Appreciate that there is a wide differences between different "qualities" of felt roof...
You basic thin "shed" felt applied as a single layer and nailed on, its unlikely to offer more than a few years of reliable service (if at all).
Whereas a hessian re-enforced 2mm thick SBR nail prep sheet glued or nailed on, then covered with 2mm SBR under felt, and finally finished with a 4mm thick SBR cap sheet with mineral finish will last decades as long as the substrate its fixed to is decent and stable.
 One of a number of trade names for a bitumen material modified with synthetic rubber or latex compounds to make it tougher and more flexible with age. Often available as "torch on" felts which are highly effective and very quick and easy to apply.
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