The roof has been lined with felt to act as a secondary barrier to rain.

Its not a lot of use having the under side of the roof lined, if there was no felt there originally then the proper way is, strip the roof of all tiles and battens, fit breathable roofing felt, batten and retile, you will have to have scaffold erected for scaffold, and allow a percentage for breakages. A lot of older properties never had felt fitted, and this wasnt a problem, if the roof was tiled correctly you shouldnt have any leakage, only maybe from wind driven rain or snow. If your in doubt about the purchase, get a couple of quotes for the above works and bear this in mind when you put an offer in for the house.

there is no problem, just means your roof is underfelted this is a good thing there is nothing to fix

If the felt is on the inside of the tiles, under the laths, then it will be known as sarking felt. This is normal practice for houses built after the war. Newer houses have lighter synthetic materials which do the same job, but will be known as Tyvek or similar. See http://www.palgrave.com/engineering/riley1/students/chapter/chapter10_1.htm for an overview of roof construction, or Google "sarking felt".

hi, in layman terms they mean that there is a covering of tiling felt on top of the rafters but below the battens and the tiles dont worry as long as it is not damaged there is no problem, this is standard practice but now most people use a breathable membrane when re-tiling a roof. hope this puts your mind at ease. shaun.

most tiled / slated roofs are underlined with a felt embrane which acts as a secondary barrier to water ingress and maintains a dust barrier - old roofs still exist with no felt lining but very rare now - providing the felt is on the outside of the rafters i.e directly underneath the tiles and the battens thenthis is perfectly normal - if however the felt has been nailed up inside the roof space then this will require further attention