Front, back, one spot or the whole run — each has a different cause, and one of them is not a gutter problem at all.
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Gutters, downspouts and the edge of the roof
A gutter is the handover point between the roof and the ground. When it stops working, the damage shows up at the fascia, the soffit and the wall — long before anybody thinks to look up.
Where the roof hands the water over
Gutters get treated as a maintenance chore and they are really a diagnostic instrument. Where a gutter overflows, and when, tells you things about the roof above it that are hard to see any other way.
A gutter that overflows only in heavy downpours but handles ordinary rain is usually a capacity or a fall problem, not a blockage. One that overflows at one specific point every time has a blockage or a sag at that point. One that overflows at the back, against the wall, is often not a gutter fault at all: water is getting behind it because the drip edge is missing or the shingles do not overhang far enough, which sends water down the fascia instead of into the trough.
What collects in there is evidence too. A gutter full of granules — coarse sand, the colour of the shingles — is reporting on the state of the roof covering, not on the gutter. A small amount after a new roof is normal. Years of accumulation with bald patches visible above is the roof telling you where it is in its life.
The consequences run downhill in a predictable sequence. Water going over the back rots the fascia, the board the gutter is fixed to; a rotten fascia loses its grip on the fixings, so the gutter starts pulling away, which makes the fall worse, which makes it overflow more. Persistent overflow at one point saturates the soffit, and a soffit that has gone soft is both a moisture problem and the end of your attic intake ventilation, because that is where the vents are.
One firm boundary on this page. Gutters and downspouts are ours up to the point where the water reaches the ground. What happens after that — water pooling against the foundation, soaking into the soil, finding its way into a basement or a crawl space — is a different subject with different causes and different specialists, and it is not covered here. We will say clearly that a downspout discharging at the base of a wall is a problem; we will not try to be a foundation site as well.
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Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.