Volodymyr Lukaniuk 
A gutter leaking at the corner almost always has an opened joint, and a seamless gutter has more of those than the name suggests. Only the straight runs are one piece. Every corner, end cap and downspout outlet is a separate connection held watertight by a bead of sealant, and that bead sits in wet leaves through sun and frost for years.
Before anyone climbs up with a tube of caulk, though, it's worth finding out whether the joint is the problem at all. Water that shows up at a corner can also be coming over the front of the gutter or down behind it, and sealant fixes neither. You can sort this out from your lawn during a steady rain.
Three ways water leaves a gutter at the corner
Stand back far enough to see the whole corner and watch where the water first appears. These three look alike from the kitchen window and need three different repairs.
Through the joint
Drips from the underside, in light rain too, and keeps dripping after the rain stops. This is the one sealant can fix.
Over the front lip
A sheet of water, only in hard rain, often under a roof valley. The trough is full, so look at a clogged downspout first.
Behind the gutter
The board behind the gutter is wet or stained above the gutter line. The trouble is at the roof edge, usually the drip edge or a run that has tipped forward.
Side views. The dashed line is the water you see from the ground.
Take a phone video while it's raining. It settles the question later, when the gutter is dry and everything looks fine.
Where a one-piece gutter still has joints
"Seamless" means the installer rolled each straight length from a coil on site, so there's no connector every 10 feet. A gutter can't be rolled around a corner, though, and it has to stop somewhere. Here is an ordinary stretch of eave seen from above.
Inside corner
Usually the first to go. Two runs empty toward it and it often sits under a roof valley, so it takes far more water than any other joint. Look for a drip line right off the point of the corner.
Outside corner
Both runs push and pull on it as they heat and cool. It tends to stain the corner trim or the siding directly below.
End cap
Leaves a dirt stripe or peeling paint straight down from the end of the run. An end cap should sit at the dry end. If this one stays wet, water is draining the wrong way, and our guide to gutter pitch and downspout placement shows what correct fall looks like.
Downspout outlet
A hole cut into the trough bottom with a sealed collar. You'll see water around the top of the downspout, or a wet patch of wall behind it.
How far an aluminum run moves in a year
A leak that comes back after every reseal usually has movement behind it. Aluminum expands about 23 millionths of its length per degree Celsius, according to the Engineering ToolBox table of expansion coefficients. That works out to about 12.8 millionths per degree Fahrenheit. Here it is over a 100 degree swing, which is modest for a gutter in northern Illinois: a below-zero January night to a July afternoon, with the metal hotter than the air.
Our arithmetic from the coefficient above. Red marks a run past the 50 ft limit described below.
All of that movement ends up at the joints, and that's why manufacturers cap the length. Berger Building Products' technical sheet on gutter and downspout sizing puts it this way: "No gutter length should exceed 50 ft without an expansion joint." An expansion joint is a deliberate break that lets two lengths move separately. The same sheet warns that it "acts as a dam in the gutter", so each side needs its own downspout. That's worth asking about if you have a long ranch or a continuous eave.
What the sealant maker says about caulking over a leak
Geocel 2320 is a common gutter sealant, and its product data sheet explains most of the quick repairs that don't last.
"When resealing over old existing sealant, remove sealant before applying"
A new bead on a cracked old one grips the old material instead of the metal, and it lets go along the same line. The sheet allows a touch-up on a damaged spot, but a joint that has failed along its length needs to be dug out.
"applied to a clean, dry surface which is free of foreign matter"
Corners collect leaves and shingle grit. The inside of the joint has to be scraped and wiped first, and that can't be done from the lawn or in a hurry.
"Do not use on joints larger than 1/4” x 1/4”."
A wooden pencil is a bit thicker than that. A corner that has pulled open wider has moved, and a fatter bead won't hold it through another winter.
"Tack Free Time 20 mins." and "Cure: 5 days"
The bead looks finished within the hour and is still curing all week. A heavy storm in those days is hitting a joint that hasn't reached full strength.
Reseal, rebuild the joint, or deal with the whole run
Which one you need comes down to the size of the gap, the condition of the metal, and how many times the joint has already failed.
Reseal
The joint is tight and the metal is sound, but the old sealant has cracked or peeled.
Dig out the old bead, clean the metal and seal it fresh.
Rebuild the joint
The gap is past 1/4 inch, the miter is bent or split, or the corner has been resealed before and still drips.
Take it apart, fit and fasten a new piece, then seal.
Re-hang or replace the run
A long run keeps pulling its corners open, holds water in low spots, or is corroded along its length.
Fix the hangers and pitch, or roll a new length.
If the run has dropped between its supports, a new corner won't last until that's sorted, and what holds a gutter up explains how to spot it. Corners and end caps are small jobs on our gutter repair list. A new run is priced like new gutter, and we keep Chicago-area figures in how much gutters cost per foot.
What Illinois law lets a roofer do if a storm caused it
Sealant that aged out over the years counts as maintenance, and homeowners policies generally exclude wear and tear. A corner crushed by a falling limb or dented in a hail storm is a different question, and our article on hail damage to gutters and downspouts shows what that damage looks like. Your carrier decides what it pays. Illinois law bars a roofing contractor from negotiating a claim, so we document the damage and write the estimate for the work. If you want someone to represent you on the claim itself, that's our partner, the licensed Illinois public adjuster State Adjusting Services.
Other questions we hear about dripping corners
Why does my gutter corner only leak in heavy rain?
More water raises the level in the trough until it reaches a gap that sits above the normal waterline. It can also mean the corner is overflowing and not leaking at all. Check for a sheet of water over the front lip versus a drip from underneath.
Can one corner be repaired without replacing the gutter?
Usually, yes. A corner, end cap or outlet can be resealed or rebuilt on its own when the rest of the run is straight, well supported and draining toward the downspout.
Why is there a leak if my gutters were installed recently?
A new joint can leak if it was sealed over dirt or moisture, if the run was pitched toward the end cap, or if a long run has no expansion joint. Sealant age is rarely the cause on a young gutter, so check those first.
If a corner of yours is dripping, you can book a free inspection or call (866) 992-2982. We work across the north and northwest Chicago suburbs from Lincolnshire, Monday to Friday 7:00 AM to 7:00 PM and Saturday and Sunday 9:00 AM to 5:00 PM. That rain video helps, so have it handy.