Hello, and welcome. I am a licensed attorney and happy to assist.
I'm sorry to hear of the situation, but happy to say that you do have recourse. Your lease provides that you have the duty to maintain the premises, but that's not the issue here. The issue is not maintenance, it is repair. A commercial landlord is under duty to repair commercial property rented to a tenant, And those repairs include the roof, electrical, plumbing, etc. Don't be confused by the portion of the clause that says "in good repair" because that is a modifying phrase that modifies the state of maintenance. Again, maintenance is a very different issue than repair of the damage you described, which has absolutely nothing to do with the ordinary maintenance that can be imparted to even a commercial tenant. The only way you would be responsible for this is if the lease unambiguously so stated. Even if the landlord points to that maintenance clause, As mentioned, it is an applicable for the reasons stated, and even in the very unlikely event that a court could pause at the ambiguity, the court would be required to resolve the ambiguity in your favor, presuming the lease was drafted by the landlord, his agents, or his attorneys. You could also be strongly argued that "promises" is not the same as "structure." Just another thing that strengthens your position. For clarification, I will point out the obvious, that repair is neither a remodel, nor a structural or any other type of improvement. Repair is simply remediation to fix the problem and return it to the previous and expected condition.
You can clarify these facts in a very pointed letter to the landlord, informing the landlord that if the condition is not fixed it could affect your business and he could be liable. A landlord will have 30 days to respond, but there is no law that specifies the amount of time the landlord has to actually fix the problem. If the landlord refuses, you could take him to court.
I hope this helps. If you need additional information or clarification, just let me know and I'll be happy to continue providing assistance. If I have addressed your issue and/or pointed you in a positive direction, please let me know that as well, and please remember to leave a positive rating when prompted, as that is the only way attorneys on this site are credited for the information we provide.