Assuming the gutters have the correct slope the main reason for overflows is that the gutters are full of debris and need to be cleaned.
Water leaking between gutter and fascia.
Water leaking in behind the facia of your roof could be caused by a couple of things.
If the gutter fills with water from organics knitting together over your downspout water can spill behind the gutter.
Unfortunately it doesn t always work that way.
1 your roof sheets have a hole in them or your screws or nails or equivalent are not watertight anymore or.
Over time rain gutters often start to pull away from the roof allowing water to drip over the gutter sides or between the gutters and the walls.
The gutters then direct the water to a downspout allowing the water to run down until it reaches the ground.
Without looking at the roof it seems that the water is missing the guttering because perhaps the previous owners may have had new fascias overlaid onto the old.
Any rainwater or in sparodic areas then simply misses the gutter.
Usually this is due to gutter fasteners rusting.
This can be frustrating and also creates a dangerous situation on porch steps during freez.
But what happens when your roof s gutters become worn out or cracked or clogged.
It s a common reason water gets between the gutter and fascia.
Have rain water dripping out from between your facia and gutters.
Water leaking between gutter and fascia when there is rain the water is expected to roll down the roof and into the gutters.
2 the more common reason is where roof sheets are cut too short or they don t overhang far enough into the gutter.
This then pushes the guttering out from the roof line by 9 17mm.
When water drips between the fascia and the gutters the problem could be a compromised drip edge but it s also possible the gutters are overflowing.