I have long decided if it deals with water getting into my house i hire a pro to do it.
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Where I am, squirrels discovered that chewed up lead tastes sweet, and have taken to eating it. This slowly poisons the squirrels and spreads lead in your garden. I recommend finding a different material.
You can see in the photo the far half of the extension already has a lead flashing and the near half just has a gap. It looks like the builders put silicone in the gap but most of that is long gone. I haven't noticed any odd behavior from squirrels, or ever seen one in the roof. This is in the U.K. btw - maybe we have different squirrels?
Much of the UK has the same gray squirrels that North America has: people deliberately moved them. Not every squirrel decides to eat lead, but it's definitely a thing.