The biggest issue here is the sun. The sun opens up cracks in pretty much every construction material, and then as soon as it rains, water comes in. I struggled for a long time with leaks in the roof at my hotel. I tried using all manners of waterproofers, but nothing seemed to fix the problem, at least not long-term.
I finally found a company in Santo Domingo called Unimper, you can search their Facebook page by entering Unimper SRL Impermeabilizantes Para Techos into Facebook. They aren't cheap, but they offer a 10 year guarantee and use an imported product which works really well. I would thoroughly recommend them, they have fixed leaks in 4 different roofs for me so far and they are due to start working on my new apartment this week. The owner is a guy called Daniel, I think he may speak some English.