I've just moved into a 1950's semi where the roof is in good condition but the bitumen building paper under the roof tiles has started to deteriorate and crumble. I'm using the central part of the loft for storage. I'm just wondering if there is any point in stapling a membrane under the rafters (and tiles) to reduce the drafts and any wind driven rain that gets past the original paper. I appreciate that the best thing would be to get the tiles lifted and the job done properly, but at the moment it's not that bad and I'd rather wait until I have to do it (and can more afford it). There would still be more than enough ventilation to stop any condensation issues, but I'm thinking it will improve the overall effectiveness of the insulation if I stop the howling gale in high winds.
Personally I wouldn't bother yet, the tiles are the waterproofer, not the felt/membrane. Lots of old homes didn't have felt, just the battens and tiles, as you're only using it for storage I wouldn't worry about it until you are in more of a position to re-roof it properly.