Think of it this way.
During heavy rain water jumpa over the gutter.
Numerous people have tried ideas but none have worked.
The second thing that causes this is either insufficiently sized downspouts for the run or else a clogged downspout often up near the top of the downspout but still in the gutter.
Drill rivet holes for the gutter splash guard.
Holes through the splash guard and the gutter.
Same goes for lack of pitch which causes the water to gather in one spot rather than flowing toward an exit.
The pitch of gutter heading towards a downspout can also cause overflow if it s too steep.
However during heavy rain water may not move quickly enough through the foam.
You can find these precut gutter splash guards about 3 in brown and white aluminum at a home center but you could easily make your own out of aluminum or sheet metal and spray paint them to match your gutters.
With surface tension covers leaves slip off the edge while water moves into the gutters.
There can be several reasons why rain is coming over the gutter instead of staying in the gutter and exiting the downspout.
Foam or sponge based inserts go into your gutters and allow water to trickle through the foam into the gutter and downspout.
A 5 inch gutter is typical but 6 inch gutters are also available.
One is that the gutter is not pitched so the area where the spillover was happening is actually a low point in the gutter.
First your gutters may be clogged with leaves.
Currently there is a diverter moved a little way up the valley and black sponge intended for putting inside a gutter has been wired onto the mess guard on top of the gutter.
Next is the size of the gutter itself.
This valley shoots rain in a flume during our frequently heavy downpours.