The old adobe house I'm working on has a flat roof that has one of those stretchable membranes on it, with seams all over the place. The roof is supposed to be pitched slightly to allow the water to drain through a hole in the side and then down the downspout.
Problem is, the water doesn't drain properly, and there is a large pool that collects in the middle, which eventually seeps through and leaks into his living room. The owner wanted someone to jack up the roof to restore the correct pitch, but the contractor refused, because he thought it would cause structural damage. Now the owner wants me to put some kind of water sealant in the area that the water collects. I don't know of any roofing product that can adhere to this plastic membrane and withstand being covered in water for weeks at a time.
Is there such a product?