Slate Clad Dormer Lead Soaker

[iCarley]

Hi, we're having a loft conversion and our builders have not put lead soakers down the external corner of our new slate clad dormer (see diagram). When I raised it they've said rain won't get in I can see the wooden batten through the tiny gaps in the corner so rain must be able to get to the battens? Should they take off the slates and add the soakers or am I over worrying and it doesn't it matter about a tiny bit of water ingress to the battens. Many thanks, Ian.