w0nderkid

I have a strip of material on top of the felt at the bottom of our roof, it seems to be breaking up/tearing and doesn't look fit for purpose. Does anyone know what this might be(doesn't look like bitumen as my neighbour suggests) and what it's purpose is? Do I need to get it replaced? Thanks in advance

Sorry I'm not very technical. There are no hips or valleys it's just a basic pitched roof with concrete tiles. Need the rafters replacing due to rot. I'm just after rough figures as I literally have no idea (am I looking at £1000? 2000? 5000? More) Thanks again for your help

As title states, any ideas roughly how much this would cost me to replace my ageing roof? Thanks in advance..