A layer of bitumous felt immediately under the tiles or slates on a pitched roof is normally found in houses.
Is this purely a precaution against rain ingress due to broken tiles or is there another reason for it?
The felt generally projects a few inches beyond the edge of the lowest course of tiles and ends within the reach of the guttering. However, this projecting portion typically perishes and didintegrates within about 20 years. Why don't they use something that lasts longer? Apart from softwood window frames, it's hard to think of any building material that is so short-lived.
Al