they are responsible for exterior maintenance,I was
out of town and in took a few day for the apt under me to show water damage by that time,my wood floor and furniture were damaged.They sent a cleaning team,to avoid further damage, and charged both owners? Is that legal?

What exactly did they charge both owners for? Generally repairs to the inside of your unit caused by damaged common elements are "no fault." The association has an obligation to fix the damaged roof and depending on your bylaws might have the obligation to repair the inside of the unit up to the original building specs. To the extent that you've upgraded your carpet or added furniture that's usually the responsibility of the owner. That's why you have homeowner's insurance.

Given that you were out of town I don't think there's any question that this constituted an emergency so the association was within its rights to enter the unit and take steps to mitigate further damage on your behalf.

I used a lot of weasel words "Generally" "might" and "usually" because a lot of it is going to depend on what your Bylaws say. Our Bylaws went on for almost three pages about these kinds of situations. Read yours carefully.