Here is a link to your state's landlord tenant laws that are applicable to this issue.
Practically speaking, you should take the following actions:
1) Review your lease carefully and follow the provisions for landlord repair notices (do so in writing every time)
2) Contact your local codes enforcement office and report the landlord for failing to keep the roof maintained
3) Contact a local attorney and have them write a letter (fairly inexpensive) to your landlord that clearly states you will sue for breach if the repair is not made within 10 days
If the landlord fails to respond to this, you may need to sue for constructive eviction and breach of contract. You will be entitled to damages that could include costs of relocation and any difference in rent you must pay a new landlord.
Please reply if I can help further.
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