If you find that this is a problem then you will need to repair your roof valley before anything serious happens such as plaster loss or even major water damage.
Patching a roof valley.
Clear any debris in the roof valley and surrounding area.
Measure the width of the sheet metal valley flashing.
Position an extension ladder against the lower edge of the roof near the valley that is repaired.
Typical valley flashing is 14 inches wide.
If the ice and water shield is missing then you will have to remove the shingles on both sides along the valley put ice and water shield in place and re shingle the roof.
A lead valley is a particularly difficult item to dispose of as lead is toxic and so a repair may involve simply patching up your lead roofing.
Take measurements of your roof flashing most flashing is 14 inches wide but yours may vary.
Move to the highest end of the valley and work your way down cleaning.