Can rotting underlay cause roof leaks?

Can rotting underlay cause roof leaks?

Absolutely, this is one of the most common causes of leaking roofs.  The underlay sits under the roof tiles and contributes to the weatherproofing of the roof.  If rain is driven by wind up under the roof tiles, then it is caught by the underlay and channelled down to the gutters at the roof edge.

A very common problem is for the underlay at the bottom to sag and rot away over time.  Once this happens, the channel to the gutter is blocked and any water that is sitting on the underlay will have to find another way out – hence the leak.