I am restoring a victorian semi at the moment and the roof has just been restored. The roofers stripped the tiles and laths and layed a water proof breathable membrain instead of felt. This membrain is so good that the house is water tight as soon as it goes on so the tiling can then be done in any weather. Make sure that if you are in a semi that your roofers strip and seal over the party wall and if you share any chimney stacks that they lead flash over next doors half as well to get a good seal.