Most gutter systems around have the troublesome ‘box miters’ on every corner. These ‘box miters’ are a pre-made piece that anyone can pick up at the hardware & install on any gutter. The problem that we have with this is that normally they do not fit very well because every gutter machine makes a gutter just a little differently. These box miters are normally made out of very thin aluminum & are easily bent out of shape. The other problem we have with the box miter is the seam issue. A box miter always wraps around the corner, so you end up with 3 seams to seal & keep from leaking. The potential for leaking problems is much higher simply because there are more seams and joints.
The correct way to make a corner is to use what we call a custom miter. This is when the gutter material itself is marked, cut to fit & fastened together. This corner is much stronger, looks much better, & has less potential to cause problems as the gutter ages or during heavy use.