It would depend on the condo community. Some condo HOA's have a covenants that the HOA will be responsible for all exterior repairs and maintenance including the roof. Some HOA's do not do the exterior repairs and only will do things like snow removal and mow the lawn. It can be very specific what they are responsible for and what the owner is responsible for. You would need to request a copy of the bylaws prior to making a purchase offer on the condo And read them.
Remember, each community is set up differently. Your Realtor should be able to provide you with a copy for each condo you are interested in. Don't assume all condo's are set up the same, it could be a costly mistake.