This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
When a roof leaks, it will generally be found that the slates or tiles are laid on laths without boarding or waterproof paper or felt Possibly the leak occurs at a broken slate or tile; the defect may be remedied by the insertion of a new one, secured by a holdfast of lead or zinc Sometimes the slates or tiles are laid with insufficient lap, and the slightest defect in the "torching" or "pointing" permits the ingress of water; in such a case the pointing must be periodically examined and repaired, or the roof-covering must be taken off and relaid in a more substantial manner, as explained in Chapter VII (Lightning-Conductors). Section II., Vol. I. In exposed situations external pointing is sometimes adopted, but it is always unsightly and does not last long.
The leadwork of roofs is a common source of leaks. Perhaps the lead was originally laid in sheets of excessive size, bo that they were not free to expand and contract without crumpling the lead into folds; when this is the ease, the cracking of the lead is only a question of time. Soldering the crack is merely a temporary remedy; the only permanent cure is to relay the lead with suitable rolls and drips, so that every piece is free to expand and contract Leaks in leadwork may be temporarily stopped by coverim: the crack with a tape well painted with white lead.
The lack of snow-boards often allows dampness beneath lead flats and gutters, as the melting snow is prevented from rinding its Way to the outlet, and as this is in all probability blocked with snow. When snow-boards are provided, not only is the snow prevented from coming into contact with the gutter or Hat. but, on melting, the water has a clear passage to the outlet. It is an easy matter to place snow-boards on roofs, as no alteration of the existing work is required
The stoppage of rain-water pipes, especially those leading from flats and gutters, is a frequent cause of dampness in the rooms beneath. Usually the entrance to the pipe is choked with leaves. The remedy is simple. and consists in inserting a cage of copper or galvanized wire into the top of the pipe, and in taking care to have the roofs regularly cleared particularly in autumn.
If, as is frequently the case in old buildings the woodwork of the roof has given way, it may sometimes be sufficient to strengthen the timbers by bolting new pieces to them. But if the yielding of the framework has resulted in considerable damage to the slates or tiles, the best way will be to remove the roof and construct a new one.