Roof felt on tiled roof 1960s

your tiles are the main protection, your under felt is your secondary protection, your roof tiles deteriorate over time they are also only about 90% efficient some water will get under your tiles through badly fitting tiles, tile deterioration movement by traffic & wind this will allow water ingress under your tiles if the felt has rotted this will allow water into your timbers which in turn will rot, there has always been a means of catching any water ingress under your roof tiles before modern felts it was lime mortar , the bit of your roof that is prone to rotting is the ends of your trusses & wall plate that supports them it can also create dampness through the brickwork, water falls, bricks are porous, under felt is an essential part of keeping your property water tight & free from damp. most modern membranes are water proof & uv resistant for only a few months if mot covered with tiles the better ones will last 30+ years the cheepo ones will last about 2yrs most breather membranes still require the loft space to be vented even the likes of tyvak insurance company's recommend you replace the under felt every 25+ yrs good luck Alex

Actually there is somthing called capillary action, this is when water tracks back under the tile hits the felt runs down back into the gutter. In the 1960s all tiles/slates used to be back pointed to soak this up as there was no felt back then. So the answer to you question is that if the correct overlap on the felt has been used it will create a waterproof layer before the tiles get laid. After the roof has been finished it also has an important job too. If the tiles are laid with the correct overlap and the pitch of the roof is correct which we can assume that it is, then the capillary action should be kept to a minmum.