Use enough water to mimic heavy rains.
How to stop a slate roof leaking.
All roof leaks must be fixed from the outside.
Have your helper stay inside the house waiting for the drip to appear.
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.
Apply tar to the area and press blocking material into the layer of tar to cover the leak.
Run water over the low side of the penetration for several minutes while the helper watches for any water droplets.
Go nuts with the vapor.
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.
Insulate the roof if you are going to finish the attic if not insulate the ceiling.
You could make a temporary patch using stick down materials or on some roofs i ve seen roof mastic used in that application.
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.
Once you ve found the problem slate you need to remove it.
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.
To do this use the slate ripper to rip off the nail that hooks the slate onto the slate roof.
With good wire cutters you might also be able to cut off the head of the nail which will help.
You can use either a dry bonding gutter or the standard bonding gutter.
Details are at slate roof leaks temporary repair.
Locate the source of the leak by examining the underside of the roof from within your attic or crawl space.
Isolate areas when you run the hose.
Insulate the smallest space possible.
You might find problems removing the nail with the slate ripper.