Nearly all old roofs were lime torched. Whether slate, clay tile or stone flags, lime was used to bed the tile, or beneath them to prevent undue wind, and hold condensation. Lime acts as a sponge to hold water, and release it as conditions change.
We constantly battle to prevent roofers from stripping entire roof pitches, just because they see torching and say "Oh - an old roof without felt - it needs felt..." That roof has survived for probably 100 years - far longer than a new roof over felt will last - so why replace it? We advocate leaving it, carrying out repairs where needed, and if you MUST replace the roof, do it properly, the way we show you here.
It won't repair damage - so if lath has rotted or moved, you need to fix it first. If the pegs are rotted, they need replacement first. Lime is applied to the underside of the tiles, with a good mix of hair to make it flexible. Its a traditional way of having roofing felt, without taking the roof off!