It's a bitumen felted roof that had some bitumen product over the years on the hairline cracks maybe, would be an idea to put a small pitch roof tiles over it then forget about it.
there are afew manufactureres of glassfibre strand coating that is like thick solvent based porrage, (Grey in colour). We put some on our roof in 1991 and its still water tight and looks to last a lot longer yet. Its about £65 for 15 ltrs but worth it.
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