A certified letter isn't just advisable, it's essential because without it the landlord can play dumb and claim he never received anything. Emails are fine as additional documentation, but the law requires a formal written notice with clear deadlines so you can then terminate the contract without notice. Without this step, you risk a judge telling you "yeah, he was right to do maintenance, but you didn't give him a chance to do it in time."
Send the certified letter with an explicit request to do the work within 15-30 days, specifying exactly what's not working. If he does nothing after that deadline, then you can terminate the contract by notifying him via certified letter as well, citing his breach. In the meantime, if the situation is really serious (like mold growing, no hot water, serious stuff), you can also contact a tenant association in your area - they often offer free consultations and sometimes send a letter to the landlord themselves, which has a pretty significant psychological effect.
One more thing: if you really don't want to wait too long and the leak is causing damage, you can also do the repairs yourself and then deduct the cost from next month's rent (keep the receipts), but this is risky if you don't know how to handle it properly, so honestly try the notice first. It usually works.