We’ve already discussed the consequences of a blocked gutter. A valley gutter in your roof creates a similar problem. Sometimes, they are an unavoidable architectural feature.
They should preferably be lined with hard wearing lead which is lapped under the adjacent tiles or slates to create a “seamless” joint.
As with guttering, they are a great place for leaves, moss and other debris to collect. If allowed to accumulate a blockage occurs which will cause rainwater to over spill resulting in damage to your wall and ceiling plaster, decorations, and in the worst cases an outbreak of Dry Rot.
So, take the trouble to ask your friendly local builder to check any valleys your roof may have every now and then to make sure they look OK.