Whenever I see a roof with box gutters, I anticipate some sort of problem.
…Especially when there are trees around. As you can see in the picture, this box gutter is full of leaves. Even though the right idea of draining the box gutter into a rainhead first – instead of into a downpipe, which is easier to block up….the leaves will still block the outlet of the box gutter. In this case, water filled up the box gutter and flooded into the building.
So whenever I am looking at a leak under a box gutter, I always ask about the volume of water that flows in. A large volume of water in a short time usually indicates that the box gutter overflowed. Where else a small trickle usually means that the box gutter is leaking – rather than overflowing.
So if you have a box gutter and you suddenly experience a large flood of water during a storm - then it is quite likely that you have a blocked outlet. You will probably find leaves to be the culprit. Just remove the leaves and the leak will disappear. But remember to regularly clean the box gutters also.
…At the same time, you should also check that the box gutter has a sufficient overflow. This overflow may actually stop any water from entering your property – even though the box gutter outlet is blocked.
For more on box guttering , rain heads and down pipes go to my new website at The Roofing Professionals Westside