Roof tiles/slates are used to to keep out any rain, there is a risk that strong winds can push rain through gaps or underneath the tiles on your roof. The importance of underlay used to carry this water to the guttering. Time and continued exposure to the rain, the underlay can begin to rot, not function effectively and finally fail, which will cause a roof to leak.
Loose, Damaged or Slipped Tiles/Slates
Slipped tiles or slates can cause large amounts of rainwater to land on the underlay thus putting it under more pressure that it can deal with. This causes pools of water that build up, where a tile or slate has slipped, a leak is more likely to happen.
Damaged Lead flashing and Roofing Valleys
The most common cause of a leaks is through the incorrect installation or damage to the lead flashing at valleys on the roof. Any damage can grow and quickly after a large amount of water and this will result in a leak.
One of the silent enemies of your home is condensation a poorly ventilated roof can lead to condensation forming on the under the underlay or tiles. This can eventually lead to droplets of water leaking through your ceiling, leaving dark patches this is a direct cause of a leaking roof.