These are caused by dirt that sits on the top edge of the eavestrough. The dirt gets washed over the front with the rain.
We have solutions for this if it is imperative that you never want to see tiger stripes. We have can use an upgraded version of the T-Rex which has a built in drip edge.
The picture shown is an example of how they can be cleaned off with great results. The key thing is to make sure it is not left too long or the dirt will stain the finish on the gutter. This gutter was almost 2 years old and cleaned up nicely.
The cleaner we used was Spray Nine which is a multi-purpose cleaner an can be found at Canadian Tire in Toronto.
Interested in a quote for eavestroughs or leafguard in Scaborough, North York or other areas in and near Toronto? Visit our site at www.eavestrough.ca