There are plain ties and there are interlocking tiles

Interlocking tiles are cheap they are heavy and don't keep out the weather very well. In fact if they were not underfelted the roof covered with interlocking tiles would leak. It also follows that when after a sustained period of wet weather the under felt if of the type 1F will rot and then the roof will leak

This happens mostly at the eaves or near a valley.

See the pictures here for a view of a 12 year old roof.

Whet happens when felt gets wet near a valley and deteriorates
or is not stretched tight. The problem here is that as the rain penetrates the roof it will run down the felt until it meets the valley and the runs down the side of the valley and puddles at the bottom if there has not been a sufficient tilt fillet installed to stop ponding.

The same thing happens where a valley discharges onto a roof, the greater amount of water penetrates causing premature failure of the under felt.

Any ponding will cause the felt to fail this happens at the eaves and work across the roof eventually rotting the tilt fillet

All we do is remove the tiles cut new plywood tilt fillet and lay a plastic dpc type membrane over dressed into the spout then at least replace the fist run of felt with a breathable membrane type felt.

Pointing along the valley will only peel of and is never the source of penetration

Plain tiles overlap that much like slate that traditionally under felt was not used.