Rainwater Runoff Issue

My question involves personal property located in the State of: Arizona

We live in a fourteen unit townhouse development that was originally built in the seventies. Our adjacent neighbor is complaining about rainwater runoff that he claims is damaging his property. We do not get much rain where we live but when it does rain it can be very heavy. Over a year ago we had our flat roof replaced and some work was done on the neighbors roof to avoid damage to his unit which he insisted on. Our unit is above his unit by about a foot and there are five scuppers along our common wall on the roof which have been there since we believe the townhouses were built. We don't know why they are there. Recently we had a very heavy rain and rainwater from our roof was pouring onto his roof and off of his roof into a gutter he has and then out to a common area. He is not pleased and would like us to do something about the runoff. We had the same roofing company out to look at the situation and they could not do anything about the runoff. The roofer proposed that the neighbor install a gutter along his side of the common wall to take the runoff to the street. Also, the roofer felt that the neighbor should pay as he may has caused the problem by having the roofer do things against their recommendation. The neighbor is now demanding that we pay for his gutter on the roof because it is our rainwater that is flowing onto his roof. The HOA board suggested an even split but he did not like that suggestion. Nothing to date has been resolved.

Question: Am I responsible for the rainwater runoff onto his roof and should I then be compelled to buy him a gutter?

Thank you, Bill

Rainwater Runoff Issue