How to spot a sag
From the street, look along each gutter run against the roofline. A healthy gutter follows a straight, gently sloping line toward its downspouts. A sag shows up as a dip, a wave or a section that tilts outward. Other signs are water spilling over the front at one spot even when the gutter is clean, standing water a day after rain, a gap between the back of the gutter and the fascia, and stains on the siding below one section. In winter, icicles concentrated at one point often mark a low spot where water collects and freezes.
Cause 1: loose spikes and ferrules
Older gutters were often hung with long spikes driven through the gutter and a tube (ferrule) into the fascia. Every season of heat, cold, wind and ice works those spikes outward a little. After years, they no longer hold, and the gutter drops between hangers. The fix is to replace spikes with hidden hangers screwed through the fascia into the rafter tails. Even on gutters that are otherwise sound, a hanger upgrade can straighten the line and stop the sag. See gutter hangers explained.
Cause 2: hangers too far apart
Hangers spaced too widely let the gutter bend between them under the weight of water, debris, snow and ice. Angi’s guidance puts typical spacing at no more than about 3 feet, and about 2 feet where heavy rain and snow are common. Adding hangers between the existing ones is a quick and inexpensive fix when the fascia is sound.
Cause 3: rotten fascia
Hangers are only as strong as the wood they are screwed into. When fascia rots, often from years of water running behind the gutter, screws pull out and the gutter leans away from the house. Re-screwing into rotten wood does not last. The rotten boards must be replaced first, and the cause, usually a missing drip edge or overflow, must be fixed. See fascia repair before new gutters.
Cause 4: weight of debris and water
A gutter full of wet leaves, needles and shingle grit is heavy, and standing water adds more. Over time that weight bends the gutter between hangers, and the bend then holds even more water. Clearing the gutter, adding hangers and installing guards breaks the cycle. See are gutter guards worth it?
Cause 5: ice and snow
In cold climates, ice that builds up in gutters and at the eaves can weigh far more than water. Ice dams and sliding snow bend gutters outward and downward, sometimes tearing them off. Close hanger spacing, heavier-gauge gutters, snow guards on metal roofs and attic air sealing reduce winter sags. See snow load and gutters and ice dams and gutters.
Cause 6: ladders and impacts
Leaning a ladder against a gutter, falling limbs and hail can bend a section. A bent gutter may still drain but no longer holds a straight line. Ladder stabilizers that rest on the wall or roof protect gutters during work. Badly bent sections should be replaced.
Cause 7: wrong slope from the start
Sometimes the gutter was simply hung with too little fall, or with fall in the wrong direction, so water sits in the middle of the run. That standing water adds weight and slowly deepens the low spot. Re-hanging the run along a properly snapped slope line, commonly about 1/4 inch per 10 feet toward the downspout, solves it. See how to measure for gutters.
Repair or replace?
A single sagging run on otherwise good gutters is a repair: new hangers, maybe a fascia board, a corrected slope. When sags appear along many runs, seams leak, fascia is rotten in several places and the gutters are near the end of their typical life, replacement usually costs less in the long run. New seamless gutters with hidden hangers at close spacing avoid most of these problems for years. See signs you need new gutters and how long do gutters last?
What repairs cost
Angi reports typical gutter repairs in the range of a few hundred dollars, with a national average around $390 in its recent data. Adding hangers to a run is at the low end; fascia replacement, work on tall homes and rebuilding several runs cost more. HomeGuide puts full seamless aluminum replacement at about $6 to $12 per linear foot installed nationally. See our cost guide.
Why it is worth fixing quickly
A sag concentrates water at one point. That spot overflows in every storm, soaks the fascia and soffit behind it, splashes the siding and drops a stream of water at the foundation. In winter it freezes into a heavy block of ice that pulls the gutter further down. Fixing a sag early is cheaper than the fascia, siding and foundation repairs that follow.
Checking a sag yourself
From the ground, run a garden hose into the high end of the gutter on a dry day and watch where the water goes. Where it pools instead of running to the downspout, there is a low spot. From a safe ladder position on a one-story house, look at the hangers near the sag: a spike sticking out, a hanger bent down or a screw that turns without biting points to the cause. Press the fascia near the sag with a screwdriver; if it is soft, the wood needs replacing before anything else.
A note on safety
Sagging gutters are often a sign of rotten wood behind them, and a gutter that is pulling away can come loose under the weight of a ladder or a person. Never lean a ladder against a sagging gutter, and leave work on two-story homes to professionals. See DIY or professional gutter installation?