Sagging And Pulling Away
A trough pulling away from the fascia is almost always telling you about the fascia rather than the trough.
Short answer
The usual cause is rotted fascia, which no longer holds a fastener. That rot came from water getting behind the trough, which came from ice, a missing drip edge, or a trough sloped the wrong way. Reattaching it to soft wood fails within a year — the fascia has to be dealt with.
The usual causes
Rotted fascia, which will not hold a screw or a spike. The most common cause by a wide margin.
Ice load in the trough, which is the Toronto particular and which pulls the hangers straight out.
Spikes and ferrules rather than hidden hangers, which work loose over years of thermal movement.
Hanger spacing too wide, so each one carries too much.
Debris weight, where a trough has been full for years and the soil in it is heavy.
And a ladder leaned against the trough, which deforms it and is how a surprising number of these start.
Fix the fascia, not the trough
A trough screwed back into soft, rotted fascia will come off again within a season. The fascia board has to be replaced where it has gone, and the reason it went — water behind the trough, no drip edge, wrong slope — has to be dealt with at the same time or the new board rots too.
What to look for
Press the fascia with a screwdriver behind the trough. Soft, spongy or crumbling means rot.
Staining down the fascia, which means water running behind the trough rather than into it.
Visible gaps between the trough and the fascia.
A trough that is level or sloping the wrong way, which holds water.
Spike heads backed out, which is the signature of a spike-and-ferrule system reaching the end.
And soffit damage, because the same water that rotted the fascia reached the soffit.
The proper repair
Remove the trough section and assess the fascia properly.
Replace rotted fascia, and check the sub-fascia and the rafter tails behind it, which are frequently affected too.
Fit drip edge if it is missing, which is the thing that stops water getting behind the trough in the first place.
Rehang with hidden hangers screwed into solid wood at proper spacing.
Set the slope correctly toward the outlet, which is the most common thing done wrong.
And deal with the ice cause — attic air sealing and fall cleaning — because otherwise the ice pulls the new hangers out as well.
Common questions
Why is my eavestrough pulling away?
Usually rotted fascia that no longer holds a fastener, plus ice load.
Can it just be screwed back on?
Not into rotted wood. It will come off again within a season.
What stops it recurring?
Drip edge, hidden hangers into solid wood, correct slope, and dealing with the attic heat loss.