slate roof leaking water

[ENGINEER]

I have a slate roof on my bungalow and can see all slates are in gud condition but I get small leaks I cannot find where the ingress is.
My question is, can I seal the edges of the slates to stop water from entering underneath, I was going to use Bostik mastic and trowel it on along the sides of the slates.
Or if theres a spray that would do the trick much faster that anyone knows about.
Thanks to anyone that can sort me out as today has been a bit of a ........rained all day long.

 

[Cubist]

Before you do anything, send us a picture of the side of the roof you think has leaks. Slate or tile roofs need no bonding materials between the them.

 

[ENGINEER]

Before you do anything, send us a picture of the side of the roof you think has leaks. Slate or tile roofs need no bonding materials between the them.

Thanks for your reply will try to get a pic.
Whats happening is, the roof has a flat roof extension attached to it, the slates are all in gud nick about 40 yrs old, the rain water is hitting these slates and getting in at the sides of the slates, then finding its way thru the slates down to the flat felt roof, the water is then entering within the multilayers of felt, making its way across the flat roof then dropping thru the ceiling. I am not looking to employ a roofer after looking at my neighbours flat roof thats not even 12 months old and its like looking at several swimming pools.

 

[a twig]

If the flat felt roof is a newer addition to the 40 year old slates there is a good chance that it hasn’t been detailed in properly up beneath the first few courses of slates.

Had the same problem when my parents had their flat felt roofed porch re-done and water was coming in within days - I was suspicious that the slates hadn’t been touched, sure enough water coming in.

Felt hadn’t been properly run up behind the slates - instead just been cut at the base of the tiles siliconed to death. Took photos and sent to roofing company, got a full refund as they literally didn’t have a leg to stand on and then got a decent company in to properly detail a fibreglass roof and stood over them while they did it. Dry as a bone since

 

[a twig]

Detailing should look something like this: