Gutter slope is the least visible specification in the house and the reason half of all "bad gutter" complaints exist. Too flat and water pools until mosquitoes and rust find it; too steep and the far end drains late while the outlet end — the visible line — looks wrong. The trade's answer is one number: 1/4 inch of fall per 10 feet.
Why exactly a quarter inch
The rule is a compromise between hydraulics and geometry. Water needs fall to move; gutters hang on a house that reads level. A quarter-inch-per-ten gives real velocity (water reaches the outlet instead of creeping) while a 40-foot run hides its single inch of drop from the eye. Hydraulic optimum would be steeper; aesthetics would be flatter; the rule splits them.
Setting it in the field
- Mark the high point — 1 inch below the roof's drip edge (or the run's center high point on 40+ ft runs sloping both ways).
- Chalk the line — string level or laser at the quarter-inch rate; the chalk is the specification.
- Hang to the chalk, hangers every 24–36 inches, screwed into rafter tails where the fascia allows.
- Prove it: two gallons of water at the high end should reach the outlet in under a minute with no standing residue.
The failure signatures
| Symptom | Cause | Fix |
|---|---|---|
| Standing water after rain | Flat spot (sag between hangers) or whole-run flat | Add hanger + re-set slope to chalk |
| Overflow at the far end in heavy rain | Too steep — water outruns and sheets past the outlet | Reduce slope toward the quarter-inch rule |
| Drips at every seam | Sectional gutters + aging seals | Seamless replacement (the slope math transfers) |
The long-run exception
Runs past 40 feet cannot hold one direction's slope without the high end reading visibly low. The professional pattern: high point at center, downspouts at both ends, each side carrying its own quarter-inch-per-ten. The slope calculator prints both the single-direction and center-high layouts.
Slope is the gutter's only moving part. Set it with chalk, prove it with two gallons, and the system — size, outlets, slope — works as designed for decades.