One of the reasons is that your shingles may be too short.
Incorrect drip edge gutter installation.
It should never be installed outside the gutter i e against the fascia board.
A gutter apron is used where shingles are not far enough from the edge of the gutter.
Home centers sell rolls of 6 in.
A gutter apron is installed on the corners above roof decking.
With what you have there you d probably be better to install a strip of metal under the drip edge and over the gutter.
The guttering should run a few inches lower than your roofline.
Use tin snips to cut the gutter apron flashing roll in two 3 in.
For proper water drainage the drip edge on the rakes should always overlap the drip edge on the eaves.
Most premade drip edge flashin.
Not sure if there s a name for it but i call it drip check.
Water stains dirt debris and dripping water behind a gutter are usually the result of poorly installed or missing drip edge.
Make sure the drip edge coming from the shingle is bending against the fascia board because when it rains it s going.
The best practice is to always install the drip edge so that it is inside the gutter.
On the eave side of a house such as this i install a 3 x3 piece of flashing to slide the gutter under.
Rain gutter installation can be a tricky job and a lot of.
Not an uncommon problem.
It is put under the shingles of an attic to flow water directly into the gutter.
If there s a drip edge installed where the fascia meets your shingles and the gutter is hung below the drip edge get some roll flashing and tuck it up under the drip edge and over the top of the gutter.
The whole purpose is to direct the water into the gutters.
There are two reasons for water getting behind the gutter.