Surface tension gutter covers conquer heavy rain.
During heavy rain water jumpa over 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.
Foam or sponge based inserts go into your gutters and allow water to trickle through the foam into the gutter and downspout.
First your gutters may be clogged with leaves.
Next is the size of the gutter itself.
Drill rivet holes for the gutter splash guard.
However during heavy rain water may not move quickly enough through the foam.
Too few downspouts will tend to make the gutters overflow during a heavy rain.
Holes through the splash guard and the gutter.
With surface tension covers leaves slip off the edge while water moves into the gutters.
Think of it this way.
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.
If the gutter is tilting away from your house it could be causing an overflow particularly during heavy storms.
A 5 inch gutter is typical but 6 inch gutters are also available.
Numerous people have tried ideas but none have worked.
One is that the gutter is not pitched so the area where the spillover was happening is actually a low point in the gutter.
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.