My roof does not have felt lining , how critical is it to have felt lining

It's not critical but it is important to stop water blowing in and damaging timber work.

Hi Luke, it is now mandatory to put a felt lining on every new pitched roof but many older roofs are still without. A felt lining provides security should the tiles/slates fail and improve the thermal efficiency of your home. The cost can vary depending on the size of your roof and the tile you want. Its possible to have a full re roof on an up and over for under three thousand using the right tile, obviously will increase with size of your home

It is not critical but is now mandatory to have breathable felt under tiles for a Plan B solution. Meaning if it goes through the tile the leak will roll down the felt into the gutter