Many thanks for your prompt advise.
Since I noticed the leakage, I used this product extensively; inside and outside of the join.
http://www.tec7.ie/
Still a bit of leak.
Maybe I should have used clear sealant.....
Or I need to give more slope.
I am installing a rain water harvesting system from two sides of double garage. Since the width of the garage is long enough, I need to connect two gutters with two down pipes installing them horizontally.
In the connecting section of these two downpipes, there's a water leak.
The downpipes...