Gutter Dripping at the Corner? A St. Louis Gutter Company on Miter Leaks

Why corners are the weak link

A run formed on site comes off the machine as one piece, end to end. No joints along the straight. But a run can't bend around the house, so at every corner two pieces get cut, fitted together and fastened. That's a joint, and every joint depends on sealant.

Aluminum moves with temperature. A long run on a south-facing wall grows on a July afternoon and shrinks on a January night. All that movement gets pushed into the corners, where it works against the sealant year after year. Eventually the bead lets go at one edge, and water finds the gap.

Strip miters and box miters

There are two common ways to build a corner. A strip miter is made by cutting both gutter ends at an angle and joining them with a narrow strip of metal riveted across the bottom seam. It's quick to make on site. It also has a long seam running right where water sits.

A box miter is a factory-formed corner piece that the two runs slide into. The seams sit up on the sides of the corner, not along its floor, so standing water has less to work on. We generally prefer box miters on replacement corners for that reason. They cost a little more, and on a corner that's never given trouble there's no reason to swap one out.

Rivets that have worked loose

Sealant isn't the only thing holding a corner together. Rivets or short screws lock the two pieces to each other. When a rivet loosens, the pieces shift a hair every time the metal expands, and that shifting tears the seal from the inside.

You'll sometimes see it as a rust-colored streak or a dark stain spreading from one rivet head. Before resealing, we drill out any loose rivet and set a new one, so the joint is tight again before fresh sealant goes on.

What a proper reseal involves

Squeezing new sealant over old sealant is the most common corner repair in St. Louis, and it rarely lasts. The new bead sticks to the old one, not to the metal, and the old one is what failed.

Done right, the corner gets scraped down to bare aluminum. Every bit of old sealant comes out. The metal is cleaned and dried, loose rivets are replaced, and a gutter-grade sealant goes in along the inside seam where water actually sits. It's slower. It holds.

Our page on leaking gutter seams covers end caps and joints along the run, which follow the same idea.

Old south city corners

On century-old brick two-stories around south city, the gutters are often small 4-inch troughs, and the corners have been patched more times than anyone remembers. We find layers of tar, roofing cement and three colors of caulk stacked on one joint. There's no clean metal left to seal to.

Those corners get replaced. On a short run it can make sense to replace the run itself. On a house where the rest of the gutter is sound, a new corner piece spliced in does the job.

Bent by a ladder, split by ice

Corners are where people lean ladders, because it feels sturdy. On thinner gutter it isn't. A bent corner opens its seams the same day. Ice is the other culprit: water trapped in a corner that won't drain freezes and pushes the joint apart from inside. Either way, a corner that's physically deformed won't seal, and we'll replace it rather than fill the gap.