This roof has reached the end of its usefulness.
The timbers have “spread” causing the ridge to sag. The clay tiles are evidently weathered. They have been coated with a bitumen type of paint. This essentially sticks them together to stop them slipping as that would allow the rain in.
It is a stop gap measure. When the tiles need replacing none of the existing ones which may have been in reasonable condition can be re-used. This adds to the cost of replacement as a whole new batch will have to be ordered.
The supporting timbers will have to be strengthened, or depending upon their condition, completely replaced. The underside of the tiles will be underlaid with a sheet membrane lining which is breathable (like a good quality rainproof coat)to help avoid condensation problems.