Three days after the last rain, there is still water sitting in a section of gutter on the back of the house. You noticed a green line of algae along the inside when you were up on the ladder for something else, or a bird has been using it. Everything about the gutter looks right. It is straight, it is attached, nothing is clogged. It just does not empty.
A gutter is supposed to be slightly crooked. That is the whole design. It has to be tilted enough that water runs downhill to the outlet, and the standard slope is about a quarter inch of drop for every ten feet of run. On a 30-foot side of a house that is three quarters of an inch of total fall, which is small enough that you cannot see it from the yard and large enough that it moves every drop of water off that run. Get it wrong by a quarter inch and you have a bathtub.
The Fascia Line Is Not Level, and That Is the Trap
Here is where a lot of gutters go wrong. The fast way to hang one is to hold it a consistent distance below the roof edge and run it that way down the wall. It looks perfect when you step back. It is also at the mercy of whether the fascia itself is level, and on a house that has stood thirty or forty years in Georgia clay, it usually is not. Foundations settle. Framing moves. A fascia run half an inch low at one end hands you a gutter half an inch low at that end no matter how carefully it was hung parallel.
If the low end happens to be where the downspout is, you got lucky and the gutter drains better than intended. If the low end is the opposite corner, you now have a gutter that runs uphill toward its outlet, and it will hold water in the far corner permanently.
The correct method is to establish the outlet as the low point, mark the high end at the right elevation with a level and a chalk line, and hang to that line rather than to the roof. Whether the resulting gutter is exactly parallel to the fascia is beside the point. It usually reads as parallel to the eye anyway, because three quarters of an inch across thirty feet is below what anybody notices.
Long runs need one more decision. Past about forty feet, pitching the whole thing one direction leaves the far end noticeably low and makes a single downspout take everything. The better arrangement is a high point in the middle with the gutter falling both ways to downspouts at each end. That halves the travel distance, halves the volume through each outlet, and keeps the visible drop small enough that nobody ever asks about it.
What Sitting Water Actually Costs You
It is tempting to treat a puddle in the gutter as cosmetic. It is not, for three reasons, and they compound.
The first is weight. Standing water is a permanent load in a place designed for an intermittent one. A five-inch gutter holds roughly a gallon and a quarter per foot when full, and even a shallow standing pool is meaningful when it never goes away. It pulls that section down, which deepens the low spot, which holds more water. A gutter with a puddle in it is actively making the puddle bigger.
The second is what it does to the seams. Sealant at a joint that stays wet year-round fails several times faster than sealant at a joint that dries out after every rain. If the low spot happens to sit over a corner or an end cap, which it often does because corners are where hangers are hardest to place, you have put the most vulnerable part of the gutter underwater.
The third is mosquitoes, and around here that is not a small thing. It takes somewhere in the range of eight to ten days of standing water for a mosquito to get from egg to adult. A gutter that holds water for a week after every rain is a nursery running all summer, roughly fifteen feet from wherever you sit outside. People spend real money on screened porches and yard treatments while a hundred feet of gutter quietly undoes the effort.
Fixing It Is Usually Cheaper Than It Sounds
Re-pitching a run of gutter does not mean replacing it. On a gutter hung with hidden hangers, it means backing out screws, resetting the line, and refastening, and a competent crew does a typical run in well under a day. On spike-and-ferrule gutter it usually turns into re-hanging with proper hangers at the same time, because the old nail holes will not hold the gutter at a new elevation anyway.
The exception is a gutter that has held water long enough to deform. Aluminum loaded in the same spot for years develops a permanent belly, and once the bottom has stretched, adjusting the ends will not pull it straight. You can tell by sighting down the inside: a pitch problem reads as a straight gutter sitting at the wrong angle, and a deformed gutter reads as a visible dip in an otherwise straight line. The first is an adjustment. The second is a replacement.