Seal dormer flashing from outside??

[Dr Rock]

I'm just wondering if it is possible to seal the edges of the flashing that lies at the join between the roof and a dormer extension, so that water goes down the flashing instead of seeping under the tiles?

The roof is about 100 years old and it doesn't leak normally, just that in heavy rain the flashing fills up and then water seeps into the inside. I'd rather have an external fix to stop water entering, than bodge from the inside.

 

[Nige F]

You can get lead sealer in cartridges.........

 

[^woody^]

It depends if the flashing is just flashing, or if it is forming a secret gutter.

Some sort of image of the flashing would help determine the problem and a possible solution