Who pays for leakage near common drain pipe area?

I live on the 11th floor of a 23-storey housing society. There’s a small balcony/flower bed outside my hall and a common vertical drain pipe built by the builder. I haven’t made any alterations, and there’s no leakage on my floor. However, the flat below is experiencing leakage around that pipe area. Who is responsible for the waterproofing costs in this case?