Re: Rain Gutters -- Handling Heavy Rains
My suspicion is that the problem lays within the downspout conduits themselves.
The most common manner of making downspouts is to take a long length of conduit and cut a "birds mouth" for each of the bends. The upper part of the conduit is folded into the lower section to direct water down the conduit. The problem with this method is that it effectively reduces the inner diameter of the conduit significantly, as well as creates places for debris to catch, further restricting the conduit and causing back-ups..
The first thing I'd recommend is that you remove the downspouts and back-flush them by running a garden hose up from the bottom. Allow the conduit to fill with water before lifting the bottom end above the top inlet and rapidly washing out the conduit. This head of water will push out any debris that is there.
This should cure the problem for the moment. If you want to fully rectify the issue, then you need to purchase 45* or 60* conduit elbows to replace the existing cut/bent elbows. Once you've got a full dimension conduit running from top to bottom, you shouldn't have any more overflow issues.
One other thing to check is how level the gutters are. I've seen some so far out of whack that one end will overflow before the water can reach the downspout on the opposite end. In cases like this, I add a downspout to the low end. If you've got downspouts on each end of long runs such as this, and you've ruled out blockages in the conduits and drains, then adding another downspout midway in the length of the gutter would be helpful.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!