Dripping Joints
Even a seamless trough has joints, and the corners and the outlets are where they are.
Short answer
Joints leak because the sealant has aged and freeze-thaw cycling has worked it, because the joint was never cleaned and primed properly at install, or because thermal movement has opened it. The repair is to clean the joint back to bare metal and reseal with a proper gutter sealant — not to add another layer over the old one.
Why they leak
Sealant age. Even a good sealant has a service life, and in Toronto it is shortened by the temperature range.
Freeze-thaw cycling, which works the joint mechanically every winter.
Thermal movement, because aluminum moves measurably between summer and winter and the joint absorbs it.
Poor preparation at install, where the metal was not cleaned and the sealant did not bond.
The wrong sealant, because ordinary silicone does not adhere well to aluminum and is the common wrong choice.
And debris sitting against the joint, which holds water against it permanently.
Where the joints are
Inside and outside corners, end caps, downspout outlets, and any splice where a run was joined. On a seamless system those are the only ones, which is the whole argument for seamless. Check all of them — a drip at the corner is almost always the corner rather than the run.
The proper repair
Clean the joint out completely, removing the old sealant back to bare metal. This is the step that gets skipped and it is the one that matters.
Dry it thoroughly, which in Toronto means picking the day.
Use a sealant made for eavestrough and compatible with aluminum, applied inside the trough rather than outside.
Work it into the seam rather than laying a bead over it.
Let it cure before the next rain, which the product datasheet specifies.
And check the rest of the joints while you are there, because they are all the same age.
When to stop repairing
When the trough is pitted or corroded through, which repeated sealing does not address.
When the same joint has been resealed several times, which means the joint is moving rather than the sealant failing.
When the trough is deformed from ice, which no sealant corrects.
When the fascia behind it is rotted, which has to be dealt with first and which means taking the trough off anyway.
When the slope is wrong, because water standing against a joint will defeat any sealant.
And when the system is old enough that replacing a run with a seamless one costs little more than a series of repairs.
Common questions
Can I just add more sealant?
Not over the old. Clean it back to bare metal first or it will not bond.
What sealant should be used?
One made for eavestrough and compatible with aluminum, applied inside the trough.
When is it time to replace the run?
When the trough is corroded or deformed, or when the same joint keeps failing.