You say that your dad coated the roof, what did he use? I would advise against using any of the roll roof products, they will not adhere to the roof deck without adding some wood fiber board. Rubber could be an option, but it also needs to be applied over wood fiber board. Another option is Elastimeric coating. It needs to be pressure washed, primmed, then top coated to be effective. Any loose organic or unbonded roof caulking needs to be cleaned off first.
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