Installing a bonding gutter this video shows a roof repair and the fitting of a bonding gutter between a slate and a tiled roof but it will also fit two slate roofs together as well.
How to stop a slate roof leaking.
Apply more tar over blocking material especially on the edges.
Begin soaking low on the roof just above where the leak is visible inside the house.
With good wire cutters you might also be able to cut off the head of the nail which will help.
How to repair a leaky roof from inside the house.
Isolate areas when you run the hose.
Details are at slate roof leaks temporary repair.
For example soak the downhill side of a chimney first then each side then the top on both sides.
Once you ve found the problem slate you need to remove it.
Apply tar to the area and press blocking material into the layer of tar to cover the leak.
To do this use the slate ripper to rip off the nail that hooks the slate onto the slate roof.
You can use either a dry bonding gutter or the standard bonding gutter.
Beware of using asphalt roof mastic on copper flashings it fixes the leak in the short run but corrodes the copper in the long run.
Use enough water to mimic heavy rains.
Have your helper stay inside the house waiting for the drip to appear.
Start low soaking the area just above where the leak appears in the house.
You might find problems removing the nail with the slate ripper.
You could make a temporary patch using stick down materials or on some roofs i ve seen roof mastic used in that application.
Shine a flashlight along the underside of the roof and look for a hole in the roof as well as areas that are wet or have been.
Insulate the smallest space possible.
Run water over the low side of the penetration for several minutes while the helper watches for any water droplets.
All roof leaks must be fixed from the outside.