roof vents?

We have find that our house lacks roof ventilation. The roof was re built, slate, some 20 years ago with non-breathing felt. The house is made up of 3 old cottages, 400 to 200 years old. The roof rafters rest directly on to the stone rubble walls.
The felt overlaps are very tight; I have looked at creating "ventilation " at the overlaps but only started to damage it.
There was water dripping down from the felt. I found a water tank in airing cupboard below the ceiling, which had no cover and many holes holes into the roof. The tank Its now fitted with thick plastic a cover, clips fitted round all pipes into/out and the holes are sealed.
The three spotlights are now fitted with plaster board covers to seal
them off and the two loft hatches now have with seal’s and "sash window
fasteners" to clamp them.
But what ever I do to prevent condensation some will get into the roof and there is no ventilation to clear what’s now there and any new developing.
The roof is in an L shape mainly facing south. There one dividing wall that is nearly to the ridge 2/3 along it the roof. There is one end wall, west, that can have air vents put in. The roof floor area is about 600sq feet, which I have found needs 1sq ft per 300sq ft. But I haven’t been able to find out any guidance on location the roof vents that will be needed. Clearly location will effect how well they work. Can any one point me towards somewhere that will help?